mbcache.c 11.7 KB
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#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>
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#include <linux/workqueue.h>
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#include <linux/mbcache.h>
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/*
 * 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
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 * mb_cache_entry_delete_block()).
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 *
 * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
 * They use hash of a block contents as a key and block number as a value.
 * That's why keys need not be unique (different xattr blocks may end up having
 * the same hash). However block number 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.
 */

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

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static struct kmem_cache *mb_entry_cache;
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static unsigned long mb_cache_shrink(struct mb_cache *cache,
				     unsigned int nr_to_scan);
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static inline bool mb_cache_entry_referenced(struct mb_cache_entry *entry)
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{
	return entry->_e_hash_list_head & 1;
}

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static inline void mb_cache_entry_set_referenced(struct mb_cache_entry *entry)
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{
	entry->_e_hash_list_head |= 1;
}

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static inline void mb_cache_entry_clear_referenced(
					struct mb_cache_entry *entry)
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{
	entry->_e_hash_list_head &= ~1;
}

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static inline struct hlist_bl_head *mb_cache_entry_head(
					struct mb_cache_entry *entry)
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{
	return (struct hlist_bl_head *)
			(entry->_e_hash_list_head & ~1);
}

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

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/*
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 * mb_cache_entry_create - create entry in cache
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 * @cache - cache where the entry should be created
 * @mask - gfp mask with which the entry should be allocated
 * @key - key of the entry
 * @block - block that contains data
 *
 * Creates entry in @cache with key @key and records that data is stored in
 * block @block. The function returns -EBUSY if entry with the same key
 * and for the same block already exists in cache. Otherwise 0 is returned.
 */
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int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
			  sector_t block)
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{
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	struct mb_cache_entry *entry, *dup;
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	struct hlist_bl_node *dup_node;
	struct hlist_bl_head *head;

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	/* 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)
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		mb_cache_shrink(cache, SYNC_SHRINK_BATCH);
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	entry = kmem_cache_alloc(mb_entry_cache, mask);
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	if (!entry)
		return -ENOMEM;

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	INIT_LIST_HEAD(&entry->e_list);
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	/* One ref for hash, one ref returned */
	atomic_set(&entry->e_refcnt, 1);
	entry->e_key = key;
	entry->e_block = block;
	head = &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
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	entry->_e_hash_list_head = (unsigned long)head;
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	hlist_bl_lock(head);
	hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
		if (dup->e_key == key && dup->e_block == block) {
			hlist_bl_unlock(head);
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			kmem_cache_free(mb_entry_cache, entry);
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			return -EBUSY;
		}
	}
	hlist_bl_add_head(&entry->e_hash_list, head);
	hlist_bl_unlock(head);

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	spin_lock(&cache->c_list_lock);
	list_add_tail(&entry->e_list, &cache->c_list);
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	/* Grab ref for LRU list */
	atomic_inc(&entry->e_refcnt);
	cache->c_entry_count++;
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	spin_unlock(&cache->c_list_lock);
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	return 0;
}
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EXPORT_SYMBOL(mb_cache_entry_create);
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void __mb_cache_entry_free(struct mb_cache_entry *entry)
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{
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	kmem_cache_free(mb_entry_cache, entry);
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}
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EXPORT_SYMBOL(__mb_cache_entry_free);
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static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
					   struct mb_cache_entry *entry,
					   u32 key)
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{
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	struct mb_cache_entry *old_entry = entry;
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	struct hlist_bl_node *node;
	struct hlist_bl_head *head;

	if (entry)
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		head = mb_cache_entry_head(entry);
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	else
		head = &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
	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) {
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		entry = hlist_bl_entry(node, struct mb_cache_entry,
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				       e_hash_list);
		if (entry->e_key == key) {
			atomic_inc(&entry->e_refcnt);
			goto out;
		}
		node = node->next;
	}
	entry = NULL;
out:
	hlist_bl_unlock(head);
	if (old_entry)
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		mb_cache_entry_put(cache, old_entry);
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	return entry;
}

/*
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 * mb_cache_entry_find_first - find the first entry in cache with given key
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 * @cache: cache where we should search
 * @key: key to look for
 *
 * Search in @cache for entry with key @key. Grabs reference to the first
 * entry found and returns the entry.
 */
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struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
						 u32 key)
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{
	return __entry_find(cache, NULL, key);
}
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EXPORT_SYMBOL(mb_cache_entry_find_first);
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/*
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 * mb_cache_entry_find_next - find next entry in cache with the same
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 * @cache: cache where we should search
 * @entry: entry to start search from
 *
 * Finds next 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 entry in the hash chain. The function
 * drops reference to @entry and returns with a reference to the found entry.
 */
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struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
						struct mb_cache_entry *entry)
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{
	return __entry_find(cache, entry, entry->e_key);
}
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EXPORT_SYMBOL(mb_cache_entry_find_next);
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/* mb_cache_entry_delete_block - remove information about block from cache
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 * @cache - cache we work with
 * @key - key of the entry to remove
 * @block - block containing data for @key
 *
 * Remove entry from cache @cache with key @key with data stored in @block.
 */
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void mb_cache_entry_delete_block(struct mb_cache *cache, u32 key,
				 sector_t block)
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{
	struct hlist_bl_node *node;
	struct hlist_bl_head *head;
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	struct mb_cache_entry *entry;
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	head = &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
	hlist_bl_lock(head);
	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
		if (entry->e_key == key && entry->e_block == block) {
			/* We keep hash list reference to keep entry alive */
			hlist_bl_del_init(&entry->e_hash_list);
			hlist_bl_unlock(head);
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			spin_lock(&cache->c_list_lock);
			if (!list_empty(&entry->e_list)) {
				list_del_init(&entry->e_list);
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				cache->c_entry_count--;
				atomic_dec(&entry->e_refcnt);
			}
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			spin_unlock(&cache->c_list_lock);
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			mb_cache_entry_put(cache, entry);
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			return;
		}
	}
	hlist_bl_unlock(head);
}
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EXPORT_SYMBOL(mb_cache_entry_delete_block);
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/* mb_cache_entry_touch - cache entry got used
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 * @cache - cache the entry belongs to
 * @entry - entry that got used
 *
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 * Marks entry as used to give hit higher chances of surviving in cache.
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 */
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void mb_cache_entry_touch(struct mb_cache *cache,
			  struct mb_cache_entry *entry)
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{
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	mb_cache_entry_set_referenced(entry);
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}
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EXPORT_SYMBOL(mb_cache_entry_touch);
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static unsigned long mb_cache_count(struct shrinker *shrink,
				    struct shrink_control *sc)
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{
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	struct mb_cache *cache = container_of(shrink, struct mb_cache,
					      c_shrink);
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	return cache->c_entry_count;
}

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

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	spin_lock(&cache->c_list_lock);
	while (nr_to_scan-- && !list_empty(&cache->c_list)) {
		entry = list_first_entry(&cache->c_list,
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					 struct mb_cache_entry, e_list);
		if (mb_cache_entry_referenced(entry)) {
			mb_cache_entry_clear_referenced(entry);
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			list_move_tail(&cache->c_list, &entry->e_list);
			continue;
		}
		list_del_init(&entry->e_list);
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		cache->c_entry_count--;
		/*
		 * We keep LRU list reference so that entry doesn't go away
		 * from under us.
		 */
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		spin_unlock(&cache->c_list_lock);
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		head = mb_cache_entry_head(entry);
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		hlist_bl_lock(head);
		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
			hlist_bl_del_init(&entry->e_hash_list);
			atomic_dec(&entry->e_refcnt);
		}
		hlist_bl_unlock(head);
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		if (mb_cache_entry_put(cache, entry))
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			shrunk++;
		cond_resched();
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		spin_lock(&cache->c_list_lock);
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	}
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	spin_unlock(&cache->c_list_lock);
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	return shrunk;
}

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

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

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

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

	if (!try_module_get(THIS_MODULE))
		return NULL;

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	cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
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	if (!cache)
		goto err_out;
	cache->c_bucket_bits = bucket_bits;
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	cache->c_max_entries = bucket_count << 4;
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	INIT_LIST_HEAD(&cache->c_list);
	spin_lock_init(&cache->c_list_lock);
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	cache->c_hash = kmalloc(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]);

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	cache->c_shrink.count_objects = mb_cache_count;
	cache->c_shrink.scan_objects = mb_cache_scan;
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	cache->c_shrink.seeks = DEFAULT_SEEKS;
	register_shrinker(&cache->c_shrink);

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	INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
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	return cache;

err_out:
	module_put(THIS_MODULE);
	return NULL;
}
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EXPORT_SYMBOL(mb_cache_create);
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/*
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 * mb_cache_destroy - destroy cache
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 * @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.
 */
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void mb_cache_destroy(struct mb_cache *cache)
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{
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	struct mb_cache_entry *entry, *next;
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	unregister_shrinker(&cache->c_shrink);

	/*
	 * We don't bother with any locking. Cache must not be used at this
	 * point.
	 */
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	list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
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		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
			hlist_bl_del_init(&entry->e_hash_list);
			atomic_dec(&entry->e_refcnt);
		} else
			WARN_ON(1);
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		list_del(&entry->e_list);
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		WARN_ON(atomic_read(&entry->e_refcnt) != 1);
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		mb_cache_entry_put(cache, entry);
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	}
	kfree(cache->c_hash);
	kfree(cache);
	module_put(THIS_MODULE);
}
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EXPORT_SYMBOL(mb_cache_destroy);
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static int __init mbcache_init(void)
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{
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	mb_entry_cache = kmem_cache_create("mbcache",
				sizeof(struct mb_cache_entry), 0,
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				SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
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	BUG_ON(!mb_entry_cache);
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	return 0;
}

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

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