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_block()).
  14  *
  15  * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
  16  * They use hash of a block contents as a key and block number as a value.
  17  * That's why keys need not be unique (different xattr blocks may end up having
  18  * the same hash). However block number always uniquely identifies a cache
  19  * entry.
  20  *
  21  * We provide functions for creation and removal of entries, search by key,
  22  * and a special "delete entry with given key-value pair" operation. Fixed
  23  * size hash table is used for fast key lookups.
  24  */
  25
  26 struct mb_cache {
  27         /* Hash table of entries */
  28         struct hlist_bl_head    *c_hash;
  29         /* log2 of hash table size */
  30         int                     c_bucket_bits;
  31         /* Maximum entries in cache to avoid degrading hash too much */
  32         int                     c_max_entries;
  33         /* Protects c_list, c_entry_count */
  34         spinlock_t              c_list_lock;
  35         struct list_head        c_list;
  36         /* Number of entries in cache */
  37         unsigned long           c_entry_count;
  38         struct shrinker         c_shrink;
  39         /* Work for shrinking when the cache has too many entries */
  40         struct work_struct      c_shrink_work;
  41 };
  42
  43 static struct kmem_cache *mb_entry_cache;
  44
  45 static unsigned long mb_cache_shrink(struct mb_cache *cache,
  46                                      unsigned int nr_to_scan);
  47
  48 static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache,
  49                                                         u32 key)
  50 {
  51         return &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
  52 }
  53
  54 /*
  55  * Number of entries to reclaim synchronously when there are too many entries
  56  * in cache
  57  */
  58 #define SYNC_SHRINK_BATCH 64
  59
  60 /*
  61  * mb_cache_entry_create - create entry in cache
  62  * @cache - cache where the entry should be created
  63  * @mask - gfp mask with which the entry should be allocated
  64  * @key - key of the entry
  65  * @block - block that contains data
  66  * @reusable - is the block reusable by other inodes?
  67  *
  68  * Creates entry in @cache with key @key and records that data is stored in
  69  * block @block. The function returns -EBUSY if entry with the same key
  70  * and for the same block already exists in cache. Otherwise 0 is returned.
  71  */
  72 int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
  73                           sector_t block, bool reusable)
  74 {
  75         struct mb_cache_entry *entry, *dup;
  76         struct hlist_bl_node *dup_node;
  77         struct hlist_bl_head *head;
  78
  79         /* Schedule background reclaim if there are too many entries */
  80         if (cache->c_entry_count >= cache->c_max_entries)
  81                 schedule_work(&cache->c_shrink_work);
  82         /* Do some sync reclaim if background reclaim cannot keep up */
  83         if (cache->c_entry_count >= 2*cache->c_max_entries)
  84                 mb_cache_shrink(cache, SYNC_SHRINK_BATCH);
  85
  86         entry = kmem_cache_alloc(mb_entry_cache, mask);
  87         if (!entry)
  88                 return -ENOMEM;
  89
  90         INIT_LIST_HEAD(&entry->e_list);
  91         /* One ref for hash, one ref returned */
  92         atomic_set(&entry->e_refcnt, 1);
  93         entry->e_key = key;
  94         entry->e_block = block;
  95         entry->e_reusable = reusable;
  96         entry->e_referenced = 0;
  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_block == block) {
 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 entry in cache with given key
 160  * @cache: cache where we should search
 161  * @key: key to look for
 162  *
 163  * Search in @cache for entry with key @key. Grabs reference to the first
 164  * 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 entry in cache with the same
 175  * @cache: cache where we should search
 176  * @entry: entry to start search from
 177  *
 178  * Finds next 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
 180  * with the search), finds the first entry in the hash chain. The function
 181  * drops 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 block number (and key)
 192  * @cache - cache we work with
 193  * @key - key of block number @block
 194  * @block - block number
 195  */
 196 struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 197                                           sector_t block)
 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_block == block) {
 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_block - remove information about block from cache
 219  * @cache - cache we work with
 220  * @key - key of block @block
 221  * @block - block number
 222  *
 223  * Remove entry from cache @cache with key @key with data stored in @block.
 224  */
 225 void mb_cache_entry_delete_block(struct mb_cache *cache, u32 key,
 226                                  sector_t block)
 227 {
 228         struct hlist_bl_node *node;
 229         struct hlist_bl_head *head;
 230         struct mb_cache_entry *entry;
 231
 232         head = mb_cache_entry_head(cache, key);
 233         hlist_bl_lock(head);
 234         hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 235                 if (entry->e_key == key && entry->e_block == block) {
 236                         /* We keep hash list reference to keep entry alive */
 237                         hlist_bl_del_init(&entry->e_hash_list);
 238                         hlist_bl_unlock(head);
 239                         spin_lock(&cache->c_list_lock);
 240                         if (!list_empty(&entry->e_list)) {
 241                                 list_del_init(&entry->e_list);
 242                                 cache->c_entry_count--;
 243                                 atomic_dec(&entry->e_refcnt);
 244                         }
 245                         spin_unlock(&cache->c_list_lock);
 246                         mb_cache_entry_put(cache, entry);
 247                         return;
 248                 }
 249         }
 250         hlist_bl_unlock(head);
 251 }
 252 EXPORT_SYMBOL(mb_cache_entry_delete_block);
 253
 254 /* mb_cache_entry_touch - cache entry got used
 255  * @cache - cache the entry belongs to
 256  * @entry - entry that got used
 257  *
 258  * Marks entry as used to give hit higher chances of surviving in cache.
 259  */
 260 void mb_cache_entry_touch(struct mb_cache *cache,
 261                           struct mb_cache_entry *entry)
 262 {
 263         entry->e_referenced = 1;
 264 }
 265 EXPORT_SYMBOL(mb_cache_entry_touch);
 266
 267 static unsigned long mb_cache_count(struct shrinker *shrink,
 268                                     struct shrink_control *sc)
 269 {
 270         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 271                                               c_shrink);
 272
 273         return cache->c_entry_count;
 274 }
 275
 276 /* Shrink number of entries in cache */
 277 static unsigned long mb_cache_shrink(struct mb_cache *cache,
 278                                      unsigned int nr_to_scan)
 279 {
 280         struct mb_cache_entry *entry;
 281         struct hlist_bl_head *head;
 282         unsigned int shrunk = 0;
 283
 284         spin_lock(&cache->c_list_lock);
 285         while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 286                 entry = list_first_entry(&cache->c_list,
 287                                          struct mb_cache_entry, e_list);
 288                 if (entry->e_referenced) {
 289                         entry->e_referenced = 0;
 290                         list_move_tail(&cache->c_list, &entry->e_list);
 291                         continue;
 292                 }
 293                 list_del_init(&entry->e_list);
 294                 cache->c_entry_count--;
 295                 /*
 296                  * We keep LRU list reference so that entry doesn't go away
 297                  * from under us.
 298                  */
 299                 spin_unlock(&cache->c_list_lock);
 300                 head = mb_cache_entry_head(cache, entry->e_key);
 301                 hlist_bl_lock(head);
 302                 if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 303                         hlist_bl_del_init(&entry->e_hash_list);
 304                         atomic_dec(&entry->e_refcnt);
 305                 }
 306                 hlist_bl_unlock(head);
 307                 if (mb_cache_entry_put(cache, entry))
 308                         shrunk++;
 309                 cond_resched();
 310                 spin_lock(&cache->c_list_lock);
 311         }
 312         spin_unlock(&cache->c_list_lock);
 313
 314         return shrunk;
 315 }
 316
 317 static unsigned long mb_cache_scan(struct shrinker *shrink,
 318                                    struct shrink_control *sc)
 319 {
 320         int nr_to_scan = sc->nr_to_scan;
 321         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 322                                               c_shrink);
 323         return mb_cache_shrink(cache, nr_to_scan);
 324 }
 325
 326 /* We shrink 1/X of the cache when we have too many entries in it */
 327 #define SHRINK_DIVISOR 16
 328
 329 static void mb_cache_shrink_worker(struct work_struct *work)
 330 {
 331         struct mb_cache *cache = container_of(work, struct mb_cache,
 332                                               c_shrink_work);
 333         mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
 334 }
 335
 336 /*
 337  * mb_cache_create - create cache
 338  * @bucket_bits: log2 of the hash table size
 339  *
 340  * Create cache for keys with 2^bucket_bits hash entries.
 341  */
 342 struct mb_cache *mb_cache_create(int bucket_bits)
 343 {
 344         struct mb_cache *cache;
 345         int bucket_count = 1 << bucket_bits;
 346         int i;
 347
 348         if (!try_module_get(THIS_MODULE))
 349                 return NULL;
 350
 351         cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
 352         if (!cache)
 353                 goto err_out;
 354         cache->c_bucket_bits = bucket_bits;
 355         cache->c_max_entries = bucket_count << 4;
 356         INIT_LIST_HEAD(&cache->c_list);
 357         spin_lock_init(&cache->c_list_lock);
 358         cache->c_hash = kmalloc(bucket_count * sizeof(struct hlist_bl_head),
 359                                 GFP_KERNEL);
 360         if (!cache->c_hash) {
 361                 kfree(cache);
 362                 goto err_out;
 363         }
 364         for (i = 0; i < bucket_count; i++)
 365                 INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
 366
 367         cache->c_shrink.count_objects = mb_cache_count;
 368         cache->c_shrink.scan_objects = mb_cache_scan;
 369         cache->c_shrink.seeks = DEFAULT_SEEKS;
 370         if (register_shrinker(&cache->c_shrink)) {
 371                 kfree(cache->c_hash);
 372                 kfree(cache);
 373                 goto err_out;
 374         }
 375
 376         INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
 377
 378         return cache;
 379
 380 err_out:
 381         module_put(THIS_MODULE);
 382         return NULL;
 383 }
 384 EXPORT_SYMBOL(mb_cache_create);
 385
 386 /*
 387  * mb_cache_destroy - destroy cache
 388  * @cache: the cache to destroy
 389  *
 390  * Free all entries in cache and cache itself. Caller must make sure nobody
 391  * (except shrinker) can reach @cache when calling this.
 392  */
 393 void mb_cache_destroy(struct mb_cache *cache)
 394 {
 395         struct mb_cache_entry *entry, *next;
 396
 397         unregister_shrinker(&cache->c_shrink);
 398
 399         /*
 400          * We don't bother with any locking. Cache must not be used at this
 401          * point.
 402          */
 403         list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
 404                 if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 405                         hlist_bl_del_init(&entry->e_hash_list);
 406                         atomic_dec(&entry->e_refcnt);
 407                 } else
 408                         WARN_ON(1);
 409                 list_del(&entry->e_list);
 410                 WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 411                 mb_cache_entry_put(cache, entry);
 412         }
 413         kfree(cache->c_hash);
 414         kfree(cache);
 415         module_put(THIS_MODULE);
 416 }
 417 EXPORT_SYMBOL(mb_cache_destroy);
 418
 419 static int __init mbcache_init(void)
 420 {
 421         mb_entry_cache = kmem_cache_create("mbcache",
 422                                 sizeof(struct mb_cache_entry), 0,
 423                                 SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
 424         BUG_ON(!mb_entry_cache);
 425         return 0;
 426 }
 427
 428 static void __exit mbcache_exit(void)
 429 {
 430         kmem_cache_destroy(mb_entry_cache);
 431 }
 432
 433 module_init(mbcache_init)
 434 module_exit(mbcache_exit)
 435
 436 MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
 437 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
 438 MODULE_LICENSE("GPL");