xref: /linux/mm/userfaultfd.c (revision e2683c8868d03382da7e1ce8453b543a043066d1)

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  mm/userfaultfd.c
 *
 *  Copyright (C) 2015  Red Hat, Inc.
 */

#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 <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 ? vma->vm_ops->uffd_ops : NULL;
}

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_wp(_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;
}

static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio)
{
	unsigned long src_addr = state->src_addr;
	void *kaddr;
	int err;

	/* retry copying with mm_lock dropped */
	mfill_put_vma(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(state);
	if (err)
		return err;

	err = mfill_establish_pmd(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;

	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;
}

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));
}

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));
}

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));
}

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;
}

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;
}


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_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);
		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_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);
	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));
	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.
 */
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)
				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;
}

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)
		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);
}

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

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;

	/* Reset ptes for the whole vma range if wr-protected */
	if (userfaultfd_wp(vma))
		uffd_wp_range(vma, start, end - start, false);

	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. */
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;

		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;
}

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);
}

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);
}