xref: /linux/mm/shrinker.c (revision fab183d632628381b466a41479489541ac0e29a0)

// SPDX-License-Identifier: GPL-2.0
#include <linux/memcontrol.h>
#include <linux/rwsem.h>
#include <linux/shrinker.h>
#include <linux/rculist.h>
#include <trace/events/vmscan.h>

#include "internal.h"

LIST_HEAD(shrinker_list);
DEFINE_MUTEX(shrinker_mutex);

#ifdef CONFIG_MEMCG
static int shrinker_nr_max;

static inline int shrinker_unit_size(int nr_items)
{
	return (DIV_ROUND_UP(nr_items, SHRINKER_UNIT_BITS) * sizeof(struct shrinker_info_unit *));
}

static inline void shrinker_unit_free(struct shrinker_info *info, int start)
{
	struct shrinker_info_unit **unit;
	int nr, i;

	if (!info)
		return;

	unit = info->unit;
	nr = DIV_ROUND_UP(info->map_nr_max, SHRINKER_UNIT_BITS);

	for (i = start; i < nr; i++) {
		if (!unit[i])
			break;

		kfree(unit[i]);
		unit[i] = NULL;
	}
}

static inline int shrinker_unit_alloc(struct shrinker_info *new,
				       struct shrinker_info *old, int nid)
{
	struct shrinker_info_unit *unit;
	int nr = DIV_ROUND_UP(new->map_nr_max, SHRINKER_UNIT_BITS);
	int start = old ? DIV_ROUND_UP(old->map_nr_max, SHRINKER_UNIT_BITS) : 0;
	int i;

	for (i = start; i < nr; i++) {
		unit = kzalloc_node(sizeof(*unit), GFP_KERNEL, nid);
		if (!unit) {
			shrinker_unit_free(new, start);
			return -ENOMEM;
		}

		new->unit[i] = unit;
	}

	return 0;
}

static void __free_shrinker_info(struct mem_cgroup *memcg)
{
	struct mem_cgroup_per_node *pn;
	struct shrinker_info *info;
	int nid;

	lockdep_assert_held(&shrinker_mutex);

	for_each_node(nid) {
		pn = memcg->nodeinfo[nid];
		info = rcu_dereference_protected(pn->shrinker_info, true);
		shrinker_unit_free(info, 0);
		kvfree(info);
		rcu_assign_pointer(pn->shrinker_info, NULL);
	}
}

void free_shrinker_info(struct mem_cgroup *memcg)
{
	mutex_lock(&shrinker_mutex);
	__free_shrinker_info(memcg);
	mutex_unlock(&shrinker_mutex);
}

int alloc_shrinker_info(struct mem_cgroup *memcg)
{
	int nid, ret = 0;
	int array_size = 0;

	mutex_lock(&shrinker_mutex);
	array_size = shrinker_unit_size(shrinker_nr_max);
	for_each_node(nid) {
		struct shrinker_info *info = kvzalloc_node(sizeof(*info) + array_size,
							   GFP_KERNEL, nid);
		if (!info)
			goto err;
		info->map_nr_max = shrinker_nr_max;
		if (shrinker_unit_alloc(info, NULL, nid)) {
			kvfree(info);
			goto err;
		}
		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
	}
	mutex_unlock(&shrinker_mutex);

	return ret;

err:
	__free_shrinker_info(memcg);
	mutex_unlock(&shrinker_mutex);
	return -ENOMEM;
}

static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
						     int nid)
{
	return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
					 lockdep_is_held(&shrinker_mutex));
}

static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_size,
				    int old_size, int new_nr_max)
{
	struct shrinker_info *new, *old;
	struct mem_cgroup_per_node *pn;
	int nid;

	for_each_node(nid) {
		pn = memcg->nodeinfo[nid];
		old = shrinker_info_protected(memcg, nid);
		/* Not yet online memcg */
		if (!old)
			return 0;

		/* Already expanded this shrinker_info */
		if (new_nr_max <= old->map_nr_max)
			continue;

		new = kvzalloc_node(sizeof(*new) + new_size, GFP_KERNEL, nid);
		if (!new)
			return -ENOMEM;

		new->map_nr_max = new_nr_max;

		memcpy(new->unit, old->unit, old_size);
		if (shrinker_unit_alloc(new, old, nid)) {
			kvfree(new);
			return -ENOMEM;
		}

		rcu_assign_pointer(pn->shrinker_info, new);
		kvfree_rcu(old, rcu);
	}

	return 0;
}

static int expand_shrinker_info(int new_id)
{
	int ret = 0;
	int new_nr_max = round_up(new_id + 1, SHRINKER_UNIT_BITS);
	int new_size, old_size = 0;
	struct mem_cgroup *memcg;

	if (!root_mem_cgroup)
		goto out;

	lockdep_assert_held(&shrinker_mutex);

	new_size = shrinker_unit_size(new_nr_max);
	old_size = shrinker_unit_size(shrinker_nr_max);

	memcg = mem_cgroup_iter(NULL, NULL, NULL);
	do {
		ret = expand_one_shrinker_info(memcg, new_size, old_size,
					       new_nr_max);
		if (ret) {
			mem_cgroup_iter_break(NULL, memcg);
			goto out;
		}
	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
out:
	if (!ret)
		shrinker_nr_max = new_nr_max;

	return ret;
}

static inline int shrinker_id_to_index(int shrinker_id)
{
	return shrinker_id / SHRINKER_UNIT_BITS;
}

static inline int shrinker_id_to_offset(int shrinker_id)
{
	return shrinker_id % SHRINKER_UNIT_BITS;
}

static inline int calc_shrinker_id(int index, int offset)
{
	return index * SHRINKER_UNIT_BITS + offset;
}

void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
{
	if (shrinker_id >= 0 && memcg && !mem_cgroup_is_root(memcg)) {
		struct shrinker_info *info;

		rcu_read_lock();
		info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
		if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
			struct shrinker_info_unit *unit;

			unit = info->unit[shrinker_id_to_index(shrinker_id)];
			/* Pairs with smp mb in shrink_slab() */
			smp_mb__before_atomic();
			set_bit(shrinker_id_to_offset(shrinker_id), unit->map);
		}
		rcu_read_unlock();
	}
}

static DEFINE_IDR(shrinker_idr);

static int shrinker_memcg_alloc(struct shrinker *shrinker)
{
	int id;

	if (mem_cgroup_disabled())
		return -ENOSYS;
	if (mem_cgroup_kmem_disabled() && !(shrinker->flags & SHRINKER_NONSLAB))
		return -ENOSYS;

	guard(mutex)(&shrinker_mutex);
	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
	if (id < 0)
		return id;

	if (id >= shrinker_nr_max) {
		if (expand_shrinker_info(id)) {
			idr_remove(&shrinker_idr, id);
			return -ENOMEM;
		}
	}
	shrinker->id = id;
	return 0;
}

static void shrinker_memcg_remove(struct shrinker *shrinker)
{
	int id = shrinker->id;

	BUG_ON(id < 0);

	lockdep_assert_held(&shrinker_mutex);

	idr_remove(&shrinker_idr, id);
}

static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker,
				   struct mem_cgroup *memcg)
{
	struct shrinker_info *info;
	struct shrinker_info_unit *unit;
	long nr_deferred;

	rcu_read_lock();
	info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
	unit = info->unit[shrinker_id_to_index(shrinker->id)];
	nr_deferred = atomic_long_xchg(&unit->nr_deferred[shrinker_id_to_offset(shrinker->id)], 0);
	rcu_read_unlock();

	return nr_deferred;
}

static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
				  struct mem_cgroup *memcg)
{
	struct shrinker_info *info;
	struct shrinker_info_unit *unit;
	long nr_deferred;

	rcu_read_lock();
	info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
	unit = info->unit[shrinker_id_to_index(shrinker->id)];
	nr_deferred =
		atomic_long_add_return(nr, &unit->nr_deferred[shrinker_id_to_offset(shrinker->id)]);
	rcu_read_unlock();

	return nr_deferred;
}

void reparent_shrinker_deferred(struct mem_cgroup *memcg)
{
	int nid, index, offset;
	long nr;
	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
	struct shrinker_info *child_info, *parent_info;
	struct shrinker_info_unit *child_unit, *parent_unit;

	/* Prevent from concurrent shrinker_info expand */
	mutex_lock(&shrinker_mutex);
	for_each_node(nid) {
		child_info = shrinker_info_protected(memcg, nid);
		parent_info = shrinker_info_protected(parent, nid);
		for (index = 0; index < shrinker_id_to_index(child_info->map_nr_max); index++) {
			child_unit = child_info->unit[index];
			parent_unit = parent_info->unit[index];
			for (offset = 0; offset < SHRINKER_UNIT_BITS; offset++) {
				nr = atomic_long_read(&child_unit->nr_deferred[offset]);
				atomic_long_add(nr, &parent_unit->nr_deferred[offset]);
			}
		}
	}
	mutex_unlock(&shrinker_mutex);
}
#else
static int shrinker_memcg_alloc(struct shrinker *shrinker)
{
	return -ENOSYS;
}

static void shrinker_memcg_remove(struct shrinker *shrinker)
{
}

static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker,
				   struct mem_cgroup *memcg)
{
	return 0;
}

static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
				  struct mem_cgroup *memcg)
{
	return 0;
}
#endif /* CONFIG_MEMCG */

static long xchg_nr_deferred(struct shrinker *shrinker,
			     struct shrink_control *sc)
{
	int nid = sc->nid;

	if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
		nid = 0;

	if (sc->memcg &&
	    (shrinker->flags & SHRINKER_MEMCG_AWARE))
		return xchg_nr_deferred_memcg(nid, shrinker,
					      sc->memcg);

	return atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
}


static long add_nr_deferred(long nr, struct shrinker *shrinker,
			    struct shrink_control *sc)
{
	int nid = sc->nid;

	if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
		nid = 0;

	if (sc->memcg &&
	    (shrinker->flags & SHRINKER_MEMCG_AWARE))
		return add_nr_deferred_memcg(nr, nid, shrinker,
					     sc->memcg);

	return atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
}

#define SHRINK_BATCH 128

static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
				    struct shrinker *shrinker, int priority)
{
	unsigned long freed = 0;
	unsigned long long delta;
	long total_scan;
	long freeable;
	long nr;
	long new_nr;
	long batch_size = shrinker->batch ? shrinker->batch
					  : SHRINK_BATCH;
	long scanned = 0, next_deferred;

	freeable = shrinker->count_objects(shrinker, shrinkctl);
	if (freeable == 0 || freeable == SHRINK_EMPTY)
		return freeable;

	/*
	 * copy the current shrinker scan count into a local variable
	 * and zero it so that other concurrent shrinker invocations
	 * don't also do this scanning work.
	 */
	nr = xchg_nr_deferred(shrinker, shrinkctl);

	if (shrinker->seeks) {
		delta = freeable >> priority;
		delta *= 4;
		do_div(delta, shrinker->seeks);
	} else {
		/*
		 * These objects don't require any IO to create. Trim
		 * them aggressively under memory pressure to keep
		 * them from causing refetches in the IO caches.
		 */
		delta = freeable / 2;
	}

	total_scan = nr >> priority;
	total_scan += delta;
	total_scan = min(total_scan, (2 * freeable));

	trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
				   freeable, delta, total_scan, priority,
				   shrinkctl->memcg);

	/*
	 * Normally, we should not scan less than batch_size objects in one
	 * pass to avoid too frequent shrinker calls, but if the slab has less
	 * than batch_size objects in total and we are really tight on memory,
	 * we will try to reclaim all available objects, otherwise we can end
	 * up failing allocations although there are plenty of reclaimable
	 * objects spread over several slabs with usage less than the
	 * batch_size.
	 *
	 * We detect the "tight on memory" situations by looking at the total
	 * number of objects we want to scan (total_scan). If it is greater
	 * than the total number of objects on slab (freeable), we must be
	 * scanning at high prio and therefore should try to reclaim as much as
	 * possible.
	 */
	while (total_scan >= batch_size ||
	       total_scan >= freeable) {
		unsigned long ret;
		unsigned long nr_to_scan = min(batch_size, total_scan);

		shrinkctl->nr_to_scan = nr_to_scan;
		shrinkctl->nr_scanned = nr_to_scan;
		ret = shrinker->scan_objects(shrinker, shrinkctl);
		if (ret == SHRINK_STOP)
			break;
		freed += ret;

		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
		total_scan -= shrinkctl->nr_scanned;
		scanned += shrinkctl->nr_scanned;

		cond_resched();
	}

	/*
	 * The deferred work is increased by any new work (delta) that wasn't
	 * done, decreased by old deferred work that was done now.
	 *
	 * And it is capped to two times of the freeable items.
	 */
	next_deferred = max_t(long, (nr + delta - scanned), 0);
	next_deferred = min(next_deferred, (2 * freeable));

	/*
	 * move the unused scan count back into the shrinker in a
	 * manner that handles concurrent updates.
	 */
	new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);

	trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan,
				 shrinkctl->memcg);
	return freed;
}

#ifdef CONFIG_MEMCG
static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
			struct mem_cgroup *memcg, int priority)
{
	struct shrinker_info *info;
	unsigned long ret, freed = 0;
	int offset, index = 0;

	if (!mem_cgroup_online(memcg))
		return 0;

	/*
	 * lockless algorithm of memcg shrink.
	 *
	 * The shrinker_info may be freed asynchronously via RCU in the
	 * expand_one_shrinker_info(), so the rcu_read_lock() needs to be used
	 * to ensure the existence of the shrinker_info.
	 *
	 * The shrinker_info_unit is never freed unless its corresponding memcg
	 * is destroyed. Here we already hold the refcount of memcg, so the
	 * memcg will not be destroyed, and of course shrinker_info_unit will
	 * not be freed.
	 *
	 * So in the memcg shrink:
	 *  step 1: use rcu_read_lock() to guarantee existence of the
	 *          shrinker_info.
	 *  step 2: after getting shrinker_info_unit we can safely release the
	 *          RCU lock.
	 *  step 3: traverse the bitmap and calculate shrinker_id
	 *  step 4: use rcu_read_lock() to guarantee existence of the shrinker.
	 *  step 5: use shrinker_id to find the shrinker, then use
	 *          shrinker_try_get() to guarantee existence of the shrinker,
	 *          then we can release the RCU lock to do do_shrink_slab() that
	 *          may sleep.
	 *  step 6: do shrinker_put() paired with step 5 to put the refcount,
	 *          if the refcount reaches 0, then wake up the waiter in
	 *          shrinker_free() by calling complete().
	 *          Note: here is different from the global shrink, we don't
	 *                need to acquire the RCU lock to guarantee existence of
	 *                the shrinker, because we don't need to use this
	 *                shrinker to traverse the next shrinker in the bitmap.
	 *  step 7: we have already exited the read-side of rcu critical section
	 *          before calling do_shrink_slab(), the shrinker_info may be
	 *          released in expand_one_shrinker_info(), so go back to step 1
	 *          to reacquire the shrinker_info.
	 */
again:
	rcu_read_lock();
	info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
	if (unlikely(!info))
		goto unlock;

	if (index < shrinker_id_to_index(info->map_nr_max)) {
		struct shrinker_info_unit *unit;

		unit = info->unit[index];

		rcu_read_unlock();

		for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) {
			struct shrink_control sc = {
				.gfp_mask = gfp_mask,
				.nid = nid,
				.memcg = memcg,
			};
			struct shrinker *shrinker;
			int shrinker_id = calc_shrinker_id(index, offset);

			rcu_read_lock();
			shrinker = idr_find(&shrinker_idr, shrinker_id);
			if (unlikely(!shrinker || !shrinker_try_get(shrinker))) {
				clear_bit(offset, unit->map);
				rcu_read_unlock();
				continue;
			}
			rcu_read_unlock();

			/* Call non-slab shrinkers even though kmem is disabled */
			if (!memcg_kmem_online() &&
			    !(shrinker->flags & SHRINKER_NONSLAB)) {
				clear_bit(offset, unit->map);
				shrinker_put(shrinker);
				continue;
			}

			ret = do_shrink_slab(&sc, shrinker, priority);
			if (ret == SHRINK_EMPTY) {
				clear_bit(offset, unit->map);
				/*
				 * After the shrinker reported that it had no objects to
				 * free, but before we cleared the corresponding bit in
				 * the memcg shrinker map, a new object might have been
				 * added. To make sure, we have the bit set in this
				 * case, we invoke the shrinker one more time and reset
				 * the bit if it reports that it is not empty anymore.
				 * The memory barrier here pairs with the barrier in
				 * set_shrinker_bit():
				 *
				 * list_lru_add()     shrink_slab_memcg()
				 *   list_add_tail()    clear_bit()
				 *   <MB>               <MB>
				 *   set_bit()          do_shrink_slab()
				 */
				smp_mb__after_atomic();
				ret = do_shrink_slab(&sc, shrinker, priority);
				if (ret == SHRINK_EMPTY)
					ret = 0;
				else
					set_shrinker_bit(memcg, nid, shrinker_id);
			}
			freed += ret;
			shrinker_put(shrinker);
		}

		index++;
		goto again;
	}
unlock:
	rcu_read_unlock();
	return freed;
}
#else /* !CONFIG_MEMCG */
static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
			struct mem_cgroup *memcg, int priority)
{
	return 0;
}
#endif /* CONFIG_MEMCG */

/**
 * shrink_slab - shrink slab caches
 * @gfp_mask: allocation context
 * @nid: node whose slab caches to target
 * @memcg: memory cgroup whose slab caches to target
 * @priority: the reclaim priority
 *
 * Call the shrink functions to age shrinkable caches.
 *
 * @nid is passed along to shrinkers with SHRINKER_NUMA_AWARE set,
 * unaware shrinkers will receive a node id of 0 instead.
 *
 * @memcg specifies the memory cgroup to target. Unaware shrinkers
 * are called only if it is the root cgroup.
 *
 * @priority is sc->priority, we take the number of objects and >> by priority
 * in order to get the scan target.
 *
 * Returns the number of reclaimed slab objects.
 */
unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
			  int priority)
{
	unsigned long ret, freed = 0;
	struct shrinker *shrinker;

	/*
	 * The root memcg might be allocated even though memcg is disabled
	 * via "cgroup_disable=memory" boot parameter.  This could make
	 * mem_cgroup_is_root() return false, then just run memcg slab
	 * shrink, but skip global shrink.  This may result in premature
	 * oom.
	 */
	if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
		return shrink_slab_memcg(gfp_mask, nid, memcg, priority);

	/*
	 * lockless algorithm of global shrink.
	 *
	 * In the unregistration setp, the shrinker will be freed asynchronously
	 * via RCU after its refcount reaches 0. So both rcu_read_lock() and
	 * shrinker_try_get() can be used to ensure the existence of the shrinker.
	 *
	 * So in the global shrink:
	 *  step 1: use rcu_read_lock() to guarantee existence of the shrinker
	 *          and the validity of the shrinker_list walk.
	 *  step 2: use shrinker_try_get() to try get the refcount, if successful,
	 *          then the existence of the shrinker can also be guaranteed,
	 *          so we can release the RCU lock to do do_shrink_slab() that
	 *          may sleep.
	 *  step 3: *MUST* to reacquire the RCU lock before calling shrinker_put(),
	 *          which ensures that neither this shrinker nor the next shrinker
	 *          will be freed in the next traversal operation.
	 *  step 4: do shrinker_put() paired with step 2 to put the refcount,
	 *          if the refcount reaches 0, then wake up the waiter in
	 *          shrinker_free() by calling complete().
	 */
	rcu_read_lock();
	list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
		struct shrink_control sc = {
			.gfp_mask = gfp_mask,
			.nid = nid,
			.memcg = memcg,
		};

		if (!shrinker_try_get(shrinker))
			continue;

		rcu_read_unlock();

		ret = do_shrink_slab(&sc, shrinker, priority);
		if (ret == SHRINK_EMPTY)
			ret = 0;
		freed += ret;

		rcu_read_lock();
		shrinker_put(shrinker);
	}

	rcu_read_unlock();
	cond_resched();
	return freed;
}

struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...)
{
	struct shrinker *shrinker;
	unsigned int size;
	va_list ap;
	int err;

	shrinker = kzalloc_obj(struct shrinker);
	if (!shrinker)
		return NULL;

	va_start(ap, fmt);
	err = shrinker_debugfs_name_alloc(shrinker, fmt, ap);
	va_end(ap);
	if (err)
		goto err_name;

	shrinker->flags = flags | SHRINKER_ALLOCATED;
	shrinker->seeks = DEFAULT_SEEKS;

	if (flags & SHRINKER_MEMCG_AWARE) {
		err = shrinker_memcg_alloc(shrinker);
		if (err == -ENOSYS) {
			/* Memcg is not supported, fallback to non-memcg-aware shrinker. */
			shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
			goto non_memcg;
		}

		if (err)
			goto err_flags;

		return shrinker;
	}

non_memcg:
	/*
	 * The nr_deferred is available on per memcg level for memcg aware
	 * shrinkers, so only allocate nr_deferred in the following cases:
	 *  - non-memcg-aware shrinkers
	 *  - !CONFIG_MEMCG
	 *  - memcg is disabled by kernel command line
	 *  - non-slab shrinkers: when memcg kmem is disabled
	 */
	size = sizeof(*shrinker->nr_deferred);
	if (flags & SHRINKER_NUMA_AWARE)
		size *= nr_node_ids;

	shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
	if (!shrinker->nr_deferred)
		goto err_flags;

	return shrinker;

err_flags:
	shrinker_debugfs_name_free(shrinker);
err_name:
	kfree(shrinker);
	return NULL;
}
EXPORT_SYMBOL_GPL(shrinker_alloc);

void shrinker_register(struct shrinker *shrinker)
{
	if (unlikely(!(shrinker->flags & SHRINKER_ALLOCATED))) {
		pr_warn("Must use shrinker_alloc() to dynamically allocate the shrinker");
		return;
	}

	mutex_lock(&shrinker_mutex);
	list_add_tail_rcu(&shrinker->list, &shrinker_list);
	shrinker->flags |= SHRINKER_REGISTERED;
	shrinker_debugfs_add(shrinker);
	mutex_unlock(&shrinker_mutex);

	init_completion(&shrinker->done);
	/*
	 * Now the shrinker is fully set up, take the first reference to it to
	 * indicate that lookup operations are now allowed to use it via
	 * shrinker_try_get().
	 */
	refcount_set(&shrinker->refcount, 1);
}
EXPORT_SYMBOL_GPL(shrinker_register);

static void shrinker_free_rcu_cb(struct rcu_head *head)
{
	struct shrinker *shrinker = container_of(head, struct shrinker, rcu);

	kfree(shrinker->nr_deferred);
	kfree(shrinker);
}

void shrinker_free(struct shrinker *shrinker)
{
	struct dentry *debugfs_entry = NULL;
	int debugfs_id;

	if (!shrinker)
		return;

	if (shrinker->flags & SHRINKER_REGISTERED) {
		/* drop the initial refcount */
		shrinker_put(shrinker);
		/*
		 * Wait for all lookups of the shrinker to complete, after that,
		 * no shrinker is running or will run again, then we can safely
		 * free it asynchronously via RCU and safely free the structure
		 * where the shrinker is located, such as super_block etc.
		 */
		wait_for_completion(&shrinker->done);
	}

	mutex_lock(&shrinker_mutex);
	if (shrinker->flags & SHRINKER_REGISTERED) {
		/*
		 * Now we can safely remove it from the shrinker_list and then
		 * free it.
		 */
		list_del_rcu(&shrinker->list);
		debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
		shrinker->flags &= ~SHRINKER_REGISTERED;
	}

	shrinker_debugfs_name_free(shrinker);

	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
		shrinker_memcg_remove(shrinker);
	mutex_unlock(&shrinker_mutex);

	if (debugfs_entry)
		shrinker_debugfs_remove(debugfs_entry, debugfs_id);

	call_rcu(&shrinker->rcu, shrinker_free_rcu_cb);
}
EXPORT_SYMBOL_GPL(shrinker_free);