mbcache.c

// SPDX-License-Identifier: GPL-2.0-only
#include <linux/spinlock.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/list_bl.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/workqueue.h>
#include <linux/mbcache.h>

/*
 * Mbcache is a simple key-value store. Keys need not be unique, however
 * key-value pairs are expected to be unique (we use this fact in
 * mb_cache_entry_delete_or_get()).
 *
 * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
 * Ext4 also uses it for deduplication of xattr values stored in inodes.
 * They use hash of data as a key and provide a value that may represent a
 * block or inode number. That's why keys need not be unique (hash of different
 * data may be the same). However user provided value always uniquely
 * identifies a cache entry.
 *
 * We provide functions for creation and removal of entries, search by key,
 * and a special "delete entry with given key-value pair" operation. Fixed
 * size hash table is used for fast key lookups.
 */

struct mb_cache {
	/* Hash table of entries */
	struct hlist_bl_head	*c_hash;
	/* log2 of hash table size */
	int			c_bucket_bits;
	/* Maximum entries in cache to avoid degrading hash too much */
	unsigned long		c_max_entries;
	/* Protects c_list, c_entry_count */
	spinlock_t		c_list_lock;
	struct list_head	c_list;
	/* Number of entries in cache */
	unsigned long		c_entry_count;
	struct shrinker		c_shrink;
	/* Work for shrinking when the cache has too many entries */
	struct work_struct	c_shrink_work;
};

static struct kmem_cache *mb_entry_cache;

static unsigned long mb_cache_shrink(struct mb_cache *cache,
				     unsigned long nr_to_scan);

static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache,
							u32 key)
{
	return &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
}

/*
 * Number of entries to reclaim synchronously when there are too many entries
 * in cache
 */
#define SYNC_SHRINK_BATCH 64

/*
 * mb_cache_entry_create - create entry in cache
 * @cache - cache where the entry should be created
 * @mask - gfp mask with which the entry should be allocated
 * @key - key of the entry
 * @value - value of the entry
 * @reusable - is the entry reusable by others?
 *
 * Creates entry in @cache with key @key and value @value. The function returns
 * -EBUSY if entry with the same key and value already exists in cache.
 * Otherwise 0 is returned.
 */
int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
			  u64 value, bool reusable)
{
	struct mb_cache_entry *entry, *dup;
	struct hlist_bl_node *dup_node;
	struct hlist_bl_head *head;

	/* Schedule background reclaim if there are too many entries */
	if (cache->c_entry_count >= cache->c_max_entries)
		schedule_work(&cache->c_shrink_work);
	/* Do some sync reclaim if background reclaim cannot keep up */
	if (cache->c_entry_count >= 2*cache->c_max_entries)
		mb_cache_shrink(cache, SYNC_SHRINK_BATCH);

	entry = kmem_cache_alloc(mb_entry_cache, mask);
	if (!entry)
		return -ENOMEM;

	INIT_LIST_HEAD(&entry->e_list);
	/*
	 * We create entry with two references. One reference is kept by the
	 * hash table, the other reference is used to protect us from
	 * mb_cache_entry_delete_or_get() until the entry is fully setup. This
	 * avoids nesting of cache->c_list_lock into hash table bit locks which
	 * is problematic for RT.
	 */
	atomic_set(&entry->e_refcnt, 2);
	entry->e_key = key;
	entry->e_value = value;
	entry->e_flags = 0;
	if (reusable)
		set_bit(MBE_REUSABLE_B, &entry->e_flags);
	head = mb_cache_entry_head(cache, key);
	hlist_bl_lock(head);
	hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
		if (dup->e_key == key && dup->e_value == value) {
			hlist_bl_unlock(head);
			kmem_cache_free(mb_entry_cache, entry);
			return -EBUSY;
		}
	}
	hlist_bl_add_head(&entry->e_hash_list, head);
	hlist_bl_unlock(head);
	spin_lock(&cache->c_list_lock);
	list_add_tail(&entry->e_list, &cache->c_list);
	cache->c_entry_count++;
	spin_unlock(&cache->c_list_lock);
	mb_cache_entry_put(cache, entry);

	return 0;
}
EXPORT_SYMBOL(mb_cache_entry_create);

void __mb_cache_entry_free(struct mb_cache *cache, struct mb_cache_entry *entry)
{
	struct hlist_bl_head *head;

	head = mb_cache_entry_head(cache, entry->e_key);
	hlist_bl_lock(head);
	hlist_bl_del(&entry->e_hash_list);
	hlist_bl_unlock(head);
	kmem_cache_free(mb_entry_cache, entry);
}
EXPORT_SYMBOL(__mb_cache_entry_free);

/*
 * mb_cache_entry_wait_unused - wait to be the last user of the entry
 *
 * @entry - entry to work on
 *
 * Wait to be the last user of the entry.
 */
void mb_cache_entry_wait_unused(struct mb_cache_entry *entry)
{
	wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 2);
}
EXPORT_SYMBOL(mb_cache_entry_wait_unused);

static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
					   struct mb_cache_entry *entry,
					   u32 key)
{
	struct mb_cache_entry *old_entry = entry;
	struct hlist_bl_node *node;
	struct hlist_bl_head *head;

	head = mb_cache_entry_head(cache, key);
	hlist_bl_lock(head);
	if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
		node = entry->e_hash_list.next;
	else
		node = hlist_bl_first(head);
	while (node) {
		entry = hlist_bl_entry(node, struct mb_cache_entry,
				       e_hash_list);
		if (entry->e_key == key &&
		    test_bit(MBE_REUSABLE_B, &entry->e_flags) &&
		    atomic_inc_not_zero(&entry->e_refcnt))
			goto out;
		node = node->next;
	}
	entry = NULL;
out:
	hlist_bl_unlock(head);
	if (old_entry)
		mb_cache_entry_put(cache, old_entry);

	return entry;
}

/*
 * mb_cache_entry_find_first - find the first reusable entry with the given key
 * @cache: cache where we should search
 * @key: key to look for
 *
 * Search in @cache for a reusable entry with key @key. Grabs reference to the
 * first reusable entry found and returns the entry.
 */
struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
						 u32 key)
{
	return __entry_find(cache, NULL, key);
}
EXPORT_SYMBOL(mb_cache_entry_find_first);

/*
 * mb_cache_entry_find_next - find next reusable entry with the same key
 * @cache: cache where we should search
 * @entry: entry to start search from
 *
 * Finds next reusable entry in the hash chain which has the same key as @entry.
 * If @entry is unhashed (which can happen when deletion of entry races with the
 * search), finds the first reusable entry in the hash chain. The function drops
 * reference to @entry and returns with a reference to the found entry.
 */
struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
						struct mb_cache_entry *entry)
{
	return __entry_find(cache, entry, entry->e_key);
}
EXPORT_SYMBOL(mb_cache_entry_find_next);

/*
 * mb_cache_entry_get - get a cache entry by value (and key)
 * @cache - cache we work with
 * @key - key
 * @value - value
 */
struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
					  u64 value)
{
	struct hlist_bl_node *node;
	struct hlist_bl_head *head;
	struct mb_cache_entry *entry;

	head = mb_cache_entry_head(cache, key);
	hlist_bl_lock(head);
	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
		if (entry->e_key == key && entry->e_value == value &&
		    atomic_inc_not_zero(&entry->e_refcnt))
			goto out;
	}
	entry = NULL;
out:
	hlist_bl_unlock(head);
	return entry;
}
EXPORT_SYMBOL(mb_cache_entry_get);

/* mb_cache_entry_delete - try to remove a cache entry
 * @cache - cache we work with
 * @key - key
 * @value - value
 *
 * Remove entry from cache @cache with key @key and value @value.
 */
void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
{
	struct hlist_bl_node *node;
	struct hlist_bl_head *head;
	struct mb_cache_entry *entry;

	head = mb_cache_entry_head(cache, key);
	hlist_bl_lock(head);
	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
		if (entry->e_key == key && entry->e_value == value) {
			/* We keep hash list reference to keep entry alive */
			hlist_bl_del_init(&entry->e_hash_list);
			hlist_bl_unlock(head);
			spin_lock(&cache->c_list_lock);
			if (!list_empty(&entry->e_list)) {
				list_del_init(&entry->e_list);
				if (!WARN_ONCE(cache->c_entry_count == 0,
		"mbcache: attempt to decrement c_entry_count past zero"))
					cache->c_entry_count--;
				atomic_dec(&entry->e_refcnt);
			}
			spin_unlock(&cache->c_list_lock);
			mb_cache_entry_put(cache, entry);
			return;
		}
	}
	hlist_bl_unlock(head);
}
EXPORT_SYMBOL(mb_cache_entry_delete);

/* mb_cache_entry_delete_or_get - remove a cache entry if it has no users
 * @cache - cache we work with
 * @key - key
 * @value - value
 *
 * Remove entry from cache @cache with key @key and value @value. The removal
 * happens only if the entry is unused. The function returns NULL in case the
 * entry was successfully removed or there's no entry in cache. Otherwise the
 * function grabs reference of the entry that we failed to delete because it
 * still has users and return it.
 */
struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,
						    u32 key, u64 value)
{
	struct mb_cache_entry *entry;

	entry = mb_cache_entry_get(cache, key, value);
	if (!entry)
		return NULL;

	/*
	 * Drop the ref we got from mb_cache_entry_get() and the initial hash
	 * ref if we are the last user
	 */
	if (atomic_cmpxchg(&entry->e_refcnt, 2, 0) != 2)
		return entry;

	spin_lock(&cache->c_list_lock);
	if (!list_empty(&entry->e_list))
		list_del_init(&entry->e_list);
	cache->c_entry_count--;
	spin_unlock(&cache->c_list_lock);
	__mb_cache_entry_free(cache, entry);
	return NULL;
}
EXPORT_SYMBOL(mb_cache_entry_delete_or_get);

/* mb_cache_entry_touch - cache entry got used
 * @cache - cache the entry belongs to
 * @entry - entry that got used
 *
 * Marks entry as used to give hit higher chances of surviving in cache.
 */
void mb_cache_entry_touch(struct mb_cache *cache,
			  struct mb_cache_entry *entry)
{
	set_bit(MBE_REFERENCED_B, &entry->e_flags);
}
EXPORT_SYMBOL(mb_cache_entry_touch);

static unsigned long mb_cache_count(struct shrinker *shrink,
				    struct shrink_control *sc)
{
	struct mb_cache *cache = container_of(shrink, struct mb_cache,
					      c_shrink);

	return cache->c_entry_count;
}

/* Shrink number of entries in cache */
static unsigned long mb_cache_shrink(struct mb_cache *cache,
				     unsigned long nr_to_scan)
{
	struct mb_cache_entry *entry;
	unsigned long shrunk = 0;

	spin_lock(&cache->c_list_lock);
	while (nr_to_scan-- && !list_empty(&cache->c_list)) {
		entry = list_first_entry(&cache->c_list,
					 struct mb_cache_entry, e_list);
		/* Drop initial hash reference if there is no user */
		if (test_bit(MBE_REFERENCED_B, &entry->e_flags) ||
		    atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
			clear_bit(MBE_REFERENCED_B, &entry->e_flags);
			list_move_tail(&entry->e_list, &cache->c_list);
			continue;
		}
		list_del_init(&entry->e_list);
		cache->c_entry_count--;
		spin_unlock(&cache->c_list_lock);
		__mb_cache_entry_free(cache, entry);
		shrunk++;
		cond_resched();
		spin_lock(&cache->c_list_lock);
	}
	spin_unlock(&cache->c_list_lock);

	return shrunk;
}

static unsigned long mb_cache_scan(struct shrinker *shrink,
				   struct shrink_control *sc)
{
	struct mb_cache *cache = container_of(shrink, struct mb_cache,
					      c_shrink);
	return mb_cache_shrink(cache, sc->nr_to_scan);
}

/* We shrink 1/X of the cache when we have too many entries in it */
#define SHRINK_DIVISOR 16

static void mb_cache_shrink_worker(struct work_struct *work)
{
	struct mb_cache *cache = container_of(work, struct mb_cache,
					      c_shrink_work);
	mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
}

/*
 * mb_cache_create - create cache
 * @bucket_bits: log2 of the hash table size
 *
 * Create cache for keys with 2^bucket_bits hash entries.
 */
struct mb_cache *mb_cache_create(int bucket_bits)
{
	struct mb_cache *cache;
	unsigned long bucket_count = 1UL << bucket_bits;
	unsigned long i;

	cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
	if (!cache)
		goto err_out;
	cache->c_bucket_bits = bucket_bits;
	cache->c_max_entries = bucket_count << 4;
	INIT_LIST_HEAD(&cache->c_list);
	spin_lock_init(&cache->c_list_lock);
	cache->c_hash = kmalloc_array(bucket_count,
				      sizeof(struct hlist_bl_head),
				      GFP_KERNEL);
	if (!cache->c_hash) {
		kfree(cache);
		goto err_out;
	}
	for (i = 0; i < bucket_count; i++)
		INIT_HLIST_BL_HEAD(&cache->c_hash[i]);

	cache->c_shrink.count_objects = mb_cache_count;
	cache->c_shrink.scan_objects = mb_cache_scan;
	cache->c_shrink.seeks = DEFAULT_SEEKS;
	if (register_shrinker(&cache->c_shrink)) {
		kfree(cache->c_hash);
		kfree(cache);
		goto err_out;
	}

	INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);

	return cache;

err_out:
	return NULL;
}
EXPORT_SYMBOL(mb_cache_create);

/*
 * mb_cache_destroy - destroy cache
 * @cache: the cache to destroy
 *
 * Free all entries in cache and cache itself. Caller must make sure nobody
 * (except shrinker) can reach @cache when calling this.
 */
void mb_cache_destroy(struct mb_cache *cache)
{
	struct mb_cache_entry *entry, *next;

	unregister_shrinker(&cache->c_shrink);

	/*
	 * We don't bother with any locking. Cache must not be used at this
	 * point.
	 */
	list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
		list_del(&entry->e_list);
		WARN_ON(atomic_read(&entry->e_refcnt) != 1);
		mb_cache_entry_put(cache, entry);
	}
	kfree(cache->c_hash);
	kfree(cache);
}
EXPORT_SYMBOL(mb_cache_destroy);

static int __init mbcache_init(void)
{
	mb_entry_cache = kmem_cache_create("mbcache",
				sizeof(struct mb_cache_entry), 0,
				SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
	if (!mb_entry_cache)
		return -ENOMEM;
	return 0;
}

static void __exit mbcache_exit(void)
{
	kmem_cache_destroy(mb_entry_cache);
}

module_init(mbcache_init)
module_exit(mbcache_exit)

MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
MODULE_LICENSE("GPL");