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