// 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_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 = shrink->private_data;
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 = shrink->private_data;
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 = shrinker_alloc(0, "mbcache-shrinker");
if (!cache->c_shrink) {
kfree(cache->c_hash);
kfree(cache);
goto err_out;
}
cache->c_shrink->count_objects = mb_cache_count;
cache->c_shrink->scan_objects = mb_cache_scan;
cache->c_shrink->private_data = cache;
shrinker_register(cache->c_shrink);
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;
shrinker_free(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(mb_cache_entry, SLAB_RECLAIM_ACCOUNT);
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 <[email protected]>");
MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
MODULE_LICENSE("GPL");
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