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         head = mb_cache_entry_head(cache, key);
  97         hlist_bl_lock(head);
  98         hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
  99                 if (dup->e_key == key && dup->e_block == block) {
 100                         hlist_bl_unlock(head);
 101                         kmem_cache_free(mb_entry_cache, entry);
 102                         return -EBUSY;
 103                 }
 104         }
 105         hlist_bl_add_head(&entry->e_hash_list, head);
 106         hlist_bl_unlock(head);
 107
 108         spin_lock(&cache->c_list_lock);
 109         list_add_tail(&entry->e_list, &cache->c_list);
 110         /* Grab ref for LRU list */
 111         atomic_inc(&entry->e_refcnt);
 112         cache->c_entry_count++;
 113         spin_unlock(&cache->c_list_lock);
 114
 115         return 0;
 116 }
 117 EXPORT_SYMBOL(mb_cache_entry_create);
 118
 119 void __mb_cache_entry_free(struct mb_cache_entry *entry)
 120 {
 121         kmem_cache_free(mb_entry_cache, entry);
 122 }
 123 EXPORT_SYMBOL(__mb_cache_entry_free);
 124
 125 static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 126                                            struct mb_cache_entry *entry,
 127                                            u32 key)
 128 {
 129         struct mb_cache_entry *old_entry = entry;
 130         struct hlist_bl_node *node;
 131         struct hlist_bl_head *head;
 132
 133         head = mb_cache_entry_head(cache, key);
 134         hlist_bl_lock(head);
 135         if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
 136                 node = entry->e_hash_list.next;
 137         else
 138                 node = hlist_bl_first(head);
 139         while (node) {
 140                 entry = hlist_bl_entry(node, struct mb_cache_entry,
 141                                        e_hash_list);
 142                 if (entry->e_key == key && entry->e_reusable) {
 143                         atomic_inc(&entry->e_refcnt);
 144                         goto out;
 145                 }
 146                 node = node->next;
 147         }
 148         entry = NULL;
 149 out:
 150         hlist_bl_unlock(head);
 151         if (old_entry)
 152                 mb_cache_entry_put(cache, old_entry);
 153
 154         return entry;
 155 }
 156
 157 /*
 158  * mb_cache_entry_find_first - find the first entry in cache with given key
 159  * @cache: cache where we should search
 160  * @key: key to look for
 161  *
 162  * Search in @cache for entry with key @key. Grabs reference to the first
 163  * entry found and returns the entry.
 164  */
 165 struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
 166                                                  u32 key)
 167 {
 168         return __entry_find(cache, NULL, key);
 169 }
 170 EXPORT_SYMBOL(mb_cache_entry_find_first);
 171
 172 /*
 173  * mb_cache_entry_find_next - find next entry in cache with the same
 174  * @cache: cache where we should search
 175  * @entry: entry to start search from
 176  *
 177  * Finds next entry in the hash chain which has the same key as @entry.
 178  * If @entry is unhashed (which can happen when deletion of entry races
 179  * with the search), finds the first entry in the hash chain. The function
 180  * drops reference to @entry and returns with a reference to the found entry.
 181  */
 182 struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
 183                                                 struct mb_cache_entry *entry)
 184 {
 185         return __entry_find(cache, entry, entry->e_key);
 186 }
 187 EXPORT_SYMBOL(mb_cache_entry_find_next);
 188
 189 /*
 190  * mb_cache_entry_get - get a cache entry by block number (and key)
 191  * @cache - cache we work with
 192  * @key - key of block number @block
 193  * @block - block number
 194  */
 195 struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 196                                           sector_t block)
 197 {
 198         struct hlist_bl_node *node;
 199         struct hlist_bl_head *head;
 200         struct mb_cache_entry *entry;
 201
 202         head = mb_cache_entry_head(cache, key);
 203         hlist_bl_lock(head);
 204         hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 205                 if (entry->e_key == key && entry->e_block == block) {
 206                         atomic_inc(&entry->e_refcnt);
 207                         goto out;
 208                 }
 209         }
 210         entry = NULL;
 211 out:
 212         hlist_bl_unlock(head);
 213         return entry;
 214 }
 215 EXPORT_SYMBOL(mb_cache_entry_get);
 216
 217 /* mb_cache_entry_delete_block - remove information about block from cache
 218  * @cache - cache we work with
 219  * @key - key of block @block
 220  * @block - block number
 221  *
 222  * Remove entry from cache @cache with key @key with data stored in @block.
 223  */
 224 void mb_cache_entry_delete_block(struct mb_cache *cache, u32 key,
 225                                  sector_t block)
 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_block == block) {
 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_block);
 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 int nr_to_scan)
 278 {
 279         struct mb_cache_entry *entry;
 280         struct hlist_bl_head *head;
 281         unsigned int 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(&cache->c_list, &entry->e_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         int nr_to_scan = sc->nr_to_scan;
 320         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 321                                               c_shrink);
 322         return mb_cache_shrink(cache, nr_to_scan);
 323 }
 324
 325 /* We shrink 1/X of the cache when we have too many entries in it */
 326 #define SHRINK_DIVISOR 16
 327
 328 static void mb_cache_shrink_worker(struct work_struct *work)
 329 {
 330         struct mb_cache *cache = container_of(work, struct mb_cache,
 331                                               c_shrink_work);
 332         mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
 333 }
 334
 335 /*
 336  * mb_cache_create - create cache
 337  * @bucket_bits: log2 of the hash table size
 338  *
 339  * Create cache for keys with 2^bucket_bits hash entries.
 340  */
 341 struct mb_cache *mb_cache_create(int bucket_bits)
 342 {
 343         struct mb_cache *cache;
 344         int bucket_count = 1 << bucket_bits;
 345         int i;
 346
 347         if (!try_module_get(THIS_MODULE))
 348                 return NULL;
 349
 350         cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
 351         if (!cache)
 352                 goto err_out;
 353         cache->c_bucket_bits = bucket_bits;
 354         cache->c_max_entries = bucket_count << 4;
 355         INIT_LIST_HEAD(&cache->c_list);
 356         spin_lock_init(&cache->c_list_lock);
 357         cache->c_hash = kmalloc(bucket_count * sizeof(struct hlist_bl_head),
 358                                 GFP_KERNEL);
 359         if (!cache->c_hash) {
 360                 kfree(cache);
 361                 goto err_out;
 362         }
 363         for (i = 0; i < bucket_count; i++)
 364                 INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
 365
 366         cache->c_shrink.count_objects = mb_cache_count;
 367         cache->c_shrink.scan_objects = mb_cache_scan;
 368         cache->c_shrink.seeks = DEFAULT_SEEKS;
 369         register_shrinker(&cache->c_shrink);
 370
 371         INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
 372
 373         return cache;
 374
 375 err_out:
 376         module_put(THIS_MODULE);
 377         return NULL;
 378 }
 379 EXPORT_SYMBOL(mb_cache_create);
 380
 381 /*
 382  * mb_cache_destroy - destroy cache
 383  * @cache: the cache to destroy
 384  *
 385  * Free all entries in cache and cache itself. Caller must make sure nobody
 386  * (except shrinker) can reach @cache when calling this.
 387  */
 388 void mb_cache_destroy(struct mb_cache *cache)
 389 {
 390         struct mb_cache_entry *entry, *next;
 391
 392         unregister_shrinker(&cache->c_shrink);
 393
 394         /*
 395          * We don't bother with any locking. Cache must not be used at this
 396          * point.
 397          */
 398         list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
 399                 if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 400                         hlist_bl_del_init(&entry->e_hash_list);
 401                         atomic_dec(&entry->e_refcnt);
 402                 } else
 403                         WARN_ON(1);
 404                 list_del(&entry->e_list);
 405                 WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 406                 mb_cache_entry_put(cache, entry);
 407         }
 408         kfree(cache->c_hash);
 409         kfree(cache);
 410         module_put(THIS_MODULE);
 411 }
 412 EXPORT_SYMBOL(mb_cache_destroy);
 413
 414 static int __init mbcache_init(void)
 415 {
 416         mb_entry_cache = kmem_cache_create("mbcache",
 417                                 sizeof(struct mb_cache_entry), 0,
 418                                 SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
 419         BUG_ON(!mb_entry_cache);
 420         return 0;
 421 }
 422
 423 static void __exit mbcache_exit(void)
 424 {
 425         kmem_cache_destroy(mb_entry_cache);
 426 }
 427
 428 module_init(mbcache_init)
 429 module_exit(mbcache_exit)
 430
 431 MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
 432 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
 433 MODULE_LICENSE("GPL");