xref: /linux/mm/userfaultfd.c (revision 546b928da0427b0d6c663cbb992bd7bfa9ac7971)

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  mm/userfaultfd.c
 *
 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
 *  Copyright (C) 2008-2009 Red Hat, Inc.
 *  Copyright (C) 2015  Red Hat, Inc.
 *
 *  Some part derived from fs/eventfd.c (anon inode setup) and
 *  mm/ksm.c (mm hashing).
 */

#include <linux/mm.h>
#include <linux/sched/signal.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/leafops.h>
#include <linux/userfaultfd_k.h>
#include <linux/mmu_notifier.h>
#include <linux/hugetlb.h>
#include <linux/list.h>
#include <linux/sched/mm.h>
#include <linux/mm_inline.h>
#include <linux/poll.h>
#include <linux/slab.h>
#include <linux/seq_file.h>
#include <linux/bug.h>
#include <linux/anon_inodes.h>
#include <linux/syscalls.h>
#include <linux/miscdevice.h>
#include <linux/uio.h>
#include <linux/file.h>
#include <linux/cleanup.h>
#include <asm/tlbflush.h>
#include <asm/tlb.h>
#include "internal.h"
#include "swap.h"

struct mfill_state {
	struct userfaultfd_ctx *ctx;
	unsigned long src_start;
	unsigned long dst_start;
	unsigned long len;
	uffd_flags_t flags;

	struct vm_area_struct *vma;
	unsigned long src_addr;
	unsigned long dst_addr;
	pmd_t *pmd;
};

static bool anon_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
{
	/* anonymous memory does not support MINOR mode */
	if (vm_flags & VM_UFFD_MINOR)
		return false;
	return true;
}

static struct folio *anon_alloc_folio(struct vm_area_struct *vma,
				      unsigned long addr)
{
	struct folio *folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma,
					      addr);

	if (!folio)
		return NULL;

	if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL)) {
		folio_put(folio);
		return NULL;
	}

	return folio;
}

static const struct vm_uffd_ops anon_uffd_ops = {
	.can_userfault	= anon_can_userfault,
	.alloc_folio	= anon_alloc_folio,
};

static const struct vm_uffd_ops *vma_uffd_ops(struct vm_area_struct *vma)
{
	if (vma_is_anonymous(vma))
		return &anon_uffd_ops;
	return vma->vm_ops->uffd_ops;
}

static __always_inline
bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
{
	/* Make sure that the dst range is fully within dst_vma. */
	if (dst_end > dst_vma->vm_end)
		return false;

	/*
	 * Check the vma is registered in uffd, this is required to
	 * enforce the VM_MAYWRITE check done at uffd registration
	 * time.
	 */
	if (!dst_vma->vm_userfaultfd_ctx.ctx)
		return false;

	return true;
}

static __always_inline
struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
						 unsigned long addr)
{
	struct vm_area_struct *vma;

	mmap_assert_locked(mm);
	vma = vma_lookup(mm, addr);
	if (!vma)
		vma = ERR_PTR(-ENOENT);
	else if (!(vma->vm_flags & VM_SHARED) &&
		 unlikely(anon_vma_prepare(vma)))
		vma = ERR_PTR(-ENOMEM);

	return vma;
}

#ifdef CONFIG_PER_VMA_LOCK
/*
 * uffd_lock_vma() - Lookup and lock vma corresponding to @address.
 * @mm: mm to search vma in.
 * @address: address that the vma should contain.
 *
 * Should be called without holding mmap_lock.
 *
 * Return: A locked vma containing @address, -ENOENT if no vma is found, or
 * -ENOMEM if anon_vma couldn't be allocated.
 */
static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
				       unsigned long address)
{
	struct vm_area_struct *vma;

	vma = lock_vma_under_rcu(mm, address);
	if (vma) {
		/*
		 * We know we're going to need to use anon_vma, so check
		 * that early.
		 */
		if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
			vma_end_read(vma);
		else
			return vma;
	}

	mmap_read_lock(mm);
	vma = find_vma_and_prepare_anon(mm, address);
	if (!IS_ERR(vma)) {
		bool locked = vma_start_read_locked(vma);

		if (!locked)
			vma = ERR_PTR(-EAGAIN);
	}

	mmap_read_unlock(mm);
	return vma;
}

static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
					      unsigned long dst_start,
					      unsigned long len)
{
	struct vm_area_struct *dst_vma;

	dst_vma = uffd_lock_vma(dst_mm, dst_start);
	if (IS_ERR(dst_vma) || validate_dst_vma(dst_vma, dst_start + len))
		return dst_vma;

	vma_end_read(dst_vma);
	return ERR_PTR(-ENOENT);
}

static void uffd_mfill_unlock(struct vm_area_struct *vma)
{
	vma_end_read(vma);
}

#else

static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
					      unsigned long dst_start,
					      unsigned long len)
{
	struct vm_area_struct *dst_vma;

	mmap_read_lock(dst_mm);
	dst_vma = find_vma_and_prepare_anon(dst_mm, dst_start);
	if (IS_ERR(dst_vma))
		goto out_unlock;

	if (validate_dst_vma(dst_vma, dst_start + len))
		return dst_vma;

	dst_vma = ERR_PTR(-ENOENT);
out_unlock:
	mmap_read_unlock(dst_mm);
	return dst_vma;
}

static void uffd_mfill_unlock(struct vm_area_struct *vma)
{
	mmap_read_unlock(vma->vm_mm);
}
#endif

static void mfill_put_vma(struct mfill_state *state)
{
	if (!state->vma)
		return;

	up_read(&state->ctx->map_changing_lock);
	uffd_mfill_unlock(state->vma);
	state->vma = NULL;
}

static int mfill_get_vma(struct mfill_state *state)
{
	struct userfaultfd_ctx *ctx = state->ctx;
	uffd_flags_t flags = state->flags;
	struct vm_area_struct *dst_vma;
	const struct vm_uffd_ops *ops;
	int err;

	/*
	 * Make sure the vma is not shared, that the dst range is
	 * both valid and fully within a single existing vma.
	 */
	dst_vma = uffd_mfill_lock(ctx->mm, state->dst_start, state->len);
	if (IS_ERR(dst_vma))
		return PTR_ERR(dst_vma);

	/*
	 * If memory mappings are changing because of non-cooperative
	 * operation (e.g. mremap) running in parallel, bail out and
	 * request the user to retry later
	 */
	down_read(&ctx->map_changing_lock);
	state->vma = dst_vma;
	err = -EAGAIN;
	if (atomic_read(&ctx->mmap_changing))
		goto out_unlock;

	err = -EINVAL;

	/*
	 * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
	 * it will overwrite vm_ops, so vma_is_anonymous must return false.
	 */
	if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
	    dst_vma->vm_flags & VM_SHARED))
		goto out_unlock;

	/*
	 * validate 'mode' now that we know the dst_vma: don't allow
	 * a wrprotect copy if the userfaultfd didn't register as WP.
	 */
	if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
		goto out_unlock;

	if (is_vm_hugetlb_page(dst_vma))
		return 0;

	ops = vma_uffd_ops(dst_vma);
	if (!ops)
		goto out_unlock;

	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE) &&
	    !ops->get_folio_noalloc)
		goto out_unlock;

	return 0;

out_unlock:
	mfill_put_vma(state);
	return err;
}

static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
{
	pgd_t *pgd;
	p4d_t *p4d;
	pud_t *pud;

	pgd = pgd_offset(mm, address);
	p4d = p4d_alloc(mm, pgd, address);
	if (!p4d)
		return NULL;
	pud = pud_alloc(mm, p4d, address);
	if (!pud)
		return NULL;
	/*
	 * Note that we didn't run this because the pmd was
	 * missing, the *pmd may be already established and in
	 * turn it may also be a trans_huge_pmd.
	 */
	return pmd_alloc(mm, pud, address);
}

static int mfill_establish_pmd(struct mfill_state *state)
{
	struct mm_struct *dst_mm = state->ctx->mm;
	pmd_t *dst_pmd, dst_pmdval;

	dst_pmd = mm_alloc_pmd(dst_mm, state->dst_addr);
	if (unlikely(!dst_pmd))
		return -ENOMEM;

	dst_pmdval = pmdp_get_lockless(dst_pmd);
	if (unlikely(pmd_none(dst_pmdval)) &&
	    unlikely(__pte_alloc(dst_mm, dst_pmd)))
		return -ENOMEM;

	dst_pmdval = pmdp_get_lockless(dst_pmd);
	/*
	 * If the dst_pmd is THP don't override it and just be strict.
	 * (This includes the case where the PMD used to be THP and
	 * changed back to none after __pte_alloc().)
	 */
	if (unlikely(!pmd_present(dst_pmdval) || pmd_leaf(dst_pmdval)))
		return -EEXIST;
	if (unlikely(pmd_bad(dst_pmdval)))
		return -EFAULT;

	state->pmd = dst_pmd;
	return 0;
}

/* Check if dst_addr is outside of file's size. Must be called with ptl held. */
static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
				 unsigned long dst_addr)
{
	struct inode *inode;
	pgoff_t offset, max_off;

	if (!dst_vma->vm_file)
		return false;

	inode = dst_vma->vm_file->f_inode;
	offset = linear_page_index(dst_vma, dst_addr);
	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
	return offset >= max_off;
}

/*
 * Install PTEs, to map dst_addr (within dst_vma) to page.
 *
 * This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
 * and anon, and for both shared and private VMAs.
 */
static int mfill_atomic_install_pte(pmd_t *dst_pmd,
				    struct vm_area_struct *dst_vma,
				    unsigned long dst_addr, struct page *page,
				    uffd_flags_t flags)
{
	int ret;
	struct mm_struct *dst_mm = dst_vma->vm_mm;
	pte_t _dst_pte, *dst_pte;
	bool writable = dst_vma->vm_flags & VM_WRITE;
	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
	spinlock_t *ptl;
	struct folio *folio = page_folio(page);
	bool page_in_cache = folio_mapping(folio);
	pte_t dst_ptep;

	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
	_dst_pte = pte_mkdirty(_dst_pte);
	if (page_in_cache && !vm_shared)
		writable = false;
	if (writable)
		_dst_pte = pte_mkwrite(_dst_pte, dst_vma);
	if (flags & MFILL_ATOMIC_WP)
		_dst_pte = pte_mkuffd(_dst_pte);

	ret = -EAGAIN;
	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
	if (!dst_pte)
		goto out;

	if (mfill_file_over_size(dst_vma, dst_addr)) {
		ret = -EFAULT;
		goto out_unlock;
	}

	ret = -EEXIST;

	dst_ptep = ptep_get(dst_pte);

	/*
	 * We are allowed to overwrite a UFFD pte marker: consider when both
	 * MISSING|WP registered, we firstly wr-protect a none pte which has no
	 * page cache page backing it, then access the page.
	 */
	if (!pte_none(dst_ptep) && !pte_is_uffd_marker(dst_ptep))
		goto out_unlock;

	if (page_in_cache) {
		folio_add_file_rmap_pte(folio, page, dst_vma);
	} else {
		folio_add_new_anon_rmap(folio, dst_vma, dst_addr, RMAP_EXCLUSIVE);
		folio_add_lru_vma(folio, dst_vma);
	}

	/*
	 * Must happen after rmap, as mm_counter() checks mapping (via
	 * PageAnon()), which is set by __page_set_anon_rmap().
	 */
	inc_mm_counter(dst_mm, mm_counter(folio));

	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

	if (page_in_cache)
		folio_unlock(folio);

	/* No need to invalidate - it was non-present before */
	update_mmu_cache(dst_vma, dst_addr, dst_pte);
	ret = 0;
out_unlock:
	pte_unmap_unlock(dst_pte, ptl);
out:
	return ret;
}

static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr)
{
	void *kaddr;
	int ret;

	kaddr = kmap_local_folio(folio, 0);
	/*
	 * The read mmap_lock is held here.  Despite the
	 * mmap_lock being read recursive a deadlock is still
	 * possible if a writer has taken a lock.  For example:
	 *
	 * process A thread 1 takes read lock on own mmap_lock
	 * process A thread 2 calls mmap, blocks taking write lock
	 * process B thread 1 takes page fault, read lock on own mmap lock
	 * process B thread 2 calls mmap, blocks taking write lock
	 * process A thread 1 blocks taking read lock on process B
	 * process B thread 1 blocks taking read lock on process A
	 *
	 * Disable page faults to prevent potential deadlock
	 * and retry the copy outside the mmap_lock.
	 */
	pagefault_disable();
	ret = copy_from_user(kaddr, (const void __user *) src_addr,
			     PAGE_SIZE);
	pagefault_enable();
	kunmap_local(kaddr);

	if (ret)
		return -EFAULT;

	flush_dcache_folio(folio);
	return ret;
}

#define MFILL_RETRY_STATE_VMA_FLAGS \
	append_vma_flags(__VMA_UFFD_FLAGS, VMA_SHARED_BIT)

/*
 * VMA state saved before dropping the locks in mfill_copy_folio_retry().
 * Used to detect VMA replacement or incompatible changes after reacquiring the
 * locks.
 */
struct mfill_retry_state {
	const struct vm_uffd_ops *ops;
	struct file *file;
	vma_flags_t flags;
	pgoff_t pgoff;
};

static void mfill_retry_state_save(struct mfill_retry_state *s,
				   struct vm_area_struct *vma)
{
	s->flags = vma_flags_and_mask(&vma->flags, MFILL_RETRY_STATE_VMA_FLAGS);
	s->ops = vma_uffd_ops(vma);
	s->pgoff = vma_start_pgoff(vma);

	if (vma->vm_file)
		s->file = get_file(vma->vm_file);
}

static bool mfill_retry_state_changed(struct mfill_retry_state *state,
				      struct vm_area_struct *vma)
{
	vma_flags_t flags = vma_flags_and_mask(&vma->flags,
					       MFILL_RETRY_STATE_VMA_FLAGS);

	/* Have any UFFD flags (missing, WP, minor) changed? */
	if (!vma_flags_same_pair(&state->flags, &flags))
		return true;

	/* VMA type or effective uffd_ops changed while the lock was dropped */
	if (state->ops != vma_uffd_ops(vma))
		return true;

	/* VMA was anonymous before; changed only if it no longer is */
	if (!state->file)
		return !vma_is_anonymous(vma);

	/* VMA was file backed, but file, inode or offset has changed */
	if (!vma->vm_file || vma->vm_file->f_inode != state->file->f_inode ||
	    state->file != vma->vm_file || vma_start_pgoff(vma) != state->pgoff)
		return true;

	return false;
}

static void mfill_retry_state_put(struct mfill_retry_state *s)
{
	if (s->file)
		fput(s->file);
}

DEFINE_FREE(retry_put, struct mfill_retry_state *,
	    if (_T) mfill_retry_state_put(_T));

static int mfill_copy_folio_retry(struct mfill_state *mfill_state,
				  struct folio *folio)
{
	struct mfill_retry_state retry_state = { 0 };
	struct mfill_retry_state *for_free __free(retry_put) = &retry_state;
	unsigned long src_addr = mfill_state->src_addr;
	void *kaddr;
	int err;

	mfill_retry_state_save(&retry_state, mfill_state->vma);

	/* retry copying with mm_lock dropped */
	mfill_put_vma(mfill_state);

	kaddr = kmap_local_folio(folio, 0);
	err = copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE);
	kunmap_local(kaddr);
	if (unlikely(err))
		return -EFAULT;

	flush_dcache_folio(folio);

	/* reget VMA and PMD, they could change underneath us */
	err = mfill_get_vma(mfill_state);
	if (err)
		return err;

	if (mfill_retry_state_changed(&retry_state, mfill_state->vma))
		return -EAGAIN;

	err = mfill_establish_pmd(mfill_state);
	if (err)
		return err;

	return 0;
}

static int __mfill_atomic_pte(struct mfill_state *state,
			      const struct vm_uffd_ops *ops)
{
	unsigned long dst_addr = state->dst_addr;
	unsigned long src_addr = state->src_addr;
	uffd_flags_t flags = state->flags;
	struct folio *folio;
	int ret;

	if (!ops) {
		VM_WARN_ONCE(1, "UFFDIO_COPY for unsupported VMA");
		return -EOPNOTSUPP;
	}

	folio = ops->alloc_folio(state->vma, state->dst_addr);
	if (!folio)
		return -ENOMEM;

	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY)) {
		ret = mfill_copy_folio_locked(folio, src_addr);
		/*
		 * Fallback to copy_from_user outside mmap_lock.
		 * If retry is successful, mfill_copy_folio_locked() returns
		 * with locks retaken by mfill_get_vma().
		 * If there was an error, we must mfill_put_vma() anyway and it
		 * will take care of unlocking if needed.
		 */
		if (unlikely(ret)) {
			ret = mfill_copy_folio_retry(state, folio);
			if (ret)
				goto err_folio_put;
		}
	} else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
		clear_user_highpage(&folio->page, state->dst_addr);
	} else {
		VM_WARN_ONCE(1, "Unknown UFFDIO operation, flags: %x", flags);
	}

	/*
	 * The memory barrier inside __folio_mark_uptodate makes sure that
	 * preceding stores to the page contents become visible before
	 * the set_pte_at() write.
	 */
	__folio_mark_uptodate(folio);

	if (ops->filemap_add) {
		ret = ops->filemap_add(folio, state->vma, state->dst_addr);
		if (ret)
			goto err_folio_put;
	}

	ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
				       &folio->page, flags);
	if (ret)
		goto err_filemap_remove;

	return 0;

err_filemap_remove:
	if (ops->filemap_remove)
		ops->filemap_remove(folio, state->vma);
err_folio_put:
	folio_put(folio);
	return ret;
}

static int mfill_atomic_pte_copy(struct mfill_state *state)
{
	const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);

	/*
	 * The normal page fault path for a MAP_PRIVATE mapping in a
	 * file-backed VMA will invoke the fault, fill the hole in the file and
	 * COW it right away. The result generates plain anonymous memory.
	 * So when we are asked to fill a hole in a MAP_PRIVATE mapping, we'll
	 * generate anonymous memory directly without actually filling the
	 * hole. For the MAP_PRIVATE case the robustness check only happens in
	 * the pagetable (to verify it's still none) and not in the page cache.
	 */
	if (!(state->vma->vm_flags & VM_SHARED))
		ops = &anon_uffd_ops;

	return __mfill_atomic_pte(state, ops);
}

static int mfill_atomic_pte_zeroed_folio(struct mfill_state *state)
{
	const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);

	return __mfill_atomic_pte(state, ops);
}

static int mfill_atomic_pte_zeropage(struct mfill_state *state)
{
	struct vm_area_struct *dst_vma = state->vma;
	unsigned long dst_addr = state->dst_addr;
	pmd_t *dst_pmd = state->pmd;
	pte_t _dst_pte, *dst_pte;
	spinlock_t *ptl;
	int ret;

	if (mm_forbids_zeropage(dst_vma->vm_mm) ||
	    (dst_vma->vm_flags & VM_SHARED))
		return mfill_atomic_pte_zeroed_folio(state);

	_dst_pte = pte_mkspecial(pfn_pte(zero_pfn(dst_addr),
					 dst_vma->vm_page_prot));
	ret = -EAGAIN;
	dst_pte = pte_offset_map_lock(dst_vma->vm_mm, dst_pmd, dst_addr, &ptl);
	if (!dst_pte)
		goto out;
	if (mfill_file_over_size(dst_vma, dst_addr)) {
		ret = -EFAULT;
		goto out_unlock;
	}
	ret = -EEXIST;
	if (!pte_none(ptep_get(dst_pte)))
		goto out_unlock;
	set_pte_at(dst_vma->vm_mm, dst_addr, dst_pte, _dst_pte);
	/* No need to invalidate - it was non-present before */
	update_mmu_cache(dst_vma, dst_addr, dst_pte);
	ret = 0;
out_unlock:
	pte_unmap_unlock(dst_pte, ptl);
out:
	return ret;
}

/* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
static int mfill_atomic_pte_continue(struct mfill_state *state)
{
	struct vm_area_struct *dst_vma = state->vma;
	const struct vm_uffd_ops *ops = vma_uffd_ops(dst_vma);
	unsigned long dst_addr = state->dst_addr;
	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
	struct inode *inode = file_inode(dst_vma->vm_file);
	uffd_flags_t flags = state->flags;
	pmd_t *dst_pmd = state->pmd;
	struct folio *folio;
	struct page *page;
	int ret;

	if (!ops) {
		VM_WARN_ONCE(1, "UFFDIO_CONTINUE for unsupported VMA");
		return -EOPNOTSUPP;
	}

	folio = ops->get_folio_noalloc(inode, pgoff);
	/* Our caller expects us to return -EFAULT if we failed to find folio */
	if (IS_ERR_OR_NULL(folio))
		return -EFAULT;

	page = folio_file_page(folio, pgoff);
	if (PageHWPoison(page)) {
		ret = -EIO;
		goto out_release;
	}

	ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
				       page, flags);
	if (ret)
		goto out_release;

	return 0;

out_release:
	folio_unlock(folio);
	folio_put(folio);
	return ret;
}

/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
static int mfill_atomic_pte_poison(struct mfill_state *state)
{
	struct vm_area_struct *dst_vma = state->vma;
	struct mm_struct *dst_mm = dst_vma->vm_mm;
	unsigned long dst_addr = state->dst_addr;
	pmd_t *dst_pmd = state->pmd;
	pte_t _dst_pte, *dst_pte;
	spinlock_t *ptl;
	int ret;

	_dst_pte = make_pte_marker(PTE_MARKER_POISONED);
	ret = -EAGAIN;
	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
	if (!dst_pte)
		goto out;

	if (mfill_file_over_size(dst_vma, dst_addr)) {
		ret = -EFAULT;
		goto out_unlock;
	}

	ret = -EEXIST;
	/* Refuse to overwrite any PTE, even a PTE marker (e.g. UFFD WP). */
	if (!pte_none(ptep_get(dst_pte)))
		goto out_unlock;

	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

	/* No need to invalidate - it was non-present before */
	update_mmu_cache(dst_vma, dst_addr, dst_pte);
	ret = 0;
out_unlock:
	pte_unmap_unlock(dst_pte, ptl);
out:
	return ret;
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * mfill_atomic processing for HUGETLB vmas.  Note that this routine is
 * called with either vma-lock or mmap_lock held, it will release the lock
 * before returning.
 */
static __always_inline ssize_t mfill_atomic_hugetlb(
					      struct userfaultfd_ctx *ctx,
					      struct vm_area_struct *dst_vma,
					      unsigned long dst_start,
					      unsigned long src_start,
					      unsigned long len,
					      uffd_flags_t flags)
{
	struct mm_struct *dst_mm = dst_vma->vm_mm;
	ssize_t err;
	pte_t *dst_pte;
	unsigned long src_addr, dst_addr;
	long copied;
	struct folio *folio;
	unsigned long vma_hpagesize;
	pgoff_t idx;
	u32 hash;
	struct address_space *mapping;

	/*
	 * There is no default zero huge page for all huge page sizes as
	 * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
	 * by THP.  Since we can not reliably insert a zero page, this
	 * feature is not supported.
	 */
	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
		up_read(&ctx->map_changing_lock);
		uffd_mfill_unlock(dst_vma);
		return -EINVAL;
	}

	src_addr = src_start;
	dst_addr = dst_start;
	copied = 0;
	folio = NULL;
	vma_hpagesize = vma_kernel_pagesize(dst_vma);

	/*
	 * Validate alignment based on huge page size
	 */
	err = -EINVAL;
	if (dst_start & (vma_hpagesize - 1) || len & (vma_hpagesize - 1))
		goto out_unlock;

retry:
	/*
	 * On routine entry dst_vma is set.  If we had to drop mmap_lock and
	 * retry, dst_vma will be set to NULL and we must lookup again.
	 */
	if (!dst_vma) {
		dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
		if (IS_ERR(dst_vma)) {
			err = PTR_ERR(dst_vma);
			goto out;
		}

		err = -ENOENT;
		if (!is_vm_hugetlb_page(dst_vma))
			goto out_unlock_vma;

		err = -EINVAL;
		if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
			goto out_unlock_vma;

		/*
		 * If memory mappings are changing because of non-cooperative
		 * operation (e.g. mremap) running in parallel, bail out and
		 * request the user to retry later
		 */
		down_read(&ctx->map_changing_lock);
		err = -EAGAIN;
		if (atomic_read(&ctx->mmap_changing))
			goto out_unlock;
	}

	while (src_addr < src_start + len) {
		VM_WARN_ON_ONCE(dst_addr >= dst_start + len);

		/*
		 * Serialize via vma_lock and hugetlb_fault_mutex.
		 * vma_lock ensures the dst_pte remains valid even
		 * in the case of shared pmds.  fault mutex prevents
		 * races with other faulting threads.
		 */
		idx = hugetlb_linear_page_index(dst_vma, dst_addr);
		mapping = dst_vma->vm_file->f_mapping;
		hash = hugetlb_fault_mutex_hash(mapping, idx);
		mutex_lock(&hugetlb_fault_mutex_table[hash]);
		hugetlb_vma_lock_read(dst_vma);

		err = -ENOMEM;
		dst_pte = huge_pte_alloc(dst_mm, dst_vma, dst_addr, vma_hpagesize);
		if (!dst_pte) {
			hugetlb_vma_unlock_read(dst_vma);
			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
			goto out_unlock;
		}

		if (!uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
			const pte_t ptep = huge_ptep_get(dst_mm, dst_addr, dst_pte);

			if (!huge_pte_none(ptep) && !pte_is_uffd_marker(ptep)) {
				err = -EEXIST;
				hugetlb_vma_unlock_read(dst_vma);
				mutex_unlock(&hugetlb_fault_mutex_table[hash]);
				goto out_unlock;
			}
		}

		err = hugetlb_mfill_atomic_pte(dst_pte, dst_vma, dst_addr,
					       src_addr, flags, &folio);

		hugetlb_vma_unlock_read(dst_vma);
		mutex_unlock(&hugetlb_fault_mutex_table[hash]);

		cond_resched();

		if (unlikely(err == -ENOENT)) {
			up_read(&ctx->map_changing_lock);
			uffd_mfill_unlock(dst_vma);
			VM_WARN_ON_ONCE(!folio);

			err = copy_folio_from_user(folio,
						   (const void __user *)src_addr, true);
			if (unlikely(err)) {
				err = -EFAULT;
				goto out;
			}

			dst_vma = NULL;
			goto retry;
		} else
			VM_WARN_ON_ONCE(folio);

		if (!err) {
			dst_addr += vma_hpagesize;
			src_addr += vma_hpagesize;
			copied += vma_hpagesize;

			if (fatal_signal_pending(current))
				err = -EINTR;
		}
		if (err)
			break;
	}

out_unlock:
	up_read(&ctx->map_changing_lock);
out_unlock_vma:
	uffd_mfill_unlock(dst_vma);
out:
	if (folio)
		folio_put(folio);
	VM_WARN_ON_ONCE(copied < 0);
	VM_WARN_ON_ONCE(err > 0);
	VM_WARN_ON_ONCE(!copied && !err);
	return copied ? copied : err;
}
#else /* !CONFIG_HUGETLB_PAGE */
/* fail at build time if gcc attempts to use this */
extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
				    struct vm_area_struct *dst_vma,
				    unsigned long dst_start,
				    unsigned long src_start,
				    unsigned long len,
				    uffd_flags_t flags);
#endif /* CONFIG_HUGETLB_PAGE */

static __always_inline ssize_t mfill_atomic_pte(struct mfill_state *state)
{
	uffd_flags_t flags = state->flags;

	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
		return mfill_atomic_pte_continue(state);
	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON))
		return mfill_atomic_pte_poison(state);
	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
		return mfill_atomic_pte_copy(state);
	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE))
		return mfill_atomic_pte_zeropage(state);

	VM_WARN_ONCE(1, "Unknown UFFDIO operation, flags: %x", flags);
	return -EOPNOTSUPP;
}

static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
					    unsigned long dst_start,
					    unsigned long src_start,
					    unsigned long len,
					    uffd_flags_t flags)
{
	struct mfill_state state = (struct mfill_state){
		.ctx = ctx,
		.dst_start = dst_start,
		.src_start = src_start,
		.flags = flags,
		.len = len,
		.src_addr = src_start,
		.dst_addr = dst_start,
	};
	long copied = 0;
	ssize_t err;

	/*
	 * Sanitize the command parameters:
	 */
	VM_WARN_ON_ONCE(dst_start & ~PAGE_MASK);
	VM_WARN_ON_ONCE(len & ~PAGE_MASK);

	/* Does the address range wrap, or is the span zero-sized? */
	VM_WARN_ON_ONCE(src_start + len <= src_start);
	VM_WARN_ON_ONCE(dst_start + len <= dst_start);

	err = mfill_get_vma(&state);
	if (err)
		goto out;

	/*
	 * If this is a HUGETLB vma, pass off to appropriate routine
	 */
	if (is_vm_hugetlb_page(state.vma))
		return  mfill_atomic_hugetlb(ctx, state.vma, dst_start,
					     src_start, len, flags);

	while (state.src_addr < src_start + len) {
		VM_WARN_ON_ONCE(state.dst_addr >= dst_start + len);

		err = mfill_establish_pmd(&state);
		if (err)
			break;

		/*
		 * For shmem mappings, khugepaged is allowed to remove page
		 * tables under us; pte_offset_map_lock() will deal with that.
		 */

		err = mfill_atomic_pte(&state);
		cond_resched();

		if (!err) {
			state.dst_addr += PAGE_SIZE;
			state.src_addr += PAGE_SIZE;
			copied += PAGE_SIZE;

			if (fatal_signal_pending(current))
				err = -EINTR;
		}
		if (err)
			break;
	}

	mfill_put_vma(&state);
out:
	VM_WARN_ON_ONCE(copied < 0);
	VM_WARN_ON_ONCE(err > 0);
	VM_WARN_ON_ONCE(!copied && !err);
	return copied ? copied : err;
}

static ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
			  unsigned long src_start, unsigned long len,
			  uffd_flags_t flags)
{
	return mfill_atomic(ctx, dst_start, src_start, len,
			    uffd_flags_set_mode(flags, MFILL_ATOMIC_COPY));
}

static ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
			      unsigned long start,
			      unsigned long len)
{
	return mfill_atomic(ctx, start, 0, len,
			    uffd_flags_set_mode(0, MFILL_ATOMIC_ZEROPAGE));
}

static ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
			      unsigned long len, uffd_flags_t flags)
{

	/*
	 * A caller might reasonably assume that UFFDIO_CONTINUE contains an
	 * smp_wmb() to ensure that any writes to the about-to-be-mapped page by
	 * the thread doing the UFFDIO_CONTINUE are guaranteed to be visible to
	 * subsequent loads from the page through the newly mapped address range.
	 */
	smp_wmb();

	return mfill_atomic(ctx, start, 0, len,
			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
}

static ssize_t mfill_atomic_poison(struct userfaultfd_ctx *ctx, unsigned long start,
			    unsigned long len, uffd_flags_t flags)
{
	return mfill_atomic(ctx, start, 0, len,
			    uffd_flags_set_mode(flags, MFILL_ATOMIC_POISON));
}

long uffd_wp_range(struct vm_area_struct *dst_vma,
		   unsigned long start, unsigned long len, bool enable_wp)
{
	unsigned int mm_cp_flags;
	struct mmu_gather tlb;
	long ret;

	VM_WARN_ONCE(start < dst_vma->vm_start || start + len > dst_vma->vm_end,
			"The address range exceeds VMA boundary.\n");
	if (enable_wp)
		mm_cp_flags = MM_CP_UFFD_WP;
	else
		mm_cp_flags = MM_CP_UFFD_WP_RESOLVE;

	/*
	 * vma->vm_page_prot already reflects that uffd-wp is enabled for this
	 * VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed
	 * to be write-protected as default whenever protection changes.
	 * Try upgrading write permissions manually.
	 */
	if (!enable_wp && vma_wants_manual_pte_write_upgrade(dst_vma))
		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
	tlb_gather_mmu(&tlb, dst_vma->vm_mm);
	ret = change_protection(&tlb, dst_vma, start, start + len, mm_cp_flags);
	tlb_finish_mmu(&tlb);

	return ret;
}

static int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
			unsigned long len, bool enable_wp)
{
	struct mm_struct *dst_mm = ctx->mm;
	unsigned long end = start + len;
	unsigned long _start, _end;
	struct vm_area_struct *dst_vma;
	unsigned long page_mask;
	long err;
	VMA_ITERATOR(vmi, dst_mm, start);

	/*
	 * Sanitize the command parameters:
	 */
	VM_WARN_ON_ONCE(start & ~PAGE_MASK);
	VM_WARN_ON_ONCE(len & ~PAGE_MASK);

	/* Does the address range wrap, or is the span zero-sized? */
	VM_WARN_ON_ONCE(start + len <= start);

	mmap_read_lock(dst_mm);

	/*
	 * If memory mappings are changing because of non-cooperative
	 * operation (e.g. mremap) running in parallel, bail out and
	 * request the user to retry later
	 */
	down_read(&ctx->map_changing_lock);
	err = -EAGAIN;
	if (atomic_read(&ctx->mmap_changing))
		goto out_unlock;

	err = -ENOENT;
	for_each_vma_range(vmi, dst_vma, end) {

		if (!userfaultfd_wp(dst_vma)) {
			err = -ENOENT;
			break;
		}

		if (is_vm_hugetlb_page(dst_vma)) {
			err = -EINVAL;
			page_mask = vma_kernel_pagesize(dst_vma) - 1;
			if ((start & page_mask) || (len & page_mask))
				break;
		}

		_start = max(dst_vma->vm_start, start);
		_end = min(dst_vma->vm_end, end);

		err = uffd_wp_range(dst_vma, _start, _end - _start, enable_wp);

		/* Return 0 on success, <0 on failures */
		if (err < 0)
			break;
		err = 0;
	}
out_unlock:
	up_read(&ctx->map_changing_lock);
	mmap_read_unlock(dst_mm);
	return err;
}

int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
		     unsigned long len, bool enable_rwp)
{
	struct mm_struct *dst_mm = ctx->mm;
	unsigned long end = start + len;
	struct vm_area_struct *dst_vma;
	unsigned int mm_cp_flags;
	struct mmu_gather tlb;
	bool found = false;
	VMA_ITERATOR(vmi, dst_mm, start);

	VM_WARN_ON_ONCE(start & ~PAGE_MASK);
	VM_WARN_ON_ONCE(len & ~PAGE_MASK);
	VM_WARN_ON_ONCE(start + len <= start);

	guard(mmap_read_lock)(dst_mm);
	guard(rwsem_read)(&ctx->map_changing_lock);

	if (atomic_read(&ctx->mmap_changing))
		return -EAGAIN;

	if (enable_rwp)
		mm_cp_flags = MM_CP_UFFD_RWP;
	else
		mm_cp_flags = MM_CP_UFFD_RWP_RESOLVE;

	/*
	 * Pre-scan the range: validate every spanned VMA before applying
	 * any change_protection() so a partial failure cannot leave the
	 * process with only a prefix of the range re-protected.
	 */
	for_each_vma_range(vmi, dst_vma, end) {
		if (!userfaultfd_rwp(dst_vma))
			return -ENOENT;

		if (is_vm_hugetlb_page(dst_vma)) {
			unsigned long page_mask;

			page_mask = vma_kernel_pagesize(dst_vma) - 1;
			if ((start & page_mask) || (len & page_mask))
				return -EINVAL;
		}
		found = true;
	}
	if (!found)
		return -ENOENT;

	vma_iter_set(&vmi, start);
	tlb_gather_mmu(&tlb, dst_mm);
	for_each_vma_range(vmi, dst_vma, end) {
		unsigned long vma_start = max(dst_vma->vm_start, start);
		unsigned long vma_end = min(dst_vma->vm_end, end);
		unsigned int flags = mm_cp_flags;

		/*
		 * On resolve, try to upgrade writability per-VMA --
		 * MM_CP_TRY_CHANGE_WRITABLE WARNs in
		 * maybe_change_pte_writable() if the VMA is not VM_WRITE,
		 * and RWP can be registered on PROT_READ-only mappings.
		 */
		if (!enable_rwp && vma_wants_manual_pte_write_upgrade(dst_vma))
			flags |= MM_CP_TRY_CHANGE_WRITABLE;

		change_protection(&tlb, dst_vma, vma_start, vma_end, flags);
	}
	tlb_finish_mmu(&tlb);

	return 0;
}

void double_pt_lock(spinlock_t *ptl1,
		    spinlock_t *ptl2)
	__acquires(ptl1)
	__acquires(ptl2)
{
	if (ptl1 > ptl2)
		swap(ptl1, ptl2);
	/* lock in virtual address order to avoid lock inversion */
	spin_lock(ptl1);
	if (ptl1 != ptl2)
		spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
	else
		__acquire(ptl2);
}

void double_pt_unlock(spinlock_t *ptl1,
		      spinlock_t *ptl2)
	__releases(ptl1)
	__releases(ptl2)
{
	spin_unlock(ptl1);
	if (ptl1 != ptl2)
		spin_unlock(ptl2);
	else
		__release(ptl2);
}

static inline bool is_pte_pages_stable(pte_t *dst_pte, pte_t *src_pte,
				       pte_t orig_dst_pte, pte_t orig_src_pte,
				       pmd_t *dst_pmd, pmd_t dst_pmdval)
{
	return pte_same(ptep_get(src_pte), orig_src_pte) &&
	       pte_same(ptep_get(dst_pte), orig_dst_pte) &&
	       pmd_same(dst_pmdval, pmdp_get_lockless(dst_pmd));
}

/*
 * Checks if the two ptes and the corresponding folio are eligible for batched
 * move. If so, then returns pointer to the locked folio. Otherwise, returns NULL.
 *
 * NOTE: folio's reference is not required as the whole operation is within
 * PTL's critical section.
 */
static struct folio *check_ptes_for_batched_move(struct vm_area_struct *src_vma,
						 unsigned long src_addr,
						 pte_t *src_pte, pte_t *dst_pte)
{
	pte_t orig_dst_pte, orig_src_pte;
	struct folio *folio;

	orig_dst_pte = ptep_get(dst_pte);
	if (!pte_none(orig_dst_pte))
		return NULL;

	orig_src_pte = ptep_get(src_pte);
	if (!pte_present(orig_src_pte) || is_zero_pfn(pte_pfn(orig_src_pte)))
		return NULL;

	folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
	if (!folio || !folio_trylock(folio))
		return NULL;
	if (!PageAnonExclusive(&folio->page) || folio_test_large(folio)) {
		folio_unlock(folio);
		return NULL;
	}
	return folio;
}

/*
 * Moves src folios to dst in a batch as long as they are not large, and can
 * successfully take the lock via folio_trylock().
 */
static long move_present_ptes(struct mm_struct *mm,
			      struct vm_area_struct *dst_vma,
			      struct vm_area_struct *src_vma,
			      unsigned long dst_addr, unsigned long src_addr,
			      pte_t *dst_pte, pte_t *src_pte,
			      pte_t orig_dst_pte, pte_t orig_src_pte,
			      pmd_t *dst_pmd, pmd_t dst_pmdval,
			      spinlock_t *dst_ptl, spinlock_t *src_ptl,
			      struct folio **first_src_folio, unsigned long len)
{
	int err = 0;
	struct folio *src_folio = *first_src_folio;
	unsigned long src_start = src_addr;
	unsigned long src_end;

	len = pmd_addr_end(dst_addr, dst_addr + len) - dst_addr;
	src_end = pmd_addr_end(src_addr, src_addr + len);
	flush_cache_range(src_vma, src_addr, src_end);
	double_pt_lock(dst_ptl, src_ptl);

	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
				 dst_pmd, dst_pmdval)) {
		err = -EAGAIN;
		goto out;
	}
	if (folio_test_large(src_folio) ||
	    folio_maybe_dma_pinned(src_folio) ||
	    !PageAnonExclusive(&src_folio->page)) {
		err = -EBUSY;
		goto out;
	}
	/* It's safe to drop the reference now as the page-table is holding one. */
	folio_put(*first_src_folio);
	*first_src_folio = NULL;
	lazy_mmu_mode_enable();

	while (true) {
		orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
		/* Folio got pinned from under us. Put it back and fail the move. */
		if (folio_maybe_dma_pinned(src_folio)) {
			set_pte_at(mm, src_addr, src_pte, orig_src_pte);
			err = -EBUSY;
			break;
		}

		folio_move_anon_rmap(src_folio, dst_vma);
		src_folio->index = linear_anon_page_index(dst_vma, dst_addr);

		orig_dst_pte = folio_mk_pte(src_folio, dst_vma->vm_page_prot);
		/* Set soft dirty bit so userspace can notice the pte was moved */
		if (pgtable_supports_soft_dirty())
			orig_dst_pte = pte_mksoft_dirty(orig_dst_pte);
		if (pte_dirty(orig_src_pte))
			orig_dst_pte = pte_mkdirty(orig_dst_pte);
		orig_dst_pte = pte_mkwrite(orig_dst_pte, dst_vma);

		/* Re-arm RWP on the moved PTE if dst_vma is RWP-registered. */
		if (userfaultfd_rwp(dst_vma)) {
			orig_dst_pte = pte_modify(orig_dst_pte, PAGE_NONE);
			orig_dst_pte = pte_mkuffd(orig_dst_pte);
		}

		set_pte_at(mm, dst_addr, dst_pte, orig_dst_pte);

		src_addr += PAGE_SIZE;
		if (src_addr == src_end)
			break;
		dst_addr += PAGE_SIZE;
		dst_pte++;
		src_pte++;

		folio_unlock(src_folio);
		src_folio = check_ptes_for_batched_move(src_vma, src_addr,
							src_pte, dst_pte);
		if (!src_folio)
			break;
	}

	lazy_mmu_mode_disable();
	if (src_addr > src_start)
		flush_tlb_range(src_vma, src_start, src_addr);

	if (src_folio)
		folio_unlock(src_folio);
out:
	double_pt_unlock(dst_ptl, src_ptl);
	return src_addr > src_start ? src_addr - src_start : err;
}

static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
			 unsigned long dst_addr, unsigned long src_addr,
			 pte_t *dst_pte, pte_t *src_pte,
			 pte_t orig_dst_pte, pte_t orig_src_pte,
			 pmd_t *dst_pmd, pmd_t dst_pmdval,
			 spinlock_t *dst_ptl, spinlock_t *src_ptl,
			 struct folio *src_folio,
			 struct swap_info_struct *si, swp_entry_t entry)
{
	/*
	 * Check if the folio still belongs to the target swap entry after
	 * acquiring the lock. Folio can be freed in the swap cache while
	 * not locked.
	 */
	if (src_folio && unlikely(!folio_test_swapcache(src_folio) ||
				  entry.val != src_folio->swap.val))
		return -EAGAIN;

	double_pt_lock(dst_ptl, src_ptl);

	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
				 dst_pmd, dst_pmdval)) {
		double_pt_unlock(dst_ptl, src_ptl);
		return -EAGAIN;
	}

	/*
	 * The src_folio resides in the swapcache, requiring an update to its
	 * index and mapping to align with the dst_vma, where a swap-in may
	 * occur and hit the swapcache after moving the PTE.
	 */
	if (src_folio) {
		folio_move_anon_rmap(src_folio, dst_vma);
		src_folio->index = linear_anon_page_index(dst_vma, dst_addr);
	} else {
		/*
		 * Check if the swap entry is cached after acquiring the src_pte
		 * lock. Otherwise, we might miss a newly loaded swap cache folio.
		 *
		 * We are trying to catch newly added swap cache, the only possible case is
		 * when a folio is swapped in and out again staying in swap cache, using the
		 * same entry before the PTE check above. The PTL is acquired and released
		 * twice, each time after updating the swap table. So holding
		 * the PTL here ensures we see the updated value.
		 */
		if (swap_cache_has_folio(entry)) {
			double_pt_unlock(dst_ptl, src_ptl);
			return -EAGAIN;
		}
	}

	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
	if (pgtable_supports_soft_dirty())
		orig_src_pte = pte_swp_mksoft_dirty(orig_src_pte);
	/* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */
	if (userfaultfd_rwp(dst_vma))
		orig_src_pte = pte_swp_mkuffd(orig_src_pte);
	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
	double_pt_unlock(dst_ptl, src_ptl);

	return PAGE_SIZE;
}

static int move_zeropage_pte(struct mm_struct *mm,
			     struct vm_area_struct *dst_vma,
			     struct vm_area_struct *src_vma,
			     unsigned long dst_addr, unsigned long src_addr,
			     pte_t *dst_pte, pte_t *src_pte,
			     pte_t orig_dst_pte, pte_t orig_src_pte,
			     pmd_t *dst_pmd, pmd_t dst_pmdval,
			     spinlock_t *dst_ptl, spinlock_t *src_ptl)
{
	pte_t zero_pte;

	double_pt_lock(dst_ptl, src_ptl);
	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
				 dst_pmd, dst_pmdval)) {
		double_pt_unlock(dst_ptl, src_ptl);
		return -EAGAIN;
	}

	zero_pte = pte_mkspecial(pfn_pte(zero_pfn(dst_addr),
					 dst_vma->vm_page_prot));

	/* Re-arm RWP on the moved PTE if dst_vma is RWP-registered. */
	if (userfaultfd_rwp(dst_vma)) {
		zero_pte = pte_modify(zero_pte, PAGE_NONE);
		zero_pte = pte_mkuffd(zero_pte);
	}

	ptep_clear_flush(src_vma, src_addr, src_pte);
	set_pte_at(mm, dst_addr, dst_pte, zero_pte);
	double_pt_unlock(dst_ptl, src_ptl);

	return PAGE_SIZE;
}


/*
 * The mmap_lock for reading is held by the caller. Just move the page(s)
 * from src_pmd to dst_pmd if possible, and return number of bytes moved.
 * On failure, an error code is returned.
 */
static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
			    struct vm_area_struct *dst_vma,
			    struct vm_area_struct *src_vma,
			    unsigned long dst_addr, unsigned long src_addr,
			    unsigned long len, __u64 mode)
{
	struct swap_info_struct *si = NULL;
	pte_t orig_src_pte, orig_dst_pte;
	pte_t src_folio_pte;
	spinlock_t *src_ptl, *dst_ptl;
	pte_t *src_pte = NULL;
	pte_t *dst_pte = NULL;
	pmd_t dummy_pmdval;
	pmd_t dst_pmdval;
	struct folio *src_folio = NULL;
	struct mmu_notifier_range range;
	long ret = 0;

	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
				src_addr, src_addr + len);
	mmu_notifier_invalidate_range_start(&range);
retry:
	/*
	 * Use the maywrite version to indicate that dst_pte will be modified,
	 * since dst_pte needs to be none, the subsequent pte_same() check
	 * cannot prevent the dst_pte page from being freed concurrently, so we
	 * also need to obtain dst_pmdval and recheck pmd_same() later.
	 */
	dst_pte = pte_offset_map_rw_nolock(mm, dst_pmd, dst_addr, &dst_pmdval,
					   &dst_ptl);

	/* Retry if a huge pmd materialized from under us */
	if (unlikely(!dst_pte)) {
		ret = -EAGAIN;
		goto out;
	}

	/*
	 * Unlike dst_pte, the subsequent pte_same() check can ensure the
	 * stability of the src_pte page, so there is no need to get pmdval,
	 * just pass a dummy variable to it.
	 */
	src_pte = pte_offset_map_rw_nolock(mm, src_pmd, src_addr, &dummy_pmdval,
					   &src_ptl);

	/*
	 * We held the mmap_lock for reading so MADV_DONTNEED
	 * can zap transparent huge pages under us, or the
	 * transparent huge page fault can establish new
	 * transparent huge pages under us.
	 */
	if (unlikely(!src_pte)) {
		ret = -EAGAIN;
		goto out;
	}

	/* Sanity checks before the operation */
	if (pmd_none(*dst_pmd) || pmd_none(*src_pmd) ||
	    pmd_trans_huge(*dst_pmd) || pmd_trans_huge(*src_pmd)) {
		ret = -EINVAL;
		goto out;
	}

	spin_lock(dst_ptl);
	orig_dst_pte = ptep_get(dst_pte);
	spin_unlock(dst_ptl);
	if (!pte_none(orig_dst_pte)) {
		ret = -EEXIST;
		goto out;
	}

	spin_lock(src_ptl);
	orig_src_pte = ptep_get(src_pte);
	spin_unlock(src_ptl);
	if (pte_none(orig_src_pte)) {
		if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES))
			ret = -ENOENT;
		else /* nothing to do to move a hole */
			ret = PAGE_SIZE;
		goto out;
	}

	/* If PTE changed after we locked the folio then start over */
	if (src_folio && unlikely(!pte_same(src_folio_pte, orig_src_pte))) {
		ret = -EAGAIN;
		goto out;
	}

	if (pte_present(orig_src_pte)) {
		if (is_zero_pfn(pte_pfn(orig_src_pte))) {
			ret = move_zeropage_pte(mm, dst_vma, src_vma,
					       dst_addr, src_addr, dst_pte, src_pte,
					       orig_dst_pte, orig_src_pte,
					       dst_pmd, dst_pmdval, dst_ptl, src_ptl);
			goto out;
		}

		/*
		 * Pin and lock source folio. Since we are in RCU read section,
		 * we can't block, so on contention have to unmap the ptes,
		 * obtain the lock and retry.
		 */
		if (!src_folio) {
			struct folio *folio;
			bool locked;

			/*
			 * Pin the page while holding the lock to be sure the
			 * page isn't freed under us
			 */
			spin_lock(src_ptl);
			if (!pte_same(orig_src_pte, ptep_get(src_pte))) {
				spin_unlock(src_ptl);
				ret = -EAGAIN;
				goto out;
			}

			folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
			if (!folio || !PageAnonExclusive(&folio->page)) {
				spin_unlock(src_ptl);
				ret = -EBUSY;
				goto out;
			}

			locked = folio_trylock(folio);
			/*
			 * We avoid waiting for folio lock with a raised
			 * refcount for large folios because extra refcounts
			 * will result in split_folio() failing later and
			 * retrying.  If multiple tasks are trying to move a
			 * large folio we can end up livelocking.
			 */
			if (!locked && folio_test_large(folio)) {
				spin_unlock(src_ptl);
				ret = -EAGAIN;
				goto out;
			}

			folio_get(folio);
			src_folio = folio;
			src_folio_pte = orig_src_pte;
			spin_unlock(src_ptl);

			if (!locked) {
				pte_unmap(src_pte);
				pte_unmap(dst_pte);
				src_pte = dst_pte = NULL;
				/* now we can block and wait */
				folio_lock(src_folio);
				goto retry;
			}

			if (WARN_ON_ONCE(!folio_test_anon(src_folio))) {
				ret = -EBUSY;
				goto out;
			}
		}

		/* at this point we have src_folio locked */
		if (folio_test_large(src_folio)) {
			/* split_folio() can block */
			pte_unmap(src_pte);
			pte_unmap(dst_pte);
			src_pte = dst_pte = NULL;
			ret = split_folio(src_folio);
			if (ret)
				goto out;
			/* have to reacquire the folio after it got split */
			folio_unlock(src_folio);
			folio_put(src_folio);
			src_folio = NULL;
			goto retry;
		}

		ret = move_present_ptes(mm, dst_vma, src_vma,
					dst_addr, src_addr, dst_pte, src_pte,
					orig_dst_pte, orig_src_pte, dst_pmd,
					dst_pmdval, dst_ptl, src_ptl, &src_folio,
					len);
	} else { /* !pte_present() */
		struct folio *folio = NULL;
		const softleaf_t entry = softleaf_from_pte(orig_src_pte);

		if (softleaf_is_migration(entry)) {
			pte_unmap(src_pte);
			pte_unmap(dst_pte);
			src_pte = dst_pte = NULL;
			migration_entry_wait(mm, src_pmd, src_addr);

			ret = -EAGAIN;
			goto out;
		} else if (!softleaf_is_swap(entry)) {
			ret = -EFAULT;
			goto out;
		}

		if (!pte_swp_exclusive(orig_src_pte)) {
			ret = -EBUSY;
			goto out;
		}

		si = get_swap_device(entry);
		if (unlikely(!si)) {
			ret = -EAGAIN;
			goto out;
		}
		/*
		 * Verify the existence of the swapcache. If present, the folio's
		 * index and mapping must be updated even when the PTE is a swap
		 * entry. The anon_vma lock is not taken during this process since
		 * the folio has already been unmapped, and the swap entry is
		 * exclusive, preventing rmap walks.
		 *
		 * For large folios, return -EBUSY immediately, as split_folio()
		 * also returns -EBUSY when attempting to split unmapped large
		 * folios in the swapcache. This issue needs to be resolved
		 * separately to allow proper handling.
		 */
		if (!src_folio)
			folio = swap_cache_get_folio(entry);
		if (folio) {
			if (folio_test_large(folio)) {
				ret = -EBUSY;
				folio_put(folio);
				goto out;
			}
			src_folio = folio;
			src_folio_pte = orig_src_pte;
			if (!folio_trylock(src_folio)) {
				pte_unmap(src_pte);
				pte_unmap(dst_pte);
				src_pte = dst_pte = NULL;
				put_swap_device(si);
				si = NULL;
				/* now we can block and wait */
				folio_lock(src_folio);
				goto retry;
			}
		}
		ret = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
				orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
				dst_ptl, src_ptl, src_folio, si, entry);
	}

out:
	if (src_folio) {
		folio_unlock(src_folio);
		folio_put(src_folio);
	}
	/*
	 * Unmap in reverse order (LIFO) to maintain proper kmap_local
	 * index ordering when CONFIG_HIGHPTE is enabled. We mapped dst_pte
	 * first, then src_pte, so we must unmap src_pte first, then dst_pte.
	 */
	if (src_pte)
		pte_unmap(src_pte);
	if (dst_pte)
		pte_unmap(dst_pte);
	mmu_notifier_invalidate_range_end(&range);
	if (si)
		put_swap_device(si);

	return ret;
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
static inline bool move_splits_huge_pmd(unsigned long dst_addr,
					unsigned long src_addr,
					unsigned long src_end)
{
	return (src_addr & ~HPAGE_PMD_MASK) || (dst_addr & ~HPAGE_PMD_MASK) ||
		src_end - src_addr < HPAGE_PMD_SIZE;
}
#else
static inline bool move_splits_huge_pmd(unsigned long dst_addr,
					unsigned long src_addr,
					unsigned long src_end)
{
	/* This is unreachable anyway, just to avoid warnings when HPAGE_PMD_SIZE==0 */
	return false;
}
#endif

static inline bool vma_move_compatible(struct vm_area_struct *vma)
{
	return !(vma->vm_flags & (VM_PFNMAP | VM_IO |  VM_HUGETLB |
				  VM_MIXEDMAP | VM_SHADOW_STACK));
}

static int validate_move_areas(struct userfaultfd_ctx *ctx,
			       struct vm_area_struct *src_vma,
			       struct vm_area_struct *dst_vma)
{
	/* Only allow moving if both have the same access and protection */
	if ((src_vma->vm_flags & VM_ACCESS_FLAGS) != (dst_vma->vm_flags & VM_ACCESS_FLAGS) ||
	    pgprot_val(src_vma->vm_page_prot) != pgprot_val(dst_vma->vm_page_prot))
		return -EINVAL;

	/* Only allow moving if both are mlocked or both aren't */
	if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED))
		return -EINVAL;

	/*
	 * For now, we keep it simple and only move between writable VMAs.
	 * Access flags are equal, therefore checking only the source is enough.
	 */
	if (!(src_vma->vm_flags & VM_WRITE))
		return -EINVAL;

	/* Check if vma flags indicate content which can be moved */
	if (!vma_move_compatible(src_vma) || !vma_move_compatible(dst_vma))
		return -EINVAL;

	/* Ensure dst_vma is registered in uffd we are operating on */
	if (!dst_vma->vm_userfaultfd_ctx.ctx ||
	    dst_vma->vm_userfaultfd_ctx.ctx != ctx)
		return -EINVAL;

	/* Only allow moving across anonymous vmas */
	if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
		return -EINVAL;

	return 0;
}

static __always_inline
int find_vmas_mm_locked(struct mm_struct *mm,
			unsigned long dst_start,
			unsigned long src_start,
			struct vm_area_struct **dst_vmap,
			struct vm_area_struct **src_vmap)
{
	struct vm_area_struct *vma;

	mmap_assert_locked(mm);
	vma = find_vma_and_prepare_anon(mm, dst_start);
	if (IS_ERR(vma))
		return PTR_ERR(vma);

	*dst_vmap = vma;
	/* Skip finding src_vma if src_start is in dst_vma */
	if (src_start >= vma->vm_start && src_start < vma->vm_end)
		goto out_success;

	vma = vma_lookup(mm, src_start);
	if (!vma)
		return -ENOENT;
out_success:
	*src_vmap = vma;
	return 0;
}

#ifdef CONFIG_PER_VMA_LOCK
static int uffd_move_lock(struct mm_struct *mm,
			  unsigned long dst_start,
			  unsigned long src_start,
			  struct vm_area_struct **dst_vmap,
			  struct vm_area_struct **src_vmap)
{
	struct vm_area_struct *vma;
	int err;

	vma = uffd_lock_vma(mm, dst_start);
	if (IS_ERR(vma))
		return PTR_ERR(vma);

	*dst_vmap = vma;
	/*
	 * Skip finding src_vma if src_start is in dst_vma. This also ensures
	 * that we don't lock the same vma twice.
	 */
	if (src_start >= vma->vm_start && src_start < vma->vm_end) {
		*src_vmap = vma;
		return 0;
	}

	/*
	 * Using uffd_lock_vma() to get src_vma can lead to following deadlock:
	 *
	 * Thread1				Thread2
	 * -------				-------
	 * vma_start_read(dst_vma)
	 *					mmap_write_lock(mm)
	 *					vma_start_write(src_vma)
	 * vma_start_read(src_vma)
	 * mmap_read_lock(mm)
	 *					vma_start_write(dst_vma)
	 */
	*src_vmap = lock_vma_under_rcu(mm, src_start);
	if (likely(*src_vmap))
		return 0;

	/* Undo any locking and retry in mmap_lock critical section */
	vma_end_read(*dst_vmap);

	mmap_read_lock(mm);
	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
	if (err)
		goto out;

	if (!vma_start_read_locked(*dst_vmap)) {
		err = -EAGAIN;
		goto out;
	}

	/* Nothing further to do if both vmas are locked. */
	if (*dst_vmap == *src_vmap)
		goto out;

	if (!vma_start_read_locked_nested(*src_vmap, SINGLE_DEPTH_NESTING)) {
		/* Undo dst_vmap locking if src_vmap failed to lock */
		vma_end_read(*dst_vmap);
		err = -EAGAIN;
	}
out:
	mmap_read_unlock(mm);
	return err;
}

static void uffd_move_unlock(struct vm_area_struct *dst_vma,
			     struct vm_area_struct *src_vma)
{
	vma_end_read(src_vma);
	if (src_vma != dst_vma)
		vma_end_read(dst_vma);
}

#else

static int uffd_move_lock(struct mm_struct *mm,
			  unsigned long dst_start,
			  unsigned long src_start,
			  struct vm_area_struct **dst_vmap,
			  struct vm_area_struct **src_vmap)
{
	int err;

	mmap_read_lock(mm);
	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
	if (err)
		mmap_read_unlock(mm);
	return err;
}

static void uffd_move_unlock(struct vm_area_struct *dst_vma,
			     struct vm_area_struct *src_vma)
{
	mmap_assert_locked(src_vma->vm_mm);
	mmap_read_unlock(dst_vma->vm_mm);
}
#endif

/**
 * move_pages - move arbitrary anonymous pages of an existing vma
 * @ctx: pointer to the userfaultfd context
 * @dst_start: start of the destination virtual memory range
 * @src_start: start of the source virtual memory range
 * @len: length of the virtual memory range
 * @mode: flags from uffdio_move.mode
 *
 * It will either use the mmap_lock in read mode or per-vma locks
 *
 * move_pages() remaps arbitrary anonymous pages atomically in zero
 * copy. It only works on non shared anonymous pages because those can
 * be relocated without generating non linear anon_vmas in the rmap
 * code.
 *
 * It provides a zero copy mechanism to handle userspace page faults.
 * The source vma pages should have mapcount == 1, which can be
 * enforced by using madvise(MADV_DONTFORK) on src vma.
 *
 * The thread receiving the page during the userland page fault
 * will receive the faulting page in the source vma through the network,
 * storage or any other I/O device (MADV_DONTFORK in the source vma
 * avoids move_pages() to fail with -EBUSY if the process forks before
 * move_pages() is called), then it will call move_pages() to map the
 * page in the faulting address in the destination vma.
 *
 * This userfaultfd command works purely via pagetables, so it's the
 * most efficient way to move physical non shared anonymous pages
 * across different virtual addresses. Unlike mremap()/mmap()/munmap()
 * it does not create any new vmas. The mapping in the destination
 * address is atomic.
 *
 * It only works if the vma protection bits are identical from the
 * source and destination vma.
 *
 * It can remap non shared anonymous pages within the same vma too.
 *
 * If the source virtual memory range has any unmapped holes, or if
 * the destination virtual memory range is not a whole unmapped hole,
 * move_pages() will fail respectively with -ENOENT or -EEXIST. This
 * provides a very strict behavior to avoid any chance of memory
 * corruption going unnoticed if there are userland race conditions.
 * Only one thread should resolve the userland page fault at any given
 * time for any given faulting address. This means that if two threads
 * try to both call move_pages() on the same destination address at the
 * same time, the second thread will get an explicit error from this
 * command.
 *
 * The command retval will return "len" is successful. The command
 * however can be interrupted by fatal signals or errors. If
 * interrupted it will return the number of bytes successfully
 * remapped before the interruption if any, or the negative error if
 * none. It will never return zero. Either it will return an error or
 * an amount of bytes successfully moved. If the retval reports a
 * "short" remap, the move_pages() command should be repeated by
 * userland with src+retval, dst+reval, len-retval if it wants to know
 * about the error that interrupted it.
 *
 * The UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES flag can be specified to
 * prevent -ENOENT errors to materialize if there are holes in the
 * source virtual range that is being remapped. The holes will be
 * accounted as successfully remapped in the retval of the
 * command. This is mostly useful to remap hugepage naturally aligned
 * virtual regions without knowing if there are transparent hugepage
 * in the regions or not, but preventing the risk of having to split
 * the hugepmd during the remap.
 */
static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
		   unsigned long src_start, unsigned long len, __u64 mode)
{
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *src_vma, *dst_vma;
	unsigned long src_addr, dst_addr, src_end;
	pmd_t *src_pmd, *dst_pmd;
	long err = -EINVAL;
	ssize_t moved = 0;

	/* Sanitize the command parameters. */
	VM_WARN_ON_ONCE(src_start & ~PAGE_MASK);
	VM_WARN_ON_ONCE(dst_start & ~PAGE_MASK);
	VM_WARN_ON_ONCE(len & ~PAGE_MASK);

	/* Does the address range wrap, or is the span zero-sized? */
	VM_WARN_ON_ONCE(src_start + len < src_start);
	VM_WARN_ON_ONCE(dst_start + len < dst_start);

	err = uffd_move_lock(mm, dst_start, src_start, &dst_vma, &src_vma);
	if (err)
		goto out;

	/* Re-check after taking map_changing_lock */
	err = -EAGAIN;
	down_read(&ctx->map_changing_lock);
	if (likely(atomic_read(&ctx->mmap_changing)))
		goto out_unlock;
	/*
	 * Make sure the vma is not shared, that the src and dst remap
	 * ranges are both valid and fully within a single existing
	 * vma.
	 */
	err = -EINVAL;
	if (src_vma->vm_flags & VM_SHARED)
		goto out_unlock;
	if (src_start + len > src_vma->vm_end)
		goto out_unlock;

	if (dst_vma->vm_flags & VM_SHARED)
		goto out_unlock;
	if (dst_start + len > dst_vma->vm_end)
		goto out_unlock;

	err = validate_move_areas(ctx, src_vma, dst_vma);
	if (err)
		goto out_unlock;

	for (src_addr = src_start, dst_addr = dst_start, src_end = src_start + len;
	     src_addr < src_end;) {
		spinlock_t *ptl;
		pmd_t dst_pmdval;
		unsigned long step_size;

		/*
		 * Below works because anonymous area would not have a
		 * transparent huge PUD. If file-backed support is added,
		 * that case would need to be handled here.
		 */
		src_pmd = mm_find_pmd(mm, src_addr);
		if (unlikely(!src_pmd)) {
			if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
				err = -ENOENT;
				break;
			}
			src_pmd = mm_alloc_pmd(mm, src_addr);
			if (unlikely(!src_pmd)) {
				err = -ENOMEM;
				break;
			}
		}
		dst_pmd = mm_alloc_pmd(mm, dst_addr);
		if (unlikely(!dst_pmd)) {
			err = -ENOMEM;
			break;
		}

		dst_pmdval = pmdp_get_lockless(dst_pmd);
		/*
		 * If the dst_pmd is mapped as THP don't override it and just
		 * be strict. If dst_pmd changes into TPH after this check, the
		 * move_pages_huge_pmd() will detect the change and retry
		 * while move_pages_pte() will detect the change and fail.
		 */
		if (unlikely(pmd_trans_huge(dst_pmdval))) {
			err = -EEXIST;
			break;
		}

		ptl = pmd_trans_huge_lock(src_pmd, src_vma);
		if (ptl) {
			/* Check if we can move the pmd without splitting it. */
			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
			    !pmd_none(dst_pmdval)) {
				/* Can be a migration entry */
				if (pmd_present(*src_pmd)) {
					struct folio *folio = pmd_folio(*src_pmd);

					if (!is_huge_zero_folio(folio) &&
					    !PageAnonExclusive(&folio->page)) {
						spin_unlock(ptl);
						err = -EBUSY;
						break;
					}
				}

				spin_unlock(ptl);
				split_huge_pmd(src_vma, src_pmd, src_addr);
				/* The folio will be split by move_pages_pte() */
				continue;
			}

			err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
						  dst_pmdval, dst_vma, src_vma,
						  dst_addr, src_addr);
			step_size = HPAGE_PMD_SIZE;
		} else {
			long ret;

			if (pmd_none(*src_pmd)) {
				if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
					err = -ENOENT;
					break;
				}
				if (unlikely(__pte_alloc(mm, src_pmd))) {
					err = -ENOMEM;
					break;
				}
			}

			if (unlikely(pte_alloc(mm, dst_pmd))) {
				err = -ENOMEM;
				break;
			}

			ret = move_pages_ptes(mm, dst_pmd, src_pmd,
					      dst_vma, src_vma, dst_addr,
					      src_addr, src_end - src_addr, mode);
			if (ret < 0)
				err = ret;
			else
				step_size = ret;
		}

		cond_resched();

		if (fatal_signal_pending(current)) {
			/* Do not override an error */
			if (!err || err == -EAGAIN)
				err = -EINTR;
			break;
		}

		if (err) {
			if (err == -EAGAIN) {
				err = 0;
				continue;
			}
			break;
		}

		/* Proceed to the next page */
		dst_addr += step_size;
		src_addr += step_size;
		moved += step_size;
	}

out_unlock:
	up_read(&ctx->map_changing_lock);
	uffd_move_unlock(dst_vma, src_vma);
out:
	VM_WARN_ON_ONCE(moved < 0);
	VM_WARN_ON_ONCE(err > 0);
	VM_WARN_ON_ONCE(!moved && !err);
	return moved ? moved : err;
}

static bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
		       bool wp_async)
{
	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);

	if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
		return false;

	if (!is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SPECIAL))
		return false;

	vm_flags &= __VM_UFFD_FLAGS;

	/*
	 * If WP is the only mode enabled and context is wp async, allow any
	 * memory type.
	 */
	if (wp_async && (vm_flags == VM_UFFD_WP))
		return true;

	/* For any other mode reject VMAs that don't implement vm_uffd_ops */
	if (!ops)
		return false;

	/*
	 * If user requested uffd-wp but not enabled pte markers for
	 * uffd-wp, then only anonymous memory is supported
	 */
	if (!uffd_supports_wp_marker() && (vm_flags & VM_UFFD_WP) &&
	    !vma_is_anonymous(vma))
		return false;

	return ops->can_userfault(vma, vm_flags);
}

static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
				     vm_flags_t vm_flags)
{
	const bool uffd_wp_changed = (vma->vm_flags ^ vm_flags) & VM_UFFD_WP;

	vm_flags_reset(vma, vm_flags);
	/*
	 * For shared mappings, we want to enable writenotify while
	 * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
	 * recalculate vma->vm_page_prot whenever userfaultfd-wp changes.
	 */
	if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed)
		vma_set_page_prot(vma);
}

static void userfaultfd_set_ctx(struct vm_area_struct *vma,
				struct userfaultfd_ctx *ctx,
				vm_flags_t vm_flags)
{
	vma_start_write(vma);
	vma->vm_userfaultfd_ctx = (struct vm_userfaultfd_ctx){ctx};
	userfaultfd_set_vm_flags(vma,
				 (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags);
}

static void userfaultfd_reset_ctx(struct vm_area_struct *vma)
{
	userfaultfd_set_ctx(vma, NULL, 0);
}

static struct vm_area_struct *userfaultfd_clear_vma(struct vma_iterator *vmi,
					     struct vm_area_struct *prev,
					     struct vm_area_struct *vma,
					     unsigned long start,
					     unsigned long end)
{
	struct vm_area_struct *ret;
	bool give_up_on_oom = false;
	vma_flags_t new_vma_flags = vma->flags;

	vma_flags_clear_mask(&new_vma_flags, __VMA_UFFD_FLAGS);

	/*
	 * If we are modifying only and not splitting, just give up on the merge
	 * if OOM prevents us from merging successfully.
	 */
	if (start == vma->vm_start && end == vma->vm_end)
		give_up_on_oom = true;

	/* Clear the uffd bit and/or restore protnone PTEs */
	if (userfaultfd_protected(vma)) {
		unsigned int mm_cp_flags = 0;
		struct mmu_gather tlb;

		if (userfaultfd_wp(vma))
			mm_cp_flags |= MM_CP_UFFD_WP_RESOLVE;
		if (userfaultfd_rwp(vma))
			mm_cp_flags |= MM_CP_UFFD_RWP_RESOLVE;
		if (vma_wants_manual_pte_write_upgrade(vma))
			mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;

		tlb_gather_mmu(&tlb, vma->vm_mm);
		change_protection(&tlb, vma, start, end, mm_cp_flags);
		tlb_finish_mmu(&tlb);
	}

	ret = vma_modify_flags_uffd(vmi, prev, vma, start, end,
				    &new_vma_flags, NULL_VM_UFFD_CTX,
				    give_up_on_oom);

	/*
	 * In the vma_merge() successful mprotect-like case 8:
	 * the next vma was merged into the current one and
	 * the current one has not been updated yet.
	 */
	if (!IS_ERR(ret))
		userfaultfd_reset_ctx(ret);

	return ret;
}

/* Assumes mmap write lock taken, and mm_struct pinned. */
static int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
			       struct vm_area_struct *vma,
			       vm_flags_t vm_flags,
			       unsigned long start, unsigned long end,
			       bool wp_async)
{
	vma_flags_t vma_flags = legacy_to_vma_flags(vm_flags);
	VMA_ITERATOR(vmi, ctx->mm, start);
	struct vm_area_struct *prev = vma_prev(&vmi);
	unsigned long vma_end;
	vma_flags_t new_vma_flags;

	if (vma->vm_start < start)
		prev = vma;

	for_each_vma_range(vmi, vma, end) {
		cond_resched();

		VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async));
		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx &&
				vma->vm_userfaultfd_ctx.ctx != ctx);
		VM_WARN_ON_ONCE(!vma_test(vma, VMA_MAYWRITE_BIT));

		/*
		 * Nothing to do: this vma is already registered into this
		 * userfaultfd and with the right tracking mode too.
		 */
		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
		    vma_test_all_mask(vma, vma_flags))
			goto skip;

		/*
		 * Pre-scan in userfaultfd_register() already rejected mode
		 * switches that would drop VM_UFFD_WP or VM_UFFD_RWP, so a
		 * stray bit here is a bug.
		 */
		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx == ctx &&
				vma->vm_flags & (VM_UFFD_WP | VM_UFFD_RWP) & ~vm_flags);

		if (vma->vm_start > start)
			start = vma->vm_start;
		vma_end = min(end, vma->vm_end);

		new_vma_flags = vma->flags;
		vma_flags_clear_mask(&new_vma_flags, __VMA_UFFD_FLAGS);
		vma_flags_set_mask(&new_vma_flags, vma_flags);

		vma = vma_modify_flags_uffd(&vmi, prev, vma, start, vma_end,
					    &new_vma_flags,
					    (struct vm_userfaultfd_ctx){ctx},
					    /* give_up_on_oom = */false);
		if (IS_ERR(vma))
			return PTR_ERR(vma);

		/*
		 * In the vma_merge() successful mprotect-like case 8:
		 * the next vma was merged into the current one and
		 * the current one has not been updated yet.
		 */
		userfaultfd_set_ctx(vma, ctx, vm_flags);

		if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
			hugetlb_unshare_all_pmds(vma);

skip:
		prev = vma;
		start = vma->vm_end;
	}

	return 0;
}

static void userfaultfd_release_new(struct userfaultfd_ctx *ctx)
{
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma;
	VMA_ITERATOR(vmi, mm, 0);

	/* the various vma->vm_userfaultfd_ctx still points to it */
	mmap_write_lock(mm);
	for_each_vma(vmi, vma) {
		if (vma->vm_userfaultfd_ctx.ctx == ctx)
			userfaultfd_reset_ctx(vma);
	}
	mmap_write_unlock(mm);
}

static void userfaultfd_release_all(struct mm_struct *mm,
			     struct userfaultfd_ctx *ctx)
{
	struct vm_area_struct *vma, *prev;
	VMA_ITERATOR(vmi, mm, 0);

	if (!mmget_not_zero(mm))
		return;

	/*
	 * Flush page faults out of all CPUs. NOTE: all page faults
	 * must be retried without returning VM_FAULT_SIGBUS if
	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
	 * changes while handle_userfault released the mmap_lock. So
	 * it's critical that released is set to true (above), before
	 * taking the mmap_lock for writing.
	 */
	mmap_write_lock(mm);
	prev = NULL;
	for_each_vma(vmi, vma) {
		cond_resched();
		VM_WARN_ON_ONCE(!!vma->vm_userfaultfd_ctx.ctx ^
				!!(vma->vm_flags & __VM_UFFD_FLAGS));
		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
			prev = vma;
			continue;
		}

		vma = userfaultfd_clear_vma(&vmi, prev, vma,
					    vma->vm_start, vma->vm_end);
		prev = vma;
	}
	mmap_write_unlock(mm);
	mmput(mm);
}

static int sysctl_unprivileged_userfaultfd __read_mostly;

#ifdef CONFIG_SYSCTL
static const struct ctl_table vm_userfaultfd_table[] = {
	{
		.procname	= "unprivileged_userfaultfd",
		.data		= &sysctl_unprivileged_userfaultfd,
		.maxlen		= sizeof(sysctl_unprivileged_userfaultfd),
		.mode		= 0644,
		.proc_handler	= proc_dointvec_minmax,
		.extra1		= SYSCTL_ZERO,
		.extra2		= SYSCTL_ONE,
	},
};
#endif

static struct kmem_cache *userfaultfd_ctx_cachep __ro_after_init;

struct userfaultfd_fork_ctx {
	struct userfaultfd_ctx *orig;
	struct userfaultfd_ctx *new;
	struct list_head list;
};

struct userfaultfd_unmap_ctx {
	struct userfaultfd_ctx *ctx;
	unsigned long start;
	unsigned long end;
	struct list_head list;
};

struct userfaultfd_wait_queue {
	struct uffd_msg msg;
	wait_queue_entry_t wq;
	struct userfaultfd_ctx *ctx;
	bool waken;
};

struct userfaultfd_wake_range {
	unsigned long start;
	unsigned long len;
};

/* internal indication that UFFD_API ioctl was successfully executed */
#define UFFD_FEATURE_INITIALIZED		(1u << 31)

/*
 * UFFDIO_SET_MODE updates ctx->features under mmap_write_lock with
 * WRITE_ONCE; readers that run outside mmap_read_lock or the per-VMA
 * lock (poll/read_iter/ioctl, fdinfo) must pair with READ_ONCE.
 */
static unsigned int userfaultfd_features(struct userfaultfd_ctx *ctx)
{
	return READ_ONCE(ctx->features);
}

static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
{
	return userfaultfd_features(ctx) & UFFD_FEATURE_INITIALIZED;
}

static bool userfaultfd_wp_async_ctx(struct userfaultfd_ctx *ctx)
{
	return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_WP_ASYNC);
}

static bool userfaultfd_rwp_async_ctx(struct userfaultfd_ctx *ctx)
{
	return ctx && (userfaultfd_features(ctx) & UFFD_FEATURE_RWP_ASYNC);
}

/*
 * Whether WP_UNPOPULATED is enabled on the uffd context.  It is only
 * meaningful when userfaultfd_wp()==true on the vma and when it's
 * anonymous.
 */
bool userfaultfd_wp_unpopulated(struct vm_area_struct *vma)
{
	struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;

	if (!ctx)
		return false;

	return userfaultfd_features(ctx) & UFFD_FEATURE_WP_UNPOPULATED;
}

static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
				     int wake_flags, void *key)
{
	struct userfaultfd_wake_range *range = key;
	int ret;
	struct userfaultfd_wait_queue *uwq;
	unsigned long start, len;

	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
	ret = 0;
	/* len == 0 means wake all */
	start = range->start;
	len = range->len;
	if (len && (start > uwq->msg.arg.pagefault.address ||
		    start + len <= uwq->msg.arg.pagefault.address))
		goto out;
	WRITE_ONCE(uwq->waken, true);
	/*
	 * The Program-Order guarantees provided by the scheduler
	 * ensure uwq->waken is visible before the task is woken.
	 */
	ret = wake_up_state(wq->private, mode);
	if (ret) {
		/*
		 * Wake only once, autoremove behavior.
		 *
		 * After the effect of list_del_init is visible to the other
		 * CPUs, the waitqueue may disappear from under us, see the
		 * !list_empty_careful() in handle_userfault().
		 *
		 * try_to_wake_up() has an implicit smp_mb(), and the
		 * wq->private is read before calling the extern function
		 * "wake_up_state" (which in turns calls try_to_wake_up).
		 */
		list_del_init(&wq->entry);
	}
out:
	return ret;
}

/**
 * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
 * context.
 * @ctx: [in] Pointer to the userfaultfd context.
 */
static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
{
	refcount_inc(&ctx->refcount);
}

/**
 * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
 * context.
 * @ctx: [in] Pointer to userfaultfd context.
 *
 * The userfaultfd context reference must have been previously acquired either
 * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
 */
static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
{
	if (refcount_dec_and_test(&ctx->refcount)) {
		VM_WARN_ON_ONCE(spin_is_locked(&ctx->fault_pending_wqh.lock));
		VM_WARN_ON_ONCE(waitqueue_active(&ctx->fault_pending_wqh));
		VM_WARN_ON_ONCE(spin_is_locked(&ctx->fault_wqh.lock));
		VM_WARN_ON_ONCE(waitqueue_active(&ctx->fault_wqh));
		VM_WARN_ON_ONCE(spin_is_locked(&ctx->event_wqh.lock));
		VM_WARN_ON_ONCE(waitqueue_active(&ctx->event_wqh));
		VM_WARN_ON_ONCE(spin_is_locked(&ctx->fd_wqh.lock));
		VM_WARN_ON_ONCE(waitqueue_active(&ctx->fd_wqh));
		mmdrop(ctx->mm);
		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
	}
}

static inline void msg_init(struct uffd_msg *msg)
{
	BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
	/*
	 * Must use memset to zero out the paddings or kernel data is
	 * leaked to userland.
	 */
	memset(msg, 0, sizeof(struct uffd_msg));
}

static inline struct uffd_msg userfault_msg(unsigned long address,
					    unsigned long real_address,
					    unsigned int flags,
					    unsigned long reason,
					    unsigned int features)
{
	struct uffd_msg msg;

	msg_init(&msg);
	msg.event = UFFD_EVENT_PAGEFAULT;

	msg.arg.pagefault.address = (features & UFFD_FEATURE_EXACT_ADDRESS) ?
				    real_address : address;

	/*
	 * These flags indicate why the userfault occurred:
	 * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
	 * - UFFD_PAGEFAULT_FLAG_MINOR indicates a minor fault.
	 * - Neither of these flags being set indicates a MISSING fault.
	 *
	 * Separately, UFFD_PAGEFAULT_FLAG_WRITE indicates it was a write
	 * fault. Otherwise, it was a read fault.
	 */
	if (flags & FAULT_FLAG_WRITE)
		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
	if (reason & VM_UFFD_WP)
		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
	if (reason & VM_UFFD_RWP)
		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_RWP;
	if (reason & VM_UFFD_MINOR)
		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR;
	if (features & UFFD_FEATURE_THREAD_ID)
		msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
	return msg;
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * Same functionality as userfaultfd_must_wait below with modifications for
 * hugepmd ranges.
 */
static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
					      struct vm_fault *vmf,
					      unsigned long reason)
{
	struct vm_area_struct *vma = vmf->vma;
	pte_t *ptep, pte;

	assert_fault_locked(vmf);

	ptep = hugetlb_walk(vma, vmf->address, vma_mmu_pagesize(vma));
	if (!ptep)
		return true;

	pte = huge_ptep_get(vma->vm_mm, vmf->address, ptep);

	/*
	 * Lockless access: we're in a wait_event so it's ok if it
	 * changes under us.
	 */

	/* Entry is still missing, wait for userspace to resolve the fault. */
	if (huge_pte_none(pte))
		return true;
	/* UFFD PTE markers require userspace to resolve the fault. */
	if (pte_is_uffd_marker(pte))
		return true;
	/*
	 * Concurrent migration may have replaced the present PTE with a
	 * non-marker swap entry between fault delivery and this lockless
	 * re-check. huge_pte_write() on a swap entry decodes random offset
	 * bits, so gate it on pte_present(). The migration completion path
	 * will re-deliver the fault if it still needs userspace.
	 */
	if (!pte_present(pte))
		return false;
	/*
	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
	 * resolve the fault.
	 */
	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
		return true;
	/*
	 * PTE is still RW-protected (protnone with uffd bit), wait for
	 * resolution. Plain PROT_NONE without the marker is not an RWP fault.
	 */
	if (pte_protnone(pte) && huge_pte_uffd(pte) && (reason & VM_UFFD_RWP))
		return true;

	return false;
}
#else
static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
					      struct vm_fault *vmf,
					      unsigned long reason)
{
	/* Should never get here. */
	VM_WARN_ON_ONCE(1);
	return false;
}
#endif /* CONFIG_HUGETLB_PAGE */

/*
 * Verify the pagetables are still not ok after having registered into
 * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
 * userfault that has already been resolved, if userfaultfd_read_iter and
 * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
 * threads.
 */
static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
					 struct vm_fault *vmf,
					 unsigned long reason)
{
	struct mm_struct *mm = ctx->mm;
	unsigned long address = vmf->address;
	pgd_t *pgd;
	p4d_t *p4d;
	pud_t *pud;
	pmd_t *pmd, _pmd;
	pte_t *pte;
	pte_t ptent;
	bool ret;

	assert_fault_locked(vmf);

	pgd = pgd_offset(mm, address);
	if (!pgd_present(*pgd))
		return true;
	p4d = p4d_offset(pgd, address);
	if (!p4d_present(*p4d))
		return true;
	pud = pud_offset(p4d, address);
	if (!pud_present(*pud))
		return true;
	pmd = pmd_offset(pud, address);
again:
	_pmd = pmdp_get_lockless(pmd);
	if (pmd_none(_pmd))
		return true;

	/*
	 * A race could arise which would result in a softleaf entry such as
	 * migration entry unexpectedly being present in the PMD, so explicitly
	 * check for this and bail out if so.
	 */
	if (!pmd_present(_pmd))
		return false;

	if (pmd_trans_huge(_pmd)) {
		if (!pmd_write(_pmd) && (reason & VM_UFFD_WP))
			return true;
		if (pmd_protnone(_pmd) && pmd_uffd(_pmd) &&
		    (reason & VM_UFFD_RWP))
			return true;
		return false;
	}

	pte = pte_offset_map(pmd, address);
	if (!pte)
		goto again;

	/*
	 * Lockless access: we're in a wait_event so it's ok if it
	 * changes under us.
	 */
	ptent = ptep_get(pte);

	ret = true;
	/* Entry is still missing, wait for userspace to resolve the fault. */
	if (pte_none(ptent))
		goto out;
	/* UFFD PTE markers require userspace to resolve the fault. */
	if (pte_is_uffd_marker(ptent))
		goto out;
	/*
	 * Concurrent swap-out / migration may have replaced the present PTE
	 * with a non-marker swap entry between fault delivery and this
	 * lockless re-check. pte_write() on a swap entry decodes random
	 * offset bits, so gate it on pte_present(). The page-in path will
	 * re-deliver the fault if it still needs userspace.
	 */
	if (!pte_present(ptent)) {
		ret = false;
		goto out;
	}
	/*
	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
	 * resolve the fault.
	 */
	if (!pte_write(ptent) && (reason & VM_UFFD_WP))
		goto out;
	/*
	 * PTE is still RW-protected (protnone with uffd bit), wait for
	 * userspace to resolve. Plain PROT_NONE without the marker is not
	 * an RWP fault.
	 */
	if (pte_protnone(ptent) && pte_uffd(ptent) && (reason & VM_UFFD_RWP))
		goto out;

	ret = false;
out:
	pte_unmap(pte);
	return ret;
}

static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
{
	if (flags & FAULT_FLAG_INTERRUPTIBLE)
		return TASK_INTERRUPTIBLE;

	if (flags & FAULT_FLAG_KILLABLE)
		return TASK_KILLABLE;

	return TASK_UNINTERRUPTIBLE;
}

/*
 * The locking rules involved in returning VM_FAULT_RETRY depending on
 * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
 * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
 * recommendation in __lock_page_or_retry is not an understatement.
 *
 * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_lock must be released
 * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
 * not set.
 *
 * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
 * set, VM_FAULT_RETRY can still be returned if and only if there are
 * fatal_signal_pending()s, and the mmap_lock must be released before
 * returning it.
 */
vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
{
	struct vm_area_struct *vma = vmf->vma;
	struct mm_struct *mm = vma->vm_mm;
	struct userfaultfd_ctx *ctx;
	struct userfaultfd_wait_queue uwq;
	vm_fault_t ret = VM_FAULT_SIGBUS;
	bool must_wait;
	unsigned int blocking_state;

	/*
	 * We don't do userfault handling for the final child pid update
	 * and when coredumping (faults triggered by get_dump_page()).
	 */
	if (current->flags & (PF_EXITING|PF_DUMPCORE))
		goto out;

	assert_fault_locked(vmf);

	ctx = vma->vm_userfaultfd_ctx.ctx;
	if (!ctx)
		goto out;

	VM_WARN_ON_ONCE(ctx->mm != mm);

	/* Any unrecognized flag is a bug. */
	VM_WARN_ON_ONCE(reason & ~__VM_UFFD_FLAGS);
	/* 0 or > 1 flags set is a bug; we expect exactly 1. */
	VM_WARN_ON_ONCE(!reason || (reason & (reason - 1)));

	if (ctx->features & UFFD_FEATURE_SIGBUS)
		goto out;
	if (!(vmf->flags & FAULT_FLAG_USER) && (ctx->flags & UFFD_USER_MODE_ONLY))
		goto out;

	/*
	 * Check that we can return VM_FAULT_RETRY.
	 *
	 * NOTE: it should become possible to return VM_FAULT_RETRY
	 * even if FAULT_FLAG_TRIED is set without leading to gup()
	 * -EBUSY failures, if the userfaultfd is to be extended for
	 * VM_UFFD_WP tracking and we intend to arm the userfault
	 * without first stopping userland access to the memory. For
	 * VM_UFFD_MISSING userfaults this is enough for now.
	 */
	if (unlikely(!(vmf->flags & FAULT_FLAG_ALLOW_RETRY))) {
		/*
		 * Validate the invariant that nowait must allow retry
		 * to be sure not to return SIGBUS erroneously on
		 * nowait invocations.
		 */
		VM_WARN_ON_ONCE(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
#ifdef CONFIG_DEBUG_VM
		if (printk_ratelimit()) {
			pr_warn("FAULT_FLAG_ALLOW_RETRY missing %x\n",
				vmf->flags);
			dump_stack();
		}
#endif
		goto out;
	}

	/*
	 * Handle nowait, not much to do other than tell it to retry
	 * and wait.
	 */
	ret = VM_FAULT_RETRY;
	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
		goto out;

	if (unlikely(READ_ONCE(ctx->released))) {
		/*
		 * If a concurrent release is detected, do not return
		 * VM_FAULT_SIGBUS or VM_FAULT_NOPAGE, but instead always
		 * return VM_FAULT_RETRY with lock released proactively.
		 *
		 * If we were to return VM_FAULT_SIGBUS here, the non
		 * cooperative manager would be instead forced to
		 * always call UFFDIO_UNREGISTER before it can safely
		 * close the uffd, to avoid involuntary SIGBUS triggered.
		 *
		 * If we were to return VM_FAULT_NOPAGE, it would work for
		 * the fault path, in which the lock will be released
		 * later.  However for GUP, faultin_page() does nothing
		 * special on NOPAGE, so GUP would spin retrying without
		 * releasing the mmap read lock, causing possible livelock.
		 *
		 * Here only VM_FAULT_RETRY would make sure the mmap lock
		 * be released immediately, so that the thread concurrently
		 * releasing the userfault would always make progress.
		 */
		release_fault_lock(vmf);
		goto out;
	}

	/* take the reference before dropping the mmap_lock */
	userfaultfd_ctx_get(ctx);

	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
	uwq.wq.private = current;
	uwq.msg = userfault_msg(vmf->address, vmf->real_address, vmf->flags,
				reason, ctx->features);
	uwq.ctx = ctx;
	uwq.waken = false;

	blocking_state = userfaultfd_get_blocking_state(vmf->flags);

	/*
	 * Take the vma lock now, in order to safely call
	 * userfaultfd_huge_must_wait() later. Since acquiring the
	 * (sleepable) vma lock can modify the current task state, that
	 * must be before explicitly calling set_current_state().
	 */
	if (is_vm_hugetlb_page(vma))
		hugetlb_vma_lock_read(vma);

	spin_lock_irq(&ctx->fault_pending_wqh.lock);
	/*
	 * After the __add_wait_queue the uwq is visible to userland
	 * through poll/read().
	 */
	__add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
	/*
	 * The smp_mb() after __set_current_state prevents the reads
	 * following the spin_unlock to happen before the list_add in
	 * __add_wait_queue.
	 */
	set_current_state(blocking_state);
	spin_unlock_irq(&ctx->fault_pending_wqh.lock);

	if (is_vm_hugetlb_page(vma)) {
		must_wait = userfaultfd_huge_must_wait(ctx, vmf, reason);
		hugetlb_vma_unlock_read(vma);
	} else {
		must_wait = userfaultfd_must_wait(ctx, vmf, reason);
	}

	release_fault_lock(vmf);

	if (likely(must_wait && !READ_ONCE(ctx->released))) {
		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
		schedule();
	}

	__set_current_state(TASK_RUNNING);

	/*
	 * Here we race with the list_del; list_add in
	 * userfaultfd_ctx_read(), however because we don't ever run
	 * list_del_init() to refile across the two lists, the prev
	 * and next pointers will never point to self. list_add also
	 * would never let any of the two pointers to point to
	 * self. So list_empty_careful won't risk to see both pointers
	 * pointing to self at any time during the list refile. The
	 * only case where list_del_init() is called is the full
	 * removal in the wake function and there we don't re-list_add
	 * and it's fine not to block on the spinlock. The uwq on this
	 * kernel stack can be released after the list_del_init.
	 */
	if (!list_empty_careful(&uwq.wq.entry)) {
		spin_lock_irq(&ctx->fault_pending_wqh.lock);
		/*
		 * No need of list_del_init(), the uwq on the stack
		 * will be freed shortly anyway.
		 */
		list_del(&uwq.wq.entry);
		spin_unlock_irq(&ctx->fault_pending_wqh.lock);
	}

	/*
	 * ctx may go away after this if the userfault pseudo fd is
	 * already released.
	 */
	userfaultfd_ctx_put(ctx);

out:
	return ret;
}

static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
					      struct userfaultfd_wait_queue *ewq)
{
	struct userfaultfd_ctx *release_new_ctx;

	if (WARN_ON_ONCE(current->flags & PF_EXITING))
		goto out;

	ewq->ctx = ctx;
	init_waitqueue_entry(&ewq->wq, current);
	release_new_ctx = NULL;

	spin_lock_irq(&ctx->event_wqh.lock);
	/*
	 * After the __add_wait_queue the uwq is visible to userland
	 * through poll/read().
	 */
	__add_wait_queue(&ctx->event_wqh, &ewq->wq);
	for (;;) {
		set_current_state(TASK_KILLABLE);
		if (ewq->msg.event == 0)
			break;
		if (READ_ONCE(ctx->released) ||
		    fatal_signal_pending(current)) {
			/*
			 * &ewq->wq may be queued in fork_event, but
			 * __remove_wait_queue ignores the head
			 * parameter. It would be a problem if it
			 * didn't.
			 */
			__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
			if (ewq->msg.event == UFFD_EVENT_FORK) {
				struct userfaultfd_ctx *new;

				new = (struct userfaultfd_ctx *)
					(unsigned long)
					ewq->msg.arg.reserved.reserved1;
				release_new_ctx = new;
			}
			break;
		}

		spin_unlock_irq(&ctx->event_wqh.lock);

		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
		schedule();

		spin_lock_irq(&ctx->event_wqh.lock);
	}
	__set_current_state(TASK_RUNNING);
	spin_unlock_irq(&ctx->event_wqh.lock);

	if (release_new_ctx) {
		userfaultfd_release_new(release_new_ctx);
		userfaultfd_ctx_put(release_new_ctx);
	}

	/*
	 * ctx may go away after this if the userfault pseudo fd is
	 * already released.
	 */
out:
	atomic_dec(&ctx->mmap_changing);
	VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0);
	userfaultfd_ctx_put(ctx);
}

static void userfaultfd_event_complete(struct userfaultfd_ctx *ctx,
				       struct userfaultfd_wait_queue *ewq)
{
	ewq->msg.event = 0;
	wake_up_locked(&ctx->event_wqh);
	__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
}

int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
{
	struct userfaultfd_ctx *ctx = NULL, *octx;
	struct userfaultfd_fork_ctx *fctx;

	octx = vma->vm_userfaultfd_ctx.ctx;
	if (!octx)
		return 0;

	if (!(octx->features & UFFD_FEATURE_EVENT_FORK)) {
		userfaultfd_reset_ctx(vma);
		return 0;
	}

	list_for_each_entry(fctx, fcs, list)
		if (fctx->orig == octx) {
			ctx = fctx->new;
			break;
		}

	if (!ctx) {
		fctx = kmalloc_obj(*fctx);
		if (!fctx)
			return -ENOMEM;

		ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
		if (!ctx) {
			kfree(fctx);
			return -ENOMEM;
		}

		refcount_set(&ctx->refcount, 1);
		ctx->flags = octx->flags;
		ctx->features = octx->features;
		ctx->released = false;
		init_rwsem(&ctx->map_changing_lock);
		atomic_set(&ctx->mmap_changing, 0);
		ctx->mm = vma->vm_mm;
		mmgrab(ctx->mm);

		userfaultfd_ctx_get(octx);
		down_write(&octx->map_changing_lock);
		atomic_inc(&octx->mmap_changing);
		up_write(&octx->map_changing_lock);
		fctx->orig = octx;
		fctx->new = ctx;
		list_add_tail(&fctx->list, fcs);
	}

	vma->vm_userfaultfd_ctx.ctx = ctx;
	return 0;
}

static void dup_fctx(struct userfaultfd_fork_ctx *fctx)
{
	struct userfaultfd_ctx *ctx = fctx->orig;
	struct userfaultfd_wait_queue ewq;

	msg_init(&ewq.msg);

	ewq.msg.event = UFFD_EVENT_FORK;
	ewq.msg.arg.reserved.reserved1 = (unsigned long)fctx->new;

	userfaultfd_event_wait_completion(ctx, &ewq);
}

void dup_userfaultfd_complete(struct list_head *fcs)
{
	struct userfaultfd_fork_ctx *fctx, *n;

	list_for_each_entry_safe(fctx, n, fcs, list) {
		dup_fctx(fctx);
		list_del(&fctx->list);
		kfree(fctx);
	}
}

void dup_userfaultfd_fail(struct list_head *fcs)
{
	struct userfaultfd_fork_ctx *fctx, *n;

	/*
	 * An error has occurred on fork, we will tear memory down, but have
	 * allocated memory for fctx's and raised reference counts for both the
	 * original and child contexts (and on the mm for each as a result).
	 *
	 * These would ordinarily be taken care of by a user handling the event,
	 * but we are no longer doing so, so manually clean up here.
	 *
	 * mm tear down will take care of cleaning up VMA contexts.
	 */
	list_for_each_entry_safe(fctx, n, fcs, list) {
		struct userfaultfd_ctx *octx = fctx->orig;
		struct userfaultfd_ctx *ctx = fctx->new;

		atomic_dec(&octx->mmap_changing);
		VM_WARN_ON_ONCE(atomic_read(&octx->mmap_changing) < 0);
		userfaultfd_ctx_put(octx);
		userfaultfd_ctx_put(ctx);

		list_del(&fctx->list);
		kfree(fctx);
	}
}

void mremap_userfaultfd_prep(struct vm_area_struct *vma,
			     struct vm_userfaultfd_ctx *vm_ctx)
{
	struct userfaultfd_ctx *ctx;

	ctx = vma->vm_userfaultfd_ctx.ctx;

	if (!ctx)
		return;

	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
		vm_ctx->ctx = ctx;
		userfaultfd_ctx_get(ctx);
		down_write(&ctx->map_changing_lock);
		atomic_inc(&ctx->mmap_changing);
		up_write(&ctx->map_changing_lock);
	} else {
		/* Drop uffd context if remap feature not enabled */
		userfaultfd_reset_ctx(vma);
	}
}

void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *vm_ctx,
				 unsigned long from, unsigned long to,
				 unsigned long len)
{
	struct userfaultfd_ctx *ctx = vm_ctx->ctx;
	struct userfaultfd_wait_queue ewq;

	if (!ctx)
		return;

	msg_init(&ewq.msg);

	ewq.msg.event = UFFD_EVENT_REMAP;
	ewq.msg.arg.remap.from = from;
	ewq.msg.arg.remap.to = to;
	ewq.msg.arg.remap.len = len;

	userfaultfd_event_wait_completion(ctx, &ewq);
}

void mremap_userfaultfd_fail(struct vm_userfaultfd_ctx *vm_ctx)
{
	struct userfaultfd_ctx *ctx = vm_ctx->ctx;

	if (!ctx)
		return;

	atomic_dec(&ctx->mmap_changing);
	VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0);
	userfaultfd_ctx_put(ctx);
}

bool userfaultfd_remove(struct vm_area_struct *vma,
			unsigned long start, unsigned long end)
{
	struct mm_struct *mm = vma->vm_mm;
	struct userfaultfd_ctx *ctx;
	struct userfaultfd_wait_queue ewq;

	ctx = vma->vm_userfaultfd_ctx.ctx;
	if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE))
		return true;

	userfaultfd_ctx_get(ctx);
	down_write(&ctx->map_changing_lock);
	atomic_inc(&ctx->mmap_changing);
	up_write(&ctx->map_changing_lock);
	mmap_read_unlock(mm);

	msg_init(&ewq.msg);

	ewq.msg.event = UFFD_EVENT_REMOVE;
	ewq.msg.arg.remove.start = start;
	ewq.msg.arg.remove.end = end;

	userfaultfd_event_wait_completion(ctx, &ewq);

	return false;
}

static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
			  unsigned long start, unsigned long end)
{
	struct userfaultfd_unmap_ctx *unmap_ctx;

	list_for_each_entry(unmap_ctx, unmaps, list)
		if (unmap_ctx->ctx == ctx && unmap_ctx->start == start &&
		    unmap_ctx->end == end)
			return true;

	return false;
}

int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start,
			   unsigned long end, struct list_head *unmaps)
{
	struct userfaultfd_unmap_ctx *unmap_ctx;
	struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;

	if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
	    has_unmap_ctx(ctx, unmaps, start, end))
		return 0;

	unmap_ctx = kzalloc_obj(*unmap_ctx);
	if (!unmap_ctx)
		return -ENOMEM;

	userfaultfd_ctx_get(ctx);
	down_write(&ctx->map_changing_lock);
	atomic_inc(&ctx->mmap_changing);
	up_write(&ctx->map_changing_lock);
	unmap_ctx->ctx = ctx;
	unmap_ctx->start = start;
	unmap_ctx->end = end;
	list_add_tail(&unmap_ctx->list, unmaps);

	return 0;
}

void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
{
	struct userfaultfd_unmap_ctx *ctx, *n;
	struct userfaultfd_wait_queue ewq;

	list_for_each_entry_safe(ctx, n, uf, list) {
		msg_init(&ewq.msg);

		ewq.msg.event = UFFD_EVENT_UNMAP;
		ewq.msg.arg.remove.start = ctx->start;
		ewq.msg.arg.remove.end = ctx->end;

		userfaultfd_event_wait_completion(ctx->ctx, &ewq);

		list_del(&ctx->list);
		kfree(ctx);
	}
}

static int userfaultfd_release(struct inode *inode, struct file *file)
{
	struct userfaultfd_ctx *ctx = file->private_data;
	struct mm_struct *mm = ctx->mm;
	/* len == 0 means wake all */
	struct userfaultfd_wake_range range = { .len = 0, };

	WRITE_ONCE(ctx->released, true);

	userfaultfd_release_all(mm, ctx);

	/*
	 * After no new page faults can wait on this fault_*wqh, flush
	 * the last page faults that may have been already waiting on
	 * the fault_*wqh.
	 */
	spin_lock_irq(&ctx->fault_pending_wqh.lock);
	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
	__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
	spin_unlock_irq(&ctx->fault_pending_wqh.lock);

	/* Flush pending events that may still wait on event_wqh */
	wake_up_all(&ctx->event_wqh);

	wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
	userfaultfd_ctx_put(ctx);
	return 0;
}

/* fault_pending_wqh.lock must be hold by the caller */
static inline struct userfaultfd_wait_queue *find_userfault_in(
		wait_queue_head_t *wqh)
{
	wait_queue_entry_t *wq;
	struct userfaultfd_wait_queue *uwq;

	lockdep_assert_held(&wqh->lock);

	uwq = NULL;
	if (!waitqueue_active(wqh))
		goto out;
	/* walk in reverse to provide FIFO behavior to read userfaults */
	wq = list_last_entry(&wqh->head, typeof(*wq), entry);
	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
out:
	return uwq;
}

static inline struct userfaultfd_wait_queue *find_userfault(
		struct userfaultfd_ctx *ctx)
{
	return find_userfault_in(&ctx->fault_pending_wqh);
}

static inline struct userfaultfd_wait_queue *find_userfault_evt(
		struct userfaultfd_ctx *ctx)
{
	return find_userfault_in(&ctx->event_wqh);
}

static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
{
	struct userfaultfd_ctx *ctx = file->private_data;
	__poll_t ret;

	poll_wait(file, &ctx->fd_wqh, wait);

	if (!userfaultfd_is_initialized(ctx))
		return EPOLLERR;

	/*
	 * poll() never guarantees that read won't block.
	 * userfaults can be waken before they're read().
	 */
	if (unlikely(!(file->f_flags & O_NONBLOCK)))
		return EPOLLERR;
	/*
	 * lockless access to see if there are pending faults
	 * __pollwait last action is the add_wait_queue but
	 * the spin_unlock would allow the waitqueue_active to
	 * pass above the actual list_add inside
	 * add_wait_queue critical section. So use a full
	 * memory barrier to serialize the list_add write of
	 * add_wait_queue() with the waitqueue_active read
	 * below.
	 */
	ret = 0;
	smp_mb();
	if (waitqueue_active(&ctx->fault_pending_wqh))
		ret = EPOLLIN;
	else if (waitqueue_active(&ctx->event_wqh))
		ret = EPOLLIN;

	return ret;
}

static const struct file_operations userfaultfd_fops;

static int resolve_userfault_fork(struct userfaultfd_ctx *new,
				  struct inode *inode,
				  struct uffd_msg *msg)
{
	int fd;

	fd = anon_inode_create_getfd("[userfaultfd]", &userfaultfd_fops, new,
			O_RDONLY | (new->flags & UFFD_SHARED_FCNTL_FLAGS), inode);
	if (fd < 0)
		return fd;

	msg->arg.reserved.reserved1 = 0;
	msg->arg.fork.ufd = fd;
	return 0;
}

static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
				    struct uffd_msg *msg, struct inode *inode)
{
	ssize_t ret;
	DECLARE_WAITQUEUE(wait, current);
	struct userfaultfd_wait_queue *uwq;
	/*
	 * Handling fork event requires sleeping operations, so
	 * we drop the event_wqh lock, then do these ops, then
	 * lock it back and wake up the waiter. While the lock is
	 * dropped the ewq may go away so we keep track of it
	 * carefully.
	 */
	LIST_HEAD(fork_event);
	struct userfaultfd_ctx *fork_nctx = NULL;

	/* always take the fd_wqh lock before the fault_pending_wqh lock */
	spin_lock_irq(&ctx->fd_wqh.lock);
	__add_wait_queue(&ctx->fd_wqh, &wait);
	for (;;) {
		set_current_state(TASK_INTERRUPTIBLE);
		spin_lock(&ctx->fault_pending_wqh.lock);
		uwq = find_userfault(ctx);
		if (uwq) {
			/*
			 * Use a seqcount to repeat the lockless check
			 * in wake_userfault() to avoid missing
			 * wakeups because during the refile both
			 * waitqueue could become empty if this is the
			 * only userfault.
			 */
			write_seqcount_begin(&ctx->refile_seq);

			/*
			 * The fault_pending_wqh.lock prevents the uwq
			 * to disappear from under us.
			 *
			 * Refile this userfault from
			 * fault_pending_wqh to fault_wqh, it's not
			 * pending anymore after we read it.
			 *
			 * Use list_del() by hand (as
			 * userfaultfd_wake_function also uses
			 * list_del_init() by hand) to be sure nobody
			 * changes __remove_wait_queue() to use
			 * list_del_init() in turn breaking the
			 * !list_empty_careful() check in
			 * handle_userfault(). The uwq->wq.head list
			 * must never be empty at any time during the
			 * refile, or the waitqueue could disappear
			 * from under us. The "wait_queue_head_t"
			 * parameter of __remove_wait_queue() is unused
			 * anyway.
			 */
			list_del(&uwq->wq.entry);
			add_wait_queue(&ctx->fault_wqh, &uwq->wq);

			write_seqcount_end(&ctx->refile_seq);

			/* careful to always initialize msg if ret == 0 */
			*msg = uwq->msg;
			spin_unlock(&ctx->fault_pending_wqh.lock);
			ret = 0;
			break;
		}
		spin_unlock(&ctx->fault_pending_wqh.lock);

		spin_lock(&ctx->event_wqh.lock);
		uwq = find_userfault_evt(ctx);
		if (uwq) {
			*msg = uwq->msg;

			if (uwq->msg.event == UFFD_EVENT_FORK) {
				fork_nctx = (struct userfaultfd_ctx *)
					(unsigned long)
					uwq->msg.arg.reserved.reserved1;
				list_move(&uwq->wq.entry, &fork_event);
				/*
				 * fork_nctx can be freed as soon as
				 * we drop the lock, unless we take a
				 * reference on it.
				 */
				userfaultfd_ctx_get(fork_nctx);
				spin_unlock(&ctx->event_wqh.lock);
				ret = 0;
				break;
			}

			userfaultfd_event_complete(ctx, uwq);
			spin_unlock(&ctx->event_wqh.lock);
			ret = 0;
			break;
		}
		spin_unlock(&ctx->event_wqh.lock);

		if (signal_pending(current)) {
			ret = -ERESTARTSYS;
			break;
		}
		if (no_wait) {
			ret = -EAGAIN;
			break;
		}
		spin_unlock_irq(&ctx->fd_wqh.lock);
		schedule();
		spin_lock_irq(&ctx->fd_wqh.lock);
	}
	__remove_wait_queue(&ctx->fd_wqh, &wait);
	__set_current_state(TASK_RUNNING);
	spin_unlock_irq(&ctx->fd_wqh.lock);

	if (!ret && msg->event == UFFD_EVENT_FORK) {
		ret = resolve_userfault_fork(fork_nctx, inode, msg);
		spin_lock_irq(&ctx->event_wqh.lock);
		if (!list_empty(&fork_event)) {
			/*
			 * The fork thread didn't abort, so we can
			 * drop the temporary refcount.
			 */
			userfaultfd_ctx_put(fork_nctx);

			uwq = list_first_entry(&fork_event,
					       typeof(*uwq),
					       wq.entry);
			/*
			 * If fork_event list wasn't empty and in turn
			 * the event wasn't already released by fork
			 * (the event is allocated on fork kernel
			 * stack), put the event back to its place in
			 * the event_wq. fork_event head will be freed
			 * as soon as we return so the event cannot
			 * stay queued there no matter the current
			 * "ret" value.
			 */
			list_del(&uwq->wq.entry);
			__add_wait_queue(&ctx->event_wqh, &uwq->wq);

			/*
			 * Leave the event in the waitqueue and report
			 * error to userland if we failed to resolve
			 * the userfault fork.
			 */
			if (likely(!ret))
				userfaultfd_event_complete(ctx, uwq);
		} else {
			/*
			 * Here the fork thread aborted and the
			 * refcount from the fork thread on fork_nctx
			 * has already been released. We still hold
			 * the reference we took before releasing the
			 * lock above. If resolve_userfault_fork
			 * failed we've to drop it because the
			 * fork_nctx has to be freed in such case. If
			 * it succeeded we'll hold it because the new
			 * uffd references it.
			 */
			if (ret)
				userfaultfd_ctx_put(fork_nctx);
		}
		spin_unlock_irq(&ctx->event_wqh.lock);
	}

	return ret;
}

static ssize_t userfaultfd_read_iter(struct kiocb *iocb, struct iov_iter *to)
{
	struct file *file = iocb->ki_filp;
	struct userfaultfd_ctx *ctx = file->private_data;
	ssize_t _ret, ret = 0;
	struct uffd_msg msg;
	struct inode *inode = file_inode(file);
	bool no_wait;

	if (!userfaultfd_is_initialized(ctx))
		return -EINVAL;

	no_wait = file->f_flags & O_NONBLOCK || iocb->ki_flags & IOCB_NOWAIT;
	for (;;) {
		if (iov_iter_count(to) < sizeof(msg))
			return ret ? ret : -EINVAL;
		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg, inode);
		if (_ret < 0)
			return ret ? ret : _ret;
		_ret = !copy_to_iter_full(&msg, sizeof(msg), to);
		if (_ret)
			return ret ? ret : -EFAULT;
		ret += sizeof(msg);
		/*
		 * Allow to read more than one fault at time but only
		 * block if waiting for the very first one.
		 */
		no_wait = true;
	}
}

static void __wake_userfault(struct userfaultfd_ctx *ctx,
			     struct userfaultfd_wake_range *range)
{
	spin_lock_irq(&ctx->fault_pending_wqh.lock);
	/* wake all in the range and autoremove */
	if (waitqueue_active(&ctx->fault_pending_wqh))
		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
				     range);
	if (waitqueue_active(&ctx->fault_wqh))
		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
	spin_unlock_irq(&ctx->fault_pending_wqh.lock);
}

static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
					   struct userfaultfd_wake_range *range)
{
	unsigned seq;
	bool need_wakeup;

	/*
	 * To be sure waitqueue_active() is not reordered by the CPU
	 * before the pagetable update, use an explicit SMP memory
	 * barrier here. PT lock release or mmap_read_unlock(mm) still
	 * have release semantics that can allow the
	 * waitqueue_active() to be reordered before the pte update.
	 */
	smp_mb();

	/*
	 * Use waitqueue_active because it's very frequent to
	 * change the address space atomically even if there are no
	 * userfaults yet. So we take the spinlock only when we're
	 * sure we've userfaults to wake.
	 */
	do {
		seq = read_seqcount_begin(&ctx->refile_seq);
		need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
			waitqueue_active(&ctx->fault_wqh);
		cond_resched();
	} while (read_seqcount_retry(&ctx->refile_seq, seq));
	if (need_wakeup)
		__wake_userfault(ctx, range);
}

static __always_inline int validate_unaligned_range(
	struct mm_struct *mm, __u64 start, __u64 len)
{
	__u64 task_size = mm->task_size;

	if (len & ~PAGE_MASK)
		return -EINVAL;
	if (!len)
		return -EINVAL;
	if (start >= task_size)
		return -EINVAL;
	if (len > task_size - start)
		return -EINVAL;
	if (start + len <= start)
		return -EINVAL;
	return 0;
}

static __always_inline int validate_range(struct mm_struct *mm,
					  __u64 start, __u64 len)
{
	if (start & ~PAGE_MASK)
		return -EINVAL;

	return validate_unaligned_range(mm, start, len);
}

static int userfaultfd_register(struct userfaultfd_ctx *ctx,
				unsigned long arg)
{
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma, *cur;
	int ret;
	struct uffdio_register uffdio_register;
	struct uffdio_register __user *user_uffdio_register;
	vm_flags_t vm_flags;
	bool found;
	bool basic_ioctls;
	unsigned long start, end;
	struct vma_iterator vmi;
	bool wp_async = userfaultfd_wp_async_ctx(ctx);

	user_uffdio_register = (struct uffdio_register __user *) arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_register, user_uffdio_register,
			   sizeof(uffdio_register)-sizeof(__u64)))
		goto out;

	ret = -EINVAL;
	if (!uffdio_register.mode)
		goto out;
	if (uffdio_register.mode & ~UFFD_API_REGISTER_MODES)
		goto out;
	vm_flags = 0;
	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
		vm_flags |= VM_UFFD_MISSING;
	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
		if (!pgtable_supports_uffd())
			goto out;

		vm_flags |= VM_UFFD_WP;
	}
	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_RWP) {
		if (!pgtable_supports_uffd() || VM_UFFD_RWP == VM_NONE)
			goto out;
		if (!(userfaultfd_features(ctx) & UFFD_FEATURE_RWP))
			goto out;
		vm_flags |= VM_UFFD_RWP;
	}

	/*
	 * WP and RWP share the uffd PTE bit and
	 * cannot coexist in the same VMA — the bit would carry ambiguous
	 * semantics. Reject the combination up front.
	 */
	if ((vm_flags & VM_UFFD_WP) && (vm_flags & VM_UFFD_RWP))
		goto out;

	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR) {
#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
		goto out;
#endif
		vm_flags |= VM_UFFD_MINOR;
	}

	ret = validate_range(mm, uffdio_register.range.start,
			     uffdio_register.range.len);
	if (ret)
		goto out;

	start = uffdio_register.range.start;
	end = start + uffdio_register.range.len;

	ret = -ENOMEM;
	if (!mmget_not_zero(mm))
		goto out;

	ret = -EINVAL;
	mmap_write_lock(mm);
	vma_iter_init(&vmi, mm, start);
	vma = vma_find(&vmi, end);
	if (!vma)
		goto out_unlock;

	/*
	 * If the first vma contains huge pages, make sure start address
	 * is aligned to huge page size.
	 */
	if (is_vm_hugetlb_page(vma)) {
		unsigned long vma_hpagesize = vma_kernel_pagesize(vma);

		if (start & (vma_hpagesize - 1))
			goto out_unlock;
	}

	/*
	 * Search for not compatible vmas.
	 */
	found = false;
	basic_ioctls = false;
	cur = vma;
	do {
		cond_resched();

		VM_WARN_ON_ONCE(!!cur->vm_userfaultfd_ctx.ctx ^
				!!(cur->vm_flags & __VM_UFFD_FLAGS));

		/* check not compatible vmas */
		ret = -EINVAL;
		if (!vma_can_userfault(cur, vm_flags, wp_async))
			goto out_unlock;

		/*
		 * RWP uses protnone as an access-tracking marker. PROT_NONE
		 * VMAs have vm_page_prot == PAGE_NONE, so RWP resolution
		 * cannot make a page accessible again. Reject at register
		 * time only: a VMA that later becomes inaccessible via
		 * mprotect() must still be unregisterable, so this is not
		 * part of vma_can_userfault().
		 */
		if ((vm_flags & VM_UFFD_RWP) && !vma_is_accessible(cur))
			goto out_unlock;

		/*
		 * UFFDIO_COPY will fill file holes even without
		 * PROT_WRITE. This check enforces that if this is a
		 * MAP_SHARED, the process has write permission to the backing
		 * file. If VM_MAYWRITE is set it also enforces that on a
		 * MAP_SHARED vma: there is no F_WRITE_SEAL and no further
		 * F_WRITE_SEAL can be taken until the vma is destroyed.
		 */
		ret = -EPERM;
		if (unlikely(!(cur->vm_flags & VM_MAYWRITE)))
			goto out_unlock;

		/*
		 * If this vma contains ending address, and huge pages
		 * check alignment.
		 */
		if (is_vm_hugetlb_page(cur) && end <= cur->vm_end &&
		    end > cur->vm_start) {
			unsigned long vma_hpagesize = vma_kernel_pagesize(cur);

			ret = -EINVAL;

			if (end & (vma_hpagesize - 1))
				goto out_unlock;
		}
		if ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_MAYWRITE))
			goto out_unlock;

		/*
		 * Check that this vma isn't already owned by a
		 * different userfaultfd. We can't allow more than one
		 * userfaultfd to own a single vma simultaneously or we
		 * wouldn't know which one to deliver the userfaults to.
		 */
		ret = -EBUSY;
		if (cur->vm_userfaultfd_ctx.ctx &&
		    cur->vm_userfaultfd_ctx.ctx != ctx)
			goto out_unlock;

		/*
		 * Mode switches that drop VM_UFFD_WP or VM_UFFD_RWP would
		 * leave PTE markers without the flag that describes them;
		 * subsequent mprotect() would then promote stale markers
		 * into the other mode. Require an unregister first.
		 */
		if (cur->vm_userfaultfd_ctx.ctx == ctx &&
		    cur->vm_flags & (VM_UFFD_WP | VM_UFFD_RWP) & ~vm_flags)
			goto out_unlock;

		/*
		 * Note vmas containing huge pages
		 */
		if (is_vm_hugetlb_page(cur))
			basic_ioctls = true;

		found = true;
	} for_each_vma_range(vmi, cur, end);
	VM_WARN_ON_ONCE(!found);

	ret = userfaultfd_register_range(ctx, vma, vm_flags, start, end,
					 wp_async);

out_unlock:
	mmap_write_unlock(mm);
	mmput(mm);
	if (!ret) {
		__u64 ioctls_out;

		ioctls_out = basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
			UFFD_API_RANGE_IOCTLS;

		/*
		 * Declare the WP ioctl only if the WP mode is
		 * specified and all checks passed with the range
		 */
		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_WP))
			ioctls_out &= ~((__u64)1 << _UFFDIO_WRITEPROTECT);

		/* CONTINUE ioctl is only supported for MINOR ranges. */
		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR))
			ioctls_out &= ~((__u64)1 << _UFFDIO_CONTINUE);

		/* RWPROTECT is only supported for RWP ranges */
		if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_RWP))
			ioctls_out &= ~((__u64)1 << _UFFDIO_RWPROTECT);

		/*
		 * Now that we scanned all vmas we can already tell
		 * userland which ioctls methods are guaranteed to
		 * succeed on this range.
		 */
		if (put_user(ioctls_out, &user_uffdio_register->ioctls))
			ret = -EFAULT;
	}
out:
	return ret;
}

static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
				  unsigned long arg)
{
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma, *prev, *cur;
	int ret;
	struct uffdio_range uffdio_unregister;
	bool found;
	unsigned long start, end, vma_end;
	const void __user *buf = (void __user *)arg;
	struct vma_iterator vmi;
	bool wp_async = userfaultfd_wp_async_ctx(ctx);

	ret = -EFAULT;
	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
		goto out;

	ret = validate_range(mm, uffdio_unregister.start,
			     uffdio_unregister.len);
	if (ret)
		goto out;

	start = uffdio_unregister.start;
	end = start + uffdio_unregister.len;

	ret = -ENOMEM;
	if (!mmget_not_zero(mm))
		goto out;

	mmap_write_lock(mm);
	ret = -EINVAL;
	vma_iter_init(&vmi, mm, start);
	vma = vma_find(&vmi, end);
	if (!vma)
		goto out_unlock;

	/*
	 * If the first vma contains huge pages, make sure start address
	 * is aligned to huge page size.
	 */
	if (is_vm_hugetlb_page(vma)) {
		unsigned long vma_hpagesize = vma_kernel_pagesize(vma);

		if (start & (vma_hpagesize - 1))
			goto out_unlock;
	}

	/*
	 * Search for not compatible vmas.
	 */
	found = false;
	cur = vma;
	do {
		cond_resched();

		VM_WARN_ON_ONCE(!!cur->vm_userfaultfd_ctx.ctx ^
				!!(cur->vm_flags & __VM_UFFD_FLAGS));

		/*
		 * Prevent unregistering through a different userfaultfd than
		 * the one used for registration.
		 */
		if (cur->vm_userfaultfd_ctx.ctx &&
		    cur->vm_userfaultfd_ctx.ctx != ctx)
			goto out_unlock;

		/*
		 * Check not compatible vmas, not strictly required
		 * here as not compatible vmas cannot have an
		 * userfaultfd_ctx registered on them, but this
		 * provides for more strict behavior to notice
		 * unregistration errors.
		 */
		if (!vma_can_userfault(cur, cur->vm_flags, wp_async))
			goto out_unlock;

		found = true;
	} for_each_vma_range(vmi, cur, end);
	VM_WARN_ON_ONCE(!found);

	vma_iter_set(&vmi, start);
	prev = vma_prev(&vmi);
	if (vma->vm_start < start)
		prev = vma;

	ret = 0;
	for_each_vma_range(vmi, vma, end) {
		cond_resched();

		/* VMA not registered with userfaultfd. */
		if (!vma->vm_userfaultfd_ctx.ctx)
			goto skip;

		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx != ctx);
		VM_WARN_ON_ONCE(!vma_can_userfault(vma, vma->vm_flags, wp_async));
		VM_WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE));

		if (vma->vm_start > start)
			start = vma->vm_start;
		vma_end = min(end, vma->vm_end);

		if (userfaultfd_missing(vma)) {
			/*
			 * Wake any concurrent pending userfault while
			 * we unregister, so they will not hang
			 * permanently and it avoids userland to call
			 * UFFDIO_WAKE explicitly.
			 */
			struct userfaultfd_wake_range range;
			range.start = start;
			range.len = vma_end - start;
			wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
		}

		vma = userfaultfd_clear_vma(&vmi, prev, vma,
					    start, vma_end);
		if (IS_ERR(vma)) {
			ret = PTR_ERR(vma);
			break;
		}

skip:
		prev = vma;
		start = vma->vm_end;
	}

out_unlock:
	mmap_write_unlock(mm);
	mmput(mm);
out:
	return ret;
}

/*
 * userfaultfd_wake may be used in combination with the
 * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
 */
static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
			    unsigned long arg)
{
	int ret;
	struct uffdio_range uffdio_wake;
	struct userfaultfd_wake_range range;
	const void __user *buf = (void __user *)arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
		goto out;

	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
	if (ret)
		goto out;

	range.start = uffdio_wake.start;
	range.len = uffdio_wake.len;

	/*
	 * len == 0 means wake all and we don't want to wake all here,
	 * so check it again to be sure.
	 */
	VM_WARN_ON_ONCE(!range.len);

	wake_userfault(ctx, &range);
	ret = 0;

out:
	return ret;
}

static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
			    unsigned long arg)
{
	__s64 ret;
	struct uffdio_copy uffdio_copy;
	struct uffdio_copy __user *user_uffdio_copy;
	struct userfaultfd_wake_range range;
	uffd_flags_t flags = 0;

	user_uffdio_copy = (struct uffdio_copy __user *) arg;

	ret = -EAGAIN;
	if (unlikely(atomic_read(&ctx->mmap_changing))) {
		if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
			return -EFAULT;
		goto out;
	}

	ret = -EFAULT;
	if (copy_from_user(&uffdio_copy, user_uffdio_copy,
			   /* don't copy "copy" last field */
			   sizeof(uffdio_copy)-sizeof(__s64)))
		goto out;

	ret = validate_unaligned_range(ctx->mm, uffdio_copy.src,
				       uffdio_copy.len);
	if (ret)
		goto out;
	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
	if (ret)
		goto out;

	ret = -EINVAL;
	if (uffdio_copy.mode & ~(UFFDIO_COPY_MODE_DONTWAKE|UFFDIO_COPY_MODE_WP))
		goto out;
	if (uffdio_copy.mode & UFFDIO_COPY_MODE_WP)
		flags |= MFILL_ATOMIC_WP;
	if (mmget_not_zero(ctx->mm)) {
		ret = mfill_atomic_copy(ctx, uffdio_copy.dst, uffdio_copy.src,
					uffdio_copy.len, flags);
		mmput(ctx->mm);
	} else {
		return -ESRCH;
	}
	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
		return -EFAULT;
	if (ret < 0)
		goto out;
	VM_WARN_ON_ONCE(!ret);
	/* len == 0 would wake all */
	range.len = ret;
	if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
		range.start = uffdio_copy.dst;
		wake_userfault(ctx, &range);
	}
	ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
out:
	return ret;
}

static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
				unsigned long arg)
{
	__s64 ret;
	struct uffdio_zeropage uffdio_zeropage;
	struct uffdio_zeropage __user *user_uffdio_zeropage;
	struct userfaultfd_wake_range range;

	user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;

	ret = -EAGAIN;
	if (unlikely(atomic_read(&ctx->mmap_changing))) {
		if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
			return -EFAULT;
		goto out;
	}

	ret = -EFAULT;
	if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
			   /* don't copy "zeropage" last field */
			   sizeof(uffdio_zeropage)-sizeof(__s64)))
		goto out;

	ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
			     uffdio_zeropage.range.len);
	if (ret)
		goto out;
	ret = -EINVAL;
	if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
		goto out;

	if (mmget_not_zero(ctx->mm)) {
		ret = mfill_atomic_zeropage(ctx, uffdio_zeropage.range.start,
					    uffdio_zeropage.range.len);
		mmput(ctx->mm);
	} else {
		return -ESRCH;
	}
	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
		return -EFAULT;
	if (ret < 0)
		goto out;
	/* len == 0 would wake all */
	VM_WARN_ON_ONCE(!ret);
	range.len = ret;
	if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
		range.start = uffdio_zeropage.range.start;
		wake_userfault(ctx, &range);
	}
	ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
out:
	return ret;
}

static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
				    unsigned long arg)
{
	int ret;
	struct uffdio_writeprotect uffdio_wp;
	struct uffdio_writeprotect __user *user_uffdio_wp;
	struct userfaultfd_wake_range range;
	bool mode_wp, mode_dontwake;

	if (atomic_read(&ctx->mmap_changing))
		return -EAGAIN;

	user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;

	if (copy_from_user(&uffdio_wp, user_uffdio_wp,
			   sizeof(struct uffdio_writeprotect)))
		return -EFAULT;

	ret = validate_range(ctx->mm, uffdio_wp.range.start,
			     uffdio_wp.range.len);
	if (ret)
		return ret;

	if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE |
			       UFFDIO_WRITEPROTECT_MODE_WP))
		return -EINVAL;

	mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
	mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;

	if (mode_wp && mode_dontwake)
		return -EINVAL;

	if (mmget_not_zero(ctx->mm)) {
		ret = mwriteprotect_range(ctx, uffdio_wp.range.start,
					  uffdio_wp.range.len, mode_wp);
		mmput(ctx->mm);
	} else {
		return -ESRCH;
	}

	if (ret)
		return ret;

	if (!mode_wp && !mode_dontwake) {
		range.start = uffdio_wp.range.start;
		range.len = uffdio_wp.range.len;
		wake_userfault(ctx, &range);
	}
	return ret;
}

static int userfaultfd_rwprotect(struct userfaultfd_ctx *ctx,
				 unsigned long arg)
{
	int ret;
	struct uffdio_rwprotect uffdio_rwp;
	struct userfaultfd_wake_range range;
	bool mode_rwp, mode_dontwake;

	if (atomic_read(&ctx->mmap_changing))
		return -EAGAIN;

	if (copy_from_user(&uffdio_rwp, (void __user *)arg,
			   sizeof(uffdio_rwp)))
		return -EFAULT;

	ret = validate_range(ctx->mm, uffdio_rwp.range.start,
			     uffdio_rwp.range.len);
	if (ret)
		return ret;

	if (uffdio_rwp.mode & ~(UFFDIO_RWPROTECT_MODE_DONTWAKE |
				UFFDIO_RWPROTECT_MODE_RWP))
		return -EINVAL;

	mode_rwp = uffdio_rwp.mode & UFFDIO_RWPROTECT_MODE_RWP;
	mode_dontwake = uffdio_rwp.mode & UFFDIO_RWPROTECT_MODE_DONTWAKE;

	if (mode_rwp && mode_dontwake)
		return -EINVAL;

	if (mmget_not_zero(ctx->mm)) {
		ret = mrwprotect_range(ctx, uffdio_rwp.range.start,
				       uffdio_rwp.range.len, mode_rwp);
		mmput(ctx->mm);
	} else {
		return -ESRCH;
	}

	if (ret)
		return ret;

	if (!mode_rwp && !mode_dontwake) {
		range.start = uffdio_rwp.range.start;
		range.len = uffdio_rwp.range.len;
		wake_userfault(ctx, &range);
	}
	return ret;
}

/* Subset of UFFD_API_FEATURES actually supported by this kernel/arch */
static __u64 uffd_api_available_features(void)
{
	__u64 f = UFFD_API_FEATURES;

	if (!IS_ENABLED(CONFIG_HAVE_ARCH_USERFAULTFD_MINOR))
		f &= ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
	if (!pgtable_supports_uffd())
		f &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP;
	if (!uffd_supports_wp_marker())
		f &= ~(UFFD_FEATURE_WP_HUGETLBFS_SHMEM |
		       UFFD_FEATURE_WP_UNPOPULATED |
		       UFFD_FEATURE_WP_ASYNC);
	/*
	 * RWP needs both PROT_NONE support and the uffd PTE bit. The
	 * VM_UFFD_RWP check covers compile-time unavailability; the
	 * pgtable_supports_uffd() check covers runtime (e.g. riscv
	 * without the SVRSW60T59B extension) where the PTE bit is declared
	 * but not actually usable.
	 */
	if (VM_UFFD_RWP == VM_NONE || !pgtable_supports_uffd())
		f &= ~(UFFD_FEATURE_RWP | UFFD_FEATURE_RWP_ASYNC);
	return f;
}

/* Async features that can be toggled at runtime via UFFDIO_SET_MODE */
#define UFFD_FEATURE_TOGGLEABLE	UFFD_FEATURE_RWP_ASYNC

static int userfaultfd_set_mode(struct userfaultfd_ctx *ctx,
				unsigned long arg)
{
	struct uffdio_set_mode mode;
	struct mm_struct *mm = ctx->mm;

	if (copy_from_user(&mode, (void __user *)arg, sizeof(mode)))
		return -EFAULT;

	/* enable and disable must not overlap */
	if (mode.enable & mode.disable)
		return -EINVAL;

	/* only toggleable features that this kernel/arch actually supports */
	if ((mode.enable | mode.disable) &
	    ~(uffd_api_available_features() & UFFD_FEATURE_TOGGLEABLE))
		return -EINVAL;

	/* RWP_ASYNC can only be enabled on contexts that negotiated RWP */
	if ((mode.enable & UFFD_FEATURE_RWP_ASYNC) &&
	    !(userfaultfd_features(ctx) & UFFD_FEATURE_RWP))
		return -EINVAL;

	if (!mmget_not_zero(mm))
		return -ESRCH;

	/*
	 * Drain in-flight faults before flipping features. mmap_write_lock()
	 * blocks new mmap_read_lock() callers, but per-VMA locked faults
	 * (lock_vma_under_rcu() + FAULT_FLAG_VMA_LOCK) that acquired before
	 * this point keep running. Calling vma_start_write() on each UFFD-
	 * armed VMA waits for those readers to drop, so no in-flight fault
	 * can observe the old features after mmap_write_unlock().
	 */
	mmap_write_lock(mm);
	{
		struct vm_area_struct *vma;
		VMA_ITERATOR(vmi, mm, 0);

		for_each_vma(vmi, vma) {
			if (vma->vm_userfaultfd_ctx.ctx == ctx)
				vma_start_write(vma);
		}
	}
	/*
	 * Single WRITE_ONCE so lockless readers (fdinfo, poll/read_iter
	 * via userfaultfd_is_initialized(), and the userfaultfd_features()
	 * helper used elsewhere) can't observe a mid-RMW intermediate
	 * value. Hot-path readers already serialise through the mmap lock
	 * + vma_start_write() drain above, so their load doesn't need an
	 * annotation.
	 */
	WRITE_ONCE(ctx->features,
		   (ctx->features | mode.enable) & ~mode.disable);
	mmap_write_unlock(mm);

	/*
	 * If switching to async, wake threads blocked in handle_userfault().
	 * They will retry the fault and auto-resolve under the new mode.
	 * len=0 means wake all pending faults on this context.
	 */
	if (mode.enable & UFFD_FEATURE_RWP_ASYNC) {
		struct userfaultfd_wake_range range = { .len = 0 };

		spin_lock_irq(&ctx->fault_pending_wqh.lock);
		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
				     &range);
		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
		spin_unlock_irq(&ctx->fault_pending_wqh.lock);
	}

	mmput(mm);
	return 0;
}

static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
{
	__s64 ret;
	struct uffdio_continue uffdio_continue;
	struct uffdio_continue __user *user_uffdio_continue;
	struct userfaultfd_wake_range range;
	uffd_flags_t flags = 0;

	user_uffdio_continue = (struct uffdio_continue __user *)arg;

	ret = -EAGAIN;
	if (unlikely(atomic_read(&ctx->mmap_changing))) {
		if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
			return -EFAULT;
		goto out;
	}

	ret = -EFAULT;
	if (copy_from_user(&uffdio_continue, user_uffdio_continue,
			   /* don't copy the output fields */
			   sizeof(uffdio_continue) - (sizeof(__s64))))
		goto out;

	ret = validate_range(ctx->mm, uffdio_continue.range.start,
			     uffdio_continue.range.len);
	if (ret)
		goto out;

	ret = -EINVAL;
	if (uffdio_continue.mode & ~(UFFDIO_CONTINUE_MODE_DONTWAKE |
				     UFFDIO_CONTINUE_MODE_WP))
		goto out;
	if (uffdio_continue.mode & UFFDIO_CONTINUE_MODE_WP)
		flags |= MFILL_ATOMIC_WP;

	if (mmget_not_zero(ctx->mm)) {
		ret = mfill_atomic_continue(ctx, uffdio_continue.range.start,
					    uffdio_continue.range.len, flags);
		mmput(ctx->mm);
	} else {
		return -ESRCH;
	}

	if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
		return -EFAULT;
	if (ret < 0)
		goto out;

	/* len == 0 would wake all */
	VM_WARN_ON_ONCE(!ret);
	range.len = ret;
	if (!(uffdio_continue.mode & UFFDIO_CONTINUE_MODE_DONTWAKE)) {
		range.start = uffdio_continue.range.start;
		wake_userfault(ctx, &range);
	}
	ret = range.len == uffdio_continue.range.len ? 0 : -EAGAIN;

out:
	return ret;
}

static inline int userfaultfd_poison(struct userfaultfd_ctx *ctx, unsigned long arg)
{
	__s64 ret;
	struct uffdio_poison uffdio_poison;
	struct uffdio_poison __user *user_uffdio_poison;
	struct userfaultfd_wake_range range;

	user_uffdio_poison = (struct uffdio_poison __user *)arg;

	ret = -EAGAIN;
	if (unlikely(atomic_read(&ctx->mmap_changing))) {
		if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
			return -EFAULT;
		goto out;
	}

	ret = -EFAULT;
	if (copy_from_user(&uffdio_poison, user_uffdio_poison,
			   /* don't copy the output fields */
			   sizeof(uffdio_poison) - (sizeof(__s64))))
		goto out;

	ret = validate_range(ctx->mm, uffdio_poison.range.start,
			     uffdio_poison.range.len);
	if (ret)
		goto out;

	ret = -EINVAL;
	if (uffdio_poison.mode & ~UFFDIO_POISON_MODE_DONTWAKE)
		goto out;

	if (mmget_not_zero(ctx->mm)) {
		ret = mfill_atomic_poison(ctx, uffdio_poison.range.start,
					  uffdio_poison.range.len, 0);
		mmput(ctx->mm);
	} else {
		return -ESRCH;
	}

	if (unlikely(put_user(ret, &user_uffdio_poison->updated)))
		return -EFAULT;
	if (ret < 0)
		goto out;

	/* len == 0 would wake all */
	VM_WARN_ON_ONCE(!ret);
	range.len = ret;
	if (!(uffdio_poison.mode & UFFDIO_POISON_MODE_DONTWAKE)) {
		range.start = uffdio_poison.range.start;
		wake_userfault(ctx, &range);
	}
	ret = range.len == uffdio_poison.range.len ? 0 : -EAGAIN;

out:
	return ret;
}

bool userfaultfd_wp_async(struct vm_area_struct *vma)
{
	return userfaultfd_wp_async_ctx(vma->vm_userfaultfd_ctx.ctx);
}

bool userfaultfd_rwp_async(struct vm_area_struct *vma)
{
	return userfaultfd_rwp_async_ctx(vma->vm_userfaultfd_ctx.ctx);
}

static inline unsigned int uffd_ctx_features(__u64 user_features)
{
	/*
	 * For the current set of features the bits just coincide. Set
	 * UFFD_FEATURE_INITIALIZED to mark the features as enabled.
	 */
	return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
}

static int userfaultfd_move(struct userfaultfd_ctx *ctx,
			    unsigned long arg)
{
	__s64 ret;
	struct uffdio_move uffdio_move;
	struct uffdio_move __user *user_uffdio_move;
	struct userfaultfd_wake_range range;
	struct mm_struct *mm = ctx->mm;

	user_uffdio_move = (struct uffdio_move __user *) arg;

	ret = -EAGAIN;
	if (unlikely(atomic_read(&ctx->mmap_changing))) {
		if (unlikely(put_user(ret, &user_uffdio_move->move)))
			return -EFAULT;
		goto out;
	}

	if (copy_from_user(&uffdio_move, user_uffdio_move,
			   /* don't copy "move" last field */
			   sizeof(uffdio_move)-sizeof(__s64)))
		return -EFAULT;

	/* Do not allow cross-mm moves. */
	if (mm != current->mm)
		return -EINVAL;

	ret = validate_range(mm, uffdio_move.dst, uffdio_move.len);
	if (ret)
		return ret;

	ret = validate_range(mm, uffdio_move.src, uffdio_move.len);
	if (ret)
		return ret;

	if (uffdio_move.mode & ~(UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES|
				 UFFDIO_MOVE_MODE_DONTWAKE))
		return -EINVAL;

	if (mmget_not_zero(mm)) {
		ret = move_pages(ctx, uffdio_move.dst, uffdio_move.src,
				 uffdio_move.len, uffdio_move.mode);
		mmput(mm);
	} else {
		return -ESRCH;
	}

	if (unlikely(put_user(ret, &user_uffdio_move->move)))
		return -EFAULT;
	if (ret < 0)
		goto out;

	/* len == 0 would wake all */
	VM_WARN_ON(!ret);
	range.len = ret;
	if (!(uffdio_move.mode & UFFDIO_MOVE_MODE_DONTWAKE)) {
		range.start = uffdio_move.dst;
		wake_userfault(ctx, &range);
	}
	ret = range.len == uffdio_move.len ? 0 : -EAGAIN;

out:
	return ret;
}

/*
 * userland asks for a certain API version and we return which bits
 * and ioctl commands are implemented in this kernel for such API
 * version or -EINVAL if unknown.
 */
static int userfaultfd_api(struct userfaultfd_ctx *ctx,
			   unsigned long arg)
{
	struct uffdio_api uffdio_api;
	void __user *buf = (void __user *)arg;
	unsigned int ctx_features;
	int ret;
	__u64 features;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
		goto out;
	features = uffdio_api.features;
	ret = -EINVAL;
	if (uffdio_api.api != UFFD_API)
		goto err_out;
	ret = -EPERM;
	if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE))
		goto err_out;

	/* WP_ASYNC relies on WP_UNPOPULATED, choose it unconditionally */
	if (features & UFFD_FEATURE_WP_ASYNC)
		features |= UFFD_FEATURE_WP_UNPOPULATED;

	ret = -EINVAL;
	/* RWP_ASYNC requires RWP */
	if ((features & UFFD_FEATURE_RWP_ASYNC) &&
	    !(features & UFFD_FEATURE_RWP))
		goto err_out;

	/* report all available features and ioctls to userland */
	uffdio_api.features = uffd_api_available_features();

	ret = -EINVAL;
	if (features & ~uffdio_api.features)
		goto err_out;

	uffdio_api.ioctls = UFFD_API_IOCTLS;
	ret = -EFAULT;
	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
		goto out;

	/* only enable the requested features for this uffd context */
	ctx_features = uffd_ctx_features(features);
	ret = -EINVAL;
	if (cmpxchg(&ctx->features, 0, ctx_features) != 0)
		goto err_out;

	ret = 0;
out:
	return ret;
err_out:
	memset(&uffdio_api, 0, sizeof(uffdio_api));
	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
		ret = -EFAULT;
	goto out;
}

static long userfaultfd_ioctl(struct file *file, unsigned cmd,
			      unsigned long arg)
{
	int ret = -EINVAL;
	struct userfaultfd_ctx *ctx = file->private_data;

	if (cmd != UFFDIO_API && !userfaultfd_is_initialized(ctx))
		return -EINVAL;

	switch (cmd) {
	case UFFDIO_API:
		ret = userfaultfd_api(ctx, arg);
		break;
	case UFFDIO_REGISTER:
		ret = userfaultfd_register(ctx, arg);
		break;
	case UFFDIO_UNREGISTER:
		ret = userfaultfd_unregister(ctx, arg);
		break;
	case UFFDIO_WAKE:
		ret = userfaultfd_wake(ctx, arg);
		break;
	case UFFDIO_COPY:
		ret = userfaultfd_copy(ctx, arg);
		break;
	case UFFDIO_ZEROPAGE:
		ret = userfaultfd_zeropage(ctx, arg);
		break;
	case UFFDIO_MOVE:
		ret = userfaultfd_move(ctx, arg);
		break;
	case UFFDIO_WRITEPROTECT:
		ret = userfaultfd_writeprotect(ctx, arg);
		break;
	case UFFDIO_CONTINUE:
		ret = userfaultfd_continue(ctx, arg);
		break;
	case UFFDIO_POISON:
		ret = userfaultfd_poison(ctx, arg);
		break;
	case UFFDIO_RWPROTECT:
		ret = userfaultfd_rwprotect(ctx, arg);
		break;
	case UFFDIO_SET_MODE:
		ret = userfaultfd_set_mode(ctx, arg);
		break;
	}
	return ret;
}

#ifdef CONFIG_PROC_FS
static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
{
	struct userfaultfd_ctx *ctx = f->private_data;
	wait_queue_entry_t *wq;
	unsigned long pending = 0, total = 0;

	spin_lock_irq(&ctx->fault_pending_wqh.lock);
	list_for_each_entry(wq, &ctx->fault_pending_wqh.head, entry) {
		pending++;
		total++;
	}
	list_for_each_entry(wq, &ctx->fault_wqh.head, entry) {
		total++;
	}
	spin_unlock_irq(&ctx->fault_pending_wqh.lock);

	/*
	 * If more protocols will be added, there will be all shown
	 * separated by a space. Like this:
	 *	protocols: aa:... bb:...
	 */
	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
		   pending, total, UFFD_API, userfaultfd_features(ctx),
		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
}
#endif

static const struct file_operations userfaultfd_fops = {
#ifdef CONFIG_PROC_FS
	.show_fdinfo	= userfaultfd_show_fdinfo,
#endif
	.release	= userfaultfd_release,
	.poll		= userfaultfd_poll,
	.read_iter	= userfaultfd_read_iter,
	.unlocked_ioctl = userfaultfd_ioctl,
	.compat_ioctl	= compat_ptr_ioctl,
	.llseek		= noop_llseek,
};

static void init_once_userfaultfd_ctx(void *mem)
{
	struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;

	init_waitqueue_head(&ctx->fault_pending_wqh);
	init_waitqueue_head(&ctx->fault_wqh);
	init_waitqueue_head(&ctx->event_wqh);
	init_waitqueue_head(&ctx->fd_wqh);
	seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
}

static int new_userfaultfd(int flags)
{
	struct userfaultfd_ctx *ctx __free(kfree) = NULL;

	VM_WARN_ON_ONCE(!current->mm);

	/* Check the UFFD_* constants for consistency. */
	BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS);

	if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY))
		return -EINVAL;

	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
	if (!ctx)
		return -ENOMEM;

	refcount_set(&ctx->refcount, 1);
	ctx->flags = flags;
	ctx->features = 0;
	ctx->released = false;
	init_rwsem(&ctx->map_changing_lock);
	atomic_set(&ctx->mmap_changing, 0);
	ctx->mm = current->mm;

	FD_PREPARE(fdf, flags & UFFD_SHARED_FCNTL_FLAGS,
		   anon_inode_create_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
					     O_RDONLY | (flags & UFFD_SHARED_FCNTL_FLAGS),
					     NULL));
	if (fdf.err)
		return fdf.err;

	/* prevent the mm struct to be freed */
	mmgrab(ctx->mm);
	fd_prepare_file(fdf)->f_mode |= FMODE_NOWAIT;
	retain_and_null_ptr(ctx);
	return fd_publish(fdf);
}

static inline bool userfaultfd_syscall_allowed(int flags)
{
	/* Userspace-only page faults are always allowed */
	if (flags & UFFD_USER_MODE_ONLY)
		return true;

	/*
	 * The user is requesting a userfaultfd which can handle kernel faults.
	 * Privileged users are always allowed to do this.
	 */
	if (capable(CAP_SYS_PTRACE))
		return true;

	/* Otherwise, access to kernel fault handling is sysctl controlled. */
	return sysctl_unprivileged_userfaultfd;
}

SYSCALL_DEFINE1(userfaultfd, int, flags)
{
	if (!userfaultfd_syscall_allowed(flags))
		return -EPERM;

	return new_userfaultfd(flags);
}

static long userfaultfd_dev_ioctl(struct file *file, unsigned int cmd, unsigned long flags)
{
	if (cmd != USERFAULTFD_IOC_NEW)
		return -EINVAL;

	return new_userfaultfd(flags);
}

static const struct file_operations userfaultfd_dev_fops = {
	.unlocked_ioctl = userfaultfd_dev_ioctl,
	.compat_ioctl = userfaultfd_dev_ioctl,
	.owner = THIS_MODULE,
	.llseek = noop_llseek,
};

static struct miscdevice userfaultfd_misc = {
	.minor = MISC_DYNAMIC_MINOR,
	.name = "userfaultfd",
	.fops = &userfaultfd_dev_fops
};

static int __init userfaultfd_init(void)
{
	int ret;

	ret = misc_register(&userfaultfd_misc);
	if (ret)
		return ret;

	userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
						sizeof(struct userfaultfd_ctx),
						0,
						SLAB_HWCACHE_ALIGN|SLAB_PANIC,
						init_once_userfaultfd_ctx);
#ifdef CONFIG_SYSCTL
	register_sysctl_init("vm", vm_userfaultfd_table);
#endif
	return 0;
}
__initcall(userfaultfd_init);