fs/mbcache.c

   1 #include <linux/spinlock.h>
   2 #include <linux/slab.h>
   3 #include <linux/list.h>
   4 #include <linux/list_bl.h>
   5 #include <linux/module.h>
   6 #include <linux/sched.h>
   7 #include <linux/workqueue.h>
   8 #include <linux/mbcache.h>
   9
  10 /*
  11  * Mbcache is a simple key-value store. Keys need not be unique, however
  12  * key-value pairs are expected to be unique (we use this fact in
  13  * mb_cache_entry_delete()).
  14  *
  15  * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
  16  * Ext4 also uses it for deduplication of xattr values stored in inodes.
  17  * They use hash of data as a key and provide a value that may represent a
  18  * block or inode number. That's why keys need not be unique (hash of different
  19  * data may be the same). However user provided value always uniquely
  20  * identifies a cache entry.
  21  *
  22  * We provide functions for creation and removal of entries, search by key,
  23  * and a special "delete entry with given key-value pair" operation. Fixed
  24  * size hash table is used for fast key lookups.
  25  */
  26
  27 struct mb_cache {
  28         /* Hash table of entries */
  29         struct hlist_bl_head    *c_hash;
  30         /* log2 of hash table size */
  31         int                     c_bucket_bits;
  32         /* Maximum entries in cache to avoid degrading hash too much */
  33         unsigned long           c_max_entries;
  34         /* Protects c_list, c_entry_count */
  35         spinlock_t              c_list_lock;
  36         struct list_head        c_list;
  37         /* Number of entries in cache */
  38         unsigned long           c_entry_count;
  39         struct shrinker         c_shrink;
  40         /* Work for shrinking when the cache has too many entries */
  41         struct work_struct      c_shrink_work;
  42 };
  43
  44 static struct kmem_cache *mb_entry_cache;
  45
  46 static unsigned long mb_cache_shrink(struct mb_cache *cache,
  47                                      unsigned long nr_to_scan);
  48
  49 static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache,
  50                                                         u32 key)
  51 {
  52         return &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
  53 }
  54
  55 /*
  56  * Number of entries to reclaim synchronously when there are too many entries
  57  * in cache
  58  */
  59 #define SYNC_SHRINK_BATCH 64
  60
  61 /*
  62  * mb_cache_entry_create - create entry in cache
  63  * @cache - cache where the entry should be created
  64  * @mask - gfp mask with which the entry should be allocated
  65  * @key - key of the entry
  66  * @value - value of the entry
  67  * @reusable - is the entry reusable by others?
  68  *
  69  * Creates entry in @cache with key @key and value @value. The function returns
  70  * -EBUSY if entry with the same key and value already exists in cache.
  71  * Otherwise 0 is returned.
  72  */
  73 int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
  74                           u64 value, bool reusable)
  75 {
  76         struct mb_cache_entry *entry, *dup;
  77         struct hlist_bl_node *dup_node;
  78         struct hlist_bl_head *head;
  79
  80         /* Schedule background reclaim if there are too many entries */
  81         if (cache->c_entry_count >= cache->c_max_entries)
  82                 schedule_work(&cache->c_shrink_work);
  83         /* Do some sync reclaim if background reclaim cannot keep up */
  84         if (cache->c_entry_count >= 2*cache->c_max_entries)
  85                 mb_cache_shrink(cache, SYNC_SHRINK_BATCH);
  86
  87         entry = kmem_cache_alloc(mb_entry_cache, mask);
  88         if (!entry)
  89                 return -ENOMEM;
  90
  91         INIT_LIST_HEAD(&entry->e_list);
  92         /* One ref for hash, one ref returned */
  93         atomic_set(&entry->e_refcnt, 1);
  94         entry->e_key = key;
  95         entry->e_value = value;
  96         entry->e_reusable = reusable;
  97         head = mb_cache_entry_head(cache, key);
  98         hlist_bl_lock(head);
  99         hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
 100                 if (dup->e_key == key && dup->e_value == value) {
 101                         hlist_bl_unlock(head);
 102                         kmem_cache_free(mb_entry_cache, entry);
 103                         return -EBUSY;
 104                 }
 105         }
 106         hlist_bl_add_head(&entry->e_hash_list, head);
 107         hlist_bl_unlock(head);
 108
 109         spin_lock(&cache->c_list_lock);
 110         list_add_tail(&entry->e_list, &cache->c_list);
 111         /* Grab ref for LRU list */
 112         atomic_inc(&entry->e_refcnt);
 113         cache->c_entry_count++;
 114         spin_unlock(&cache->c_list_lock);
 115
 116         return 0;
 117 }
 118 EXPORT_SYMBOL(mb_cache_entry_create);
 119
 120 void __mb_cache_entry_free(struct mb_cache_entry *entry)
 121 {
 122         kmem_cache_free(mb_entry_cache, entry);
 123 }
 124 EXPORT_SYMBOL(__mb_cache_entry_free);
 125
 126 static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 127                                            struct mb_cache_entry *entry,
 128                                            u32 key)
 129 {
 130         struct mb_cache_entry *old_entry = entry;
 131         struct hlist_bl_node *node;
 132         struct hlist_bl_head *head;
 133
 134         head = mb_cache_entry_head(cache, key);
 135         hlist_bl_lock(head);
 136         if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
 137                 node = entry->e_hash_list.next;
 138         else
 139                 node = hlist_bl_first(head);
 140         while (node) {
 141                 entry = hlist_bl_entry(node, struct mb_cache_entry,
 142                                        e_hash_list);
 143                 if (entry->e_key == key && entry->e_reusable) {
 144                         atomic_inc(&entry->e_refcnt);
 145                         goto out;
 146                 }
 147                 node = node->next;
 148         }
 149         entry = NULL;
 150 out:
 151         hlist_bl_unlock(head);
 152         if (old_entry)
 153                 mb_cache_entry_put(cache, old_entry);
 154
 155         return entry;
 156 }
 157
 158 /*
 159  * mb_cache_entry_find_first - find the first reusable entry with the given key
 160  * @cache: cache where we should search
 161  * @key: key to look for
 162  *
 163  * Search in @cache for a reusable entry with key @key. Grabs reference to the
 164  * first reusable entry found and returns the entry.
 165  */
 166 struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
 167                                                  u32 key)
 168 {
 169         return __entry_find(cache, NULL, key);
 170 }
 171 EXPORT_SYMBOL(mb_cache_entry_find_first);
 172
 173 /*
 174  * mb_cache_entry_find_next - find next reusable entry with the same key
 175  * @cache: cache where we should search
 176  * @entry: entry to start search from
 177  *
 178  * Finds next reusable entry in the hash chain which has the same key as @entry.
 179  * If @entry is unhashed (which can happen when deletion of entry races with the
 180  * search), finds the first reusable entry in the hash chain. The function drops
 181  * reference to @entry and returns with a reference to the found entry.
 182  */
 183 struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
 184                                                 struct mb_cache_entry *entry)
 185 {
 186         return __entry_find(cache, entry, entry->e_key);
 187 }
 188 EXPORT_SYMBOL(mb_cache_entry_find_next);
 189
 190 /*
 191  * mb_cache_entry_get - get a cache entry by value (and key)
 192  * @cache - cache we work with
 193  * @key - key
 194  * @value - value
 195  */
 196 struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 197                                           u64 value)
 198 {
 199         struct hlist_bl_node *node;
 200         struct hlist_bl_head *head;
 201         struct mb_cache_entry *entry;
 202
 203         head = mb_cache_entry_head(cache, key);
 204         hlist_bl_lock(head);
 205         hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 206                 if (entry->e_key == key && entry->e_value == value) {
 207                         atomic_inc(&entry->e_refcnt);
 208                         goto out;
 209                 }
 210         }
 211         entry = NULL;
 212 out:
 213         hlist_bl_unlock(head);
 214         return entry;
 215 }
 216 EXPORT_SYMBOL(mb_cache_entry_get);
 217
 218 /* mb_cache_entry_delete - remove a cache entry
 219  * @cache - cache we work with
 220  * @key - key
 221  * @value - value
 222  *
 223  * Remove entry from cache @cache with key @key and value @value.
 224  */
 225 void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
 226 {
 227         struct hlist_bl_node *node;
 228         struct hlist_bl_head *head;
 229         struct mb_cache_entry *entry;
 230
 231         head = mb_cache_entry_head(cache, key);
 232         hlist_bl_lock(head);
 233         hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 234                 if (entry->e_key == key && entry->e_value == value) {
 235                         /* We keep hash list reference to keep entry alive */
 236                         hlist_bl_del_init(&entry->e_hash_list);
 237                         hlist_bl_unlock(head);
 238                         spin_lock(&cache->c_list_lock);
 239                         if (!list_empty(&entry->e_list)) {
 240                                 list_del_init(&entry->e_list);
 241                                 cache->c_entry_count--;
 242                                 atomic_dec(&entry->e_refcnt);
 243                         }
 244                         spin_unlock(&cache->c_list_lock);
 245                         mb_cache_entry_put(cache, entry);
 246                         return;
 247                 }
 248         }
 249         hlist_bl_unlock(head);
 250 }
 251 EXPORT_SYMBOL(mb_cache_entry_delete);
 252
 253 /* mb_cache_entry_touch - cache entry got used
 254  * @cache - cache the entry belongs to
 255  * @entry - entry that got used
 256  *
 257  * Marks entry as used to give hit higher chances of surviving in cache.
 258  */
 259 void mb_cache_entry_touch(struct mb_cache *cache,
 260                           struct mb_cache_entry *entry)
 261 {
 262         entry->e_referenced = 1;
 263 }
 264 EXPORT_SYMBOL(mb_cache_entry_touch);
 265
 266 static unsigned long mb_cache_count(struct shrinker *shrink,
 267                                     struct shrink_control *sc)
 268 {
 269         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 270                                               c_shrink);
 271
 272         return cache->c_entry_count;
 273 }
 274
 275 /* Shrink number of entries in cache */
 276 static unsigned long mb_cache_shrink(struct mb_cache *cache,
 277                                      unsigned long nr_to_scan)
 278 {
 279         struct mb_cache_entry *entry;
 280         struct hlist_bl_head *head;
 281         unsigned long shrunk = 0;
 282
 283         spin_lock(&cache->c_list_lock);
 284         while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 285                 entry = list_first_entry(&cache->c_list,
 286                                          struct mb_cache_entry, e_list);
 287                 if (entry->e_referenced) {
 288                         entry->e_referenced = 0;
 289                         list_move_tail(&entry->e_list, &cache->c_list);
 290                         continue;
 291                 }
 292                 list_del_init(&entry->e_list);
 293                 cache->c_entry_count--;
 294                 /*
 295                  * We keep LRU list reference so that entry doesn't go away
 296                  * from under us.
 297                  */
 298                 spin_unlock(&cache->c_list_lock);
 299                 head = mb_cache_entry_head(cache, entry->e_key);
 300                 hlist_bl_lock(head);
 301                 if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 302                         hlist_bl_del_init(&entry->e_hash_list);
 303                         atomic_dec(&entry->e_refcnt);
 304                 }
 305                 hlist_bl_unlock(head);
 306                 if (mb_cache_entry_put(cache, entry))
 307                         shrunk++;
 308                 cond_resched();
 309                 spin_lock(&cache->c_list_lock);
 310         }
 311         spin_unlock(&cache->c_list_lock);
 312
 313         return shrunk;
 314 }
 315
 316 static unsigned long mb_cache_scan(struct shrinker *shrink,
 317                                    struct shrink_control *sc)
 318 {
 319         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 320                                               c_shrink);
 321         return mb_cache_shrink(cache, sc->nr_to_scan);
 322 }
 323
 324 /* We shrink 1/X of the cache when we have too many entries in it */
 325 #define SHRINK_DIVISOR 16
 326
 327 static void mb_cache_shrink_worker(struct work_struct *work)
 328 {
 329         struct mb_cache *cache = container_of(work, struct mb_cache,
 330                                               c_shrink_work);
 331         mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
 332 }
 333
 334 /*
 335  * mb_cache_create - create cache
 336  * @bucket_bits: log2 of the hash table size
 337  *
 338  * Create cache for keys with 2^bucket_bits hash entries.
 339  */
 340 struct mb_cache *mb_cache_create(int bucket_bits)
 341 {
 342         struct mb_cache *cache;
 343         unsigned long bucket_count = 1UL << bucket_bits;
 344         unsigned long i;
 345
 346         cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
 347         if (!cache)
 348                 goto err_out;
 349         cache->c_bucket_bits = bucket_bits;
 350         cache->c_max_entries = bucket_count << 4;
 351         INIT_LIST_HEAD(&cache->c_list);
 352         spin_lock_init(&cache->c_list_lock);
 353         cache->c_hash = kmalloc(bucket_count * sizeof(struct hlist_bl_head),
 354                                 GFP_KERNEL);
 355         if (!cache->c_hash) {
 356                 kfree(cache);
 357                 goto err_out;
 358         }
 359         for (i = 0; i < bucket_count; i++)
 360                 INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
 361
 362         cache->c_shrink.count_objects = mb_cache_count;
 363         cache->c_shrink.scan_objects = mb_cache_scan;
 364         cache->c_shrink.seeks = DEFAULT_SEEKS;
 365         if (register_shrinker(&cache->c_shrink)) {
 366                 kfree(cache->c_hash);
 367                 kfree(cache);
 368                 goto err_out;
 369         }
 370
 371         INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
 372
 373         return cache;
 374
 375 err_out:
 376         return NULL;
 377 }
 378 EXPORT_SYMBOL(mb_cache_create);
 379
 380 /*
 381  * mb_cache_destroy - destroy cache
 382  * @cache: the cache to destroy
 383  *
 384  * Free all entries in cache and cache itself. Caller must make sure nobody
 385  * (except shrinker) can reach @cache when calling this.
 386  */
 387 void mb_cache_destroy(struct mb_cache *cache)
 388 {
 389         struct mb_cache_entry *entry, *next;
 390
 391         unregister_shrinker(&cache->c_shrink);
 392
 393         /*
 394          * We don't bother with any locking. Cache must not be used at this
 395          * point.
 396          */
 397         list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
 398                 if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 399                         hlist_bl_del_init(&entry->e_hash_list);
 400                         atomic_dec(&entry->e_refcnt);
 401                 } else
 402                         WARN_ON(1);
 403                 list_del(&entry->e_list);
 404                 WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 405                 mb_cache_entry_put(cache, entry);
 406         }
 407         kfree(cache->c_hash);
 408         kfree(cache);
 409 }
 410 EXPORT_SYMBOL(mb_cache_destroy);
 411
 412 static int __init mbcache_init(void)
 413 {
 414         mb_entry_cache = kmem_cache_create("mbcache",
 415                                 sizeof(struct mb_cache_entry), 0,
 416                                 SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
 417         if (!mb_entry_cache)
 418                 return -ENOMEM;
 419         return 0;
 420 }
 421
 422 static void __exit mbcache_exit(void)
 423 {
 424         kmem_cache_destroy(mb_entry_cache);
 425 }
 426
 427 module_init(mbcache_init)
 428 module_exit(mbcache_exit)
 429
 430 MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
 431 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
 432 MODULE_LICENSE("GPL");