linux/mm/shrinker.c

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

#include "internal.h"

LIST_HEAD(shrinker_list);
DECLARE_RWSEM(shrinker_rwsem);

#ifdef CONFIG_MEMCG
static int shrinker_nr_max;

/* The shrinker_info is expanded in a batch of BITS_PER_LONG */
static inline int shrinker_map_size(int nr_items)
{
	return (DIV_ROUND_UP(nr_items, BITS_PER_LONG) * sizeof(unsigned long));
}

static inline int shrinker_defer_size(int nr_items)
{
	return (round_up(nr_items, BITS_PER_LONG) * sizeof(atomic_long_t));
}

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

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

int alloc_shrinker_info(struct mem_cgroup *memcg)
{
	struct shrinker_info *info;
	int nid, size, ret = 0;
	int map_size, defer_size = 0;

	down_write(&shrinker_rwsem);
	map_size = shrinker_map_size(shrinker_nr_max);
	defer_size = shrinker_defer_size(shrinker_nr_max);
	size = map_size + defer_size;
	for_each_node(nid) {
		info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
		if (!info) {
			free_shrinker_info(memcg);
			ret = -ENOMEM;
			break;
		}
		info->nr_deferred = (atomic_long_t *)(info + 1);
		info->map = (void *)info->nr_deferred + defer_size;
		info->map_nr_max = shrinker_nr_max;
		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
	}
	up_write(&shrinker_rwsem);

	return ret;
}

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_rwsem));
}

static int expand_one_shrinker_info(struct mem_cgroup *memcg,
				    int map_size, int defer_size,
				    int old_map_size, int old_defer_size,
				    int new_nr_max)
{
	struct shrinker_info *new, *old;
	struct mem_cgroup_per_node *pn;
	int nid;
	int size = map_size + defer_size;

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

		new->nr_deferred = (atomic_long_t *)(new + 1);
		new->map = (void *)new->nr_deferred + defer_size;
		new->map_nr_max = new_nr_max;

		/* map: set all old bits, clear all new bits */
		memset(new->map, (int)0xff, old_map_size);
		memset((void *)new->map + old_map_size, 0, map_size - old_map_size);
		/* nr_deferred: copy old values, clear all new values */
		memcpy(new->nr_deferred, old->nr_deferred, old_defer_size);
		memset((void *)new->nr_deferred + old_defer_size, 0,
		       defer_size - old_defer_size);

		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, BITS_PER_LONG);
	int map_size, defer_size = 0;
	int old_map_size, old_defer_size = 0;
	struct mem_cgroup *memcg;

	if (!root_mem_cgroup)
		goto out;

	lockdep_assert_held(&shrinker_rwsem);

	map_size = shrinker_map_size(new_nr_max);
	defer_size = shrinker_defer_size(new_nr_max);
	old_map_size = shrinker_map_size(shrinker_nr_max);
	old_defer_size = shrinker_defer_size(shrinker_nr_max);

	memcg = mem_cgroup_iter(NULL, NULL, NULL);
	do {
		ret = expand_one_shrinker_info(memcg, map_size, defer_size,
					       old_map_size, old_defer_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;
}

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)) {
			/* Pairs with smp mb in shrink_slab() */
			smp_mb__before_atomic();
			set_bit(shrinker_id, info->map);
		}
		rcu_read_unlock();
	}
}

static DEFINE_IDR(shrinker_idr);

static int prealloc_memcg_shrinker(struct shrinker *shrinker)
{
	int id, ret = -ENOMEM;

	if (mem_cgroup_disabled())
		return -ENOSYS;

	down_write(&shrinker_rwsem);
	/* This may call shrinker, so it must use down_read_trylock() */
	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
	if (id < 0)
		goto unlock;

	if (id >= shrinker_nr_max) {
		if (expand_shrinker_info(id)) {
			idr_remove(&shrinker_idr, id);
			goto unlock;
		}
	}
	shrinker->id = id;
	ret = 0;
unlock:
	up_write(&shrinker_rwsem);
	return ret;
}

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

	BUG_ON(id < 0);

	lockdep_assert_held(&shrinker_rwsem);

	idr_remove(&shrinker_idr, id);
}

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

	info = shrinker_info_protected(memcg, nid);
	return atomic_long_xchg(&info->nr_deferred[shrinker->id], 0);
}

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

	info = shrinker_info_protected(memcg, nid);
	return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
}

void reparent_shrinker_deferred(struct mem_cgroup *memcg)
{
	int i, nid;
	long nr;
	struct mem_cgroup *parent;
	struct shrinker_info *child_info, *parent_info;

	parent = parent_mem_cgroup(memcg);
	if (!parent)
		parent = root_mem_cgroup;

	/* Prevent from concurrent shrinker_info expand */
	down_read(&shrinker_rwsem);
	for_each_node(nid) {
		child_info = shrinker_info_protected(memcg, nid);
		parent_info = shrinker_info_protected(parent, nid);
		for (i = 0; i < child_info->map_nr_max; i++) {
			nr = atomic_long_read(&child_info->nr_deferred[i]);
			atomic_long_add(nr, &parent_info->nr_deferred[i]);
		}
	}
	up_read(&shrinker_rwsem);
}
#else
static int prealloc_memcg_shrinker(struct shrinker *shrinker)
{
	return -ENOSYS;
}

static void unregister_memcg_shrinker(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);

	/*
	 * 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);
	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 i;

	if (!mem_cgroup_online(memcg))
		return 0;

	if (!down_read_trylock(&shrinker_rwsem))
		return 0;

	info = shrinker_info_protected(memcg, nid);
	if (unlikely(!info))
		goto unlock;

	for_each_set_bit(i, info->map, info->map_nr_max) {
		struct shrink_control sc = {
			.gfp_mask = gfp_mask,
			.nid = nid,
			.memcg = memcg,
		};
		struct shrinker *shrinker;

		shrinker = idr_find(&shrinker_idr, i);
		if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) {
			if (!shrinker)
				clear_bit(i, info->map);
			continue;
		}

		/* Call non-slab shrinkers even though kmem is disabled */
		if (!memcg_kmem_online() &&
		    !(shrinker->flags & SHRINKER_NONSLAB))
			continue;

		ret = do_shrink_slab(&sc, shrinker, priority);
		if (ret == SHRINK_EMPTY) {
			clear_bit(i, info->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, i);
		}
		freed += ret;

		if (rwsem_is_contended(&shrinker_rwsem)) {
			freed = freed ? : 1;
			break;
		}
	}
unlock:
	up_read(&shrinker_rwsem);
	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);

	if (!down_read_trylock(&shrinker_rwsem))
		goto out;

	list_for_each_entry(shrinker, &shrinker_list, list) {
		struct shrink_control sc = {
			.gfp_mask = gfp_mask,
			.nid = nid,
			.memcg = memcg,
		};

		ret = do_shrink_slab(&sc, shrinker, priority);
		if (ret == SHRINK_EMPTY)
			ret = 0;
		freed += ret;
		/*
		 * Bail out if someone want to register a new shrinker to
		 * prevent the registration from being stalled for long periods
		 * by parallel ongoing shrinking.
		 */
		if (rwsem_is_contended(&shrinker_rwsem)) {
			freed = freed ? : 1;
			break;
		}
	}

	up_read(&shrinker_rwsem);
out:
	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(sizeof(struct shrinker), GFP_KERNEL);
	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 = prealloc_memcg_shrinker(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
	 */
	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;
	}

	down_write(&shrinker_rwsem);
	list_add_tail(&shrinker->list, &shrinker_list);
	shrinker->flags |= SHRINKER_REGISTERED;
	shrinker_debugfs_add(shrinker);
	up_write(&shrinker_rwsem);
}
EXPORT_SYMBOL_GPL(shrinker_register);

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

	if (!shrinker)
		return;

	down_write(&shrinker_rwsem);
	if (shrinker->flags & SHRINKER_REGISTERED) {
		list_del(&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)
		unregister_memcg_shrinker(shrinker);
	up_write(&shrinker_rwsem);

	if (debugfs_entry)
		shrinker_debugfs_remove(debugfs_entry, debugfs_id);

	kfree(shrinker->nr_deferred);
	shrinker->nr_deferred = NULL;

	kfree(shrinker);
}
EXPORT_SYMBOL_GPL(shrinker_free);