fs/mbcache.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 #include <linux/spinlock.h>
   3 #include <linux/slab.h>
   4 #include <linux/list.h>
   5 #include <linux/list_bl.h>
   6 #include <linux/module.h>
   7 #include <linux/sched.h>
   8 #include <linux/workqueue.h>
   9 #include <linux/mbcache.h>
  10
  11 /*
  12  * Mbcache is a simple key-value store. Keys need not be unique, however
  13  * key-value pairs are expected to be unique (we use this fact in
  14  * mb_cache_entry_delete_or_get()).
  15  *
  16  * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
  17  * Ext4 also uses it for deduplication of xattr values stored in inodes.
  18  * They use hash of data as a key and provide a value that may represent a
  19  * block or inode number. That's why keys need not be unique (hash of different
  20  * data may be the same). However user provided value always uniquely
  21  * identifies a cache entry.
  22  *
  23  * We provide functions for creation and removal of entries, search by key,
  24  * and a special "delete entry with given key-value pair" operation. Fixed
  25  * size hash table is used for fast key lookups.
  26  */
  27
  28 struct mb_cache {
  29         /* Hash table of entries */
  30         struct hlist_bl_head    *c_hash;
  31         /* log2 of hash table size */
  32         int                     c_bucket_bits;
  33         /* Maximum entries in cache to avoid degrading hash too much */
  34         unsigned long           c_max_entries;
  35         /* Protects c_list, c_entry_count */
  36         spinlock_t              c_list_lock;
  37         struct list_head        c_list;
  38         /* Number of entries in cache */
  39         unsigned long           c_entry_count;
  40         struct shrinker         *c_shrink;
  41         /* Work for shrinking when the cache has too many entries */
  42         struct work_struct      c_shrink_work;
  43 };
  44
  45 static struct kmem_cache *mb_entry_cache;
  46
  47 static unsigned long mb_cache_shrink(struct mb_cache *cache,
  48                                      unsigned long nr_to_scan);
  49
  50 static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache,
  51                                                         u32 key)
  52 {
  53         return &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
  54 }
  55
  56 /*
  57  * Number of entries to reclaim synchronously when there are too many entries
  58  * in cache
  59  */
  60 #define SYNC_SHRINK_BATCH 64
  61
  62 /*
  63  * mb_cache_entry_create - create entry in cache
  64  * @cache - cache where the entry should be created
  65  * @mask - gfp mask with which the entry should be allocated
  66  * @key - key of the entry
  67  * @value - value of the entry
  68  * @reusable - is the entry reusable by others?
  69  *
  70  * Creates entry in @cache with key @key and value @value. The function returns
  71  * -EBUSY if entry with the same key and value already exists in cache.
  72  * Otherwise 0 is returned.
  73  */
  74 int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
  75                           u64 value, bool reusable)
  76 {
  77         struct mb_cache_entry *entry, *dup;
  78         struct hlist_bl_node *dup_node;
  79         struct hlist_bl_head *head;
  80
  81         /* Schedule background reclaim if there are too many entries */
  82         if (cache->c_entry_count >= cache->c_max_entries)
  83                 schedule_work(&cache->c_shrink_work);
  84         /* Do some sync reclaim if background reclaim cannot keep up */
  85         if (cache->c_entry_count >= 2*cache->c_max_entries)
  86                 mb_cache_shrink(cache, SYNC_SHRINK_BATCH);
  87
  88         entry = kmem_cache_alloc(mb_entry_cache, mask);
  89         if (!entry)
  90                 return -ENOMEM;
  91
  92         INIT_LIST_HEAD(&entry->e_list);
  93         /*
  94          * We create entry with two references. One reference is kept by the
  95          * hash table, the other reference is used to protect us from
  96          * mb_cache_entry_delete_or_get() until the entry is fully setup. This
  97          * avoids nesting of cache->c_list_lock into hash table bit locks which
  98          * is problematic for RT.
  99          */
 100         atomic_set(&entry->e_refcnt, 2);
 101         entry->e_key = key;
 102         entry->e_value = value;
 103         entry->e_flags = 0;
 104         if (reusable)
 105                 set_bit(MBE_REUSABLE_B, &entry->e_flags);
 106         head = mb_cache_entry_head(cache, key);
 107         hlist_bl_lock(head);
 108         hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
 109                 if (dup->e_key == key && dup->e_value == value) {
 110                         hlist_bl_unlock(head);
 111                         kmem_cache_free(mb_entry_cache, entry);
 112                         return -EBUSY;
 113                 }
 114         }
 115         hlist_bl_add_head(&entry->e_hash_list, head);
 116         hlist_bl_unlock(head);
 117         spin_lock(&cache->c_list_lock);
 118         list_add_tail(&entry->e_list, &cache->c_list);
 119         cache->c_entry_count++;
 120         spin_unlock(&cache->c_list_lock);
 121         mb_cache_entry_put(cache, entry);
 122
 123         return 0;
 124 }
 125 EXPORT_SYMBOL(mb_cache_entry_create);
 126
 127 void __mb_cache_entry_free(struct mb_cache *cache, struct mb_cache_entry *entry)
 128 {
 129         struct hlist_bl_head *head;
 130
 131         head = mb_cache_entry_head(cache, entry->e_key);
 132         hlist_bl_lock(head);
 133         hlist_bl_del(&entry->e_hash_list);
 134         hlist_bl_unlock(head);
 135         kmem_cache_free(mb_entry_cache, entry);
 136 }
 137 EXPORT_SYMBOL(__mb_cache_entry_free);
 138
 139 /*
 140  * mb_cache_entry_wait_unused - wait to be the last user of the entry
 141  *
 142  * @entry - entry to work on
 143  *
 144  * Wait to be the last user of the entry.
 145  */
 146 void mb_cache_entry_wait_unused(struct mb_cache_entry *entry)
 147 {
 148         wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 2);
 149 }
 150 EXPORT_SYMBOL(mb_cache_entry_wait_unused);
 151
 152 static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 153                                            struct mb_cache_entry *entry,
 154                                            u32 key)
 155 {
 156         struct mb_cache_entry *old_entry = entry;
 157         struct hlist_bl_node *node;
 158         struct hlist_bl_head *head;
 159
 160         head = mb_cache_entry_head(cache, key);
 161         hlist_bl_lock(head);
 162         if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
 163                 node = entry->e_hash_list.next;
 164         else
 165                 node = hlist_bl_first(head);
 166         while (node) {
 167                 entry = hlist_bl_entry(node, struct mb_cache_entry,
 168                                        e_hash_list);
 169                 if (entry->e_key == key &&
 170                     test_bit(MBE_REUSABLE_B, &entry->e_flags) &&
 171                     atomic_inc_not_zero(&entry->e_refcnt))
 172                         goto out;
 173                 node = node->next;
 174         }
 175         entry = NULL;
 176 out:
 177         hlist_bl_unlock(head);
 178         if (old_entry)
 179                 mb_cache_entry_put(cache, old_entry);
 180
 181         return entry;
 182 }
 183
 184 /*
 185  * mb_cache_entry_find_first - find the first reusable entry with the given key
 186  * @cache: cache where we should search
 187  * @key: key to look for
 188  *
 189  * Search in @cache for a reusable entry with key @key. Grabs reference to the
 190  * first reusable entry found and returns the entry.
 191  */
 192 struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
 193                                                  u32 key)
 194 {
 195         return __entry_find(cache, NULL, key);
 196 }
 197 EXPORT_SYMBOL(mb_cache_entry_find_first);
 198
 199 /*
 200  * mb_cache_entry_find_next - find next reusable entry with the same key
 201  * @cache: cache where we should search
 202  * @entry: entry to start search from
 203  *
 204  * Finds next reusable entry in the hash chain which has the same key as @entry.
 205  * If @entry is unhashed (which can happen when deletion of entry races with the
 206  * search), finds the first reusable entry in the hash chain. The function drops
 207  * reference to @entry and returns with a reference to the found entry.
 208  */
 209 struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
 210                                                 struct mb_cache_entry *entry)
 211 {
 212         return __entry_find(cache, entry, entry->e_key);
 213 }
 214 EXPORT_SYMBOL(mb_cache_entry_find_next);
 215
 216 /*
 217  * mb_cache_entry_get - get a cache entry by value (and key)
 218  * @cache - cache we work with
 219  * @key - key
 220  * @value - value
 221  */
 222 struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 223                                           u64 value)
 224 {
 225         struct hlist_bl_node *node;
 226         struct hlist_bl_head *head;
 227         struct mb_cache_entry *entry;
 228
 229         head = mb_cache_entry_head(cache, key);
 230         hlist_bl_lock(head);
 231         hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 232                 if (entry->e_key == key && entry->e_value == value &&
 233                     atomic_inc_not_zero(&entry->e_refcnt))
 234                         goto out;
 235         }
 236         entry = NULL;
 237 out:
 238         hlist_bl_unlock(head);
 239         return entry;
 240 }
 241 EXPORT_SYMBOL(mb_cache_entry_get);
 242
 243 /* mb_cache_entry_delete_or_get - remove a cache entry if it has no users
 244  * @cache - cache we work with
 245  * @key - key
 246  * @value - value
 247  *
 248  * Remove entry from cache @cache with key @key and value @value. The removal
 249  * happens only if the entry is unused. The function returns NULL in case the
 250  * entry was successfully removed or there's no entry in cache. Otherwise the
 251  * function grabs reference of the entry that we failed to delete because it
 252  * still has users and return it.
 253  */
 254 struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,
 255                                                     u32 key, u64 value)
 256 {
 257         struct mb_cache_entry *entry;
 258
 259         entry = mb_cache_entry_get(cache, key, value);
 260         if (!entry)
 261                 return NULL;
 262
 263         /*
 264          * Drop the ref we got from mb_cache_entry_get() and the initial hash
 265          * ref if we are the last user
 266          */
 267         if (atomic_cmpxchg(&entry->e_refcnt, 2, 0) != 2)
 268                 return entry;
 269
 270         spin_lock(&cache->c_list_lock);
 271         if (!list_empty(&entry->e_list))
 272                 list_del_init(&entry->e_list);
 273         cache->c_entry_count--;
 274         spin_unlock(&cache->c_list_lock);
 275         __mb_cache_entry_free(cache, entry);
 276         return NULL;
 277 }
 278 EXPORT_SYMBOL(mb_cache_entry_delete_or_get);
 279
 280 /* mb_cache_entry_touch - cache entry got used
 281  * @cache - cache the entry belongs to
 282  * @entry - entry that got used
 283  *
 284  * Marks entry as used to give hit higher chances of surviving in cache.
 285  */
 286 void mb_cache_entry_touch(struct mb_cache *cache,
 287                           struct mb_cache_entry *entry)
 288 {
 289         set_bit(MBE_REFERENCED_B, &entry->e_flags);
 290 }
 291 EXPORT_SYMBOL(mb_cache_entry_touch);
 292
 293 static unsigned long mb_cache_count(struct shrinker *shrink,
 294                                     struct shrink_control *sc)
 295 {
 296         struct mb_cache *cache = shrink->private_data;
 297
 298         return cache->c_entry_count;
 299 }
 300
 301 /* Shrink number of entries in cache */
 302 static unsigned long mb_cache_shrink(struct mb_cache *cache,
 303                                      unsigned long nr_to_scan)
 304 {
 305         struct mb_cache_entry *entry;
 306         unsigned long shrunk = 0;
 307
 308         spin_lock(&cache->c_list_lock);
 309         while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 310                 entry = list_first_entry(&cache->c_list,
 311                                          struct mb_cache_entry, e_list);
 312                 /* Drop initial hash reference if there is no user */
 313                 if (test_bit(MBE_REFERENCED_B, &entry->e_flags) ||
 314                     atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
 315                         clear_bit(MBE_REFERENCED_B, &entry->e_flags);
 316                         list_move_tail(&entry->e_list, &cache->c_list);
 317                         continue;
 318                 }
 319                 list_del_init(&entry->e_list);
 320                 cache->c_entry_count--;
 321                 spin_unlock(&cache->c_list_lock);
 322                 __mb_cache_entry_free(cache, entry);
 323                 shrunk++;
 324                 cond_resched();
 325                 spin_lock(&cache->c_list_lock);
 326         }
 327         spin_unlock(&cache->c_list_lock);
 328
 329         return shrunk;
 330 }
 331
 332 static unsigned long mb_cache_scan(struct shrinker *shrink,
 333                                    struct shrink_control *sc)
 334 {
 335         struct mb_cache *cache = shrink->private_data;
 336         return mb_cache_shrink(cache, sc->nr_to_scan);
 337 }
 338
 339 /* We shrink 1/X of the cache when we have too many entries in it */
 340 #define SHRINK_DIVISOR 16
 341
 342 static void mb_cache_shrink_worker(struct work_struct *work)
 343 {
 344         struct mb_cache *cache = container_of(work, struct mb_cache,
 345                                               c_shrink_work);
 346         mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
 347 }
 348
 349 /*
 350  * mb_cache_create - create cache
 351  * @bucket_bits: log2 of the hash table size
 352  *
 353  * Create cache for keys with 2^bucket_bits hash entries.
 354  */
 355 struct mb_cache *mb_cache_create(int bucket_bits)
 356 {
 357         struct mb_cache *cache;
 358         unsigned long bucket_count = 1UL << bucket_bits;
 359         unsigned long i;
 360
 361         cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
 362         if (!cache)
 363                 goto err_out;
 364         cache->c_bucket_bits = bucket_bits;
 365         cache->c_max_entries = bucket_count << 4;
 366         INIT_LIST_HEAD(&cache->c_list);
 367         spin_lock_init(&cache->c_list_lock);
 368         cache->c_hash = kmalloc_array(bucket_count,
 369                                       sizeof(struct hlist_bl_head),
 370                                       GFP_KERNEL);
 371         if (!cache->c_hash) {
 372                 kfree(cache);
 373                 goto err_out;
 374         }
 375         for (i = 0; i < bucket_count; i++)
 376                 INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
 377
 378         cache->c_shrink = shrinker_alloc(0, "mbcache-shrinker");
 379         if (!cache->c_shrink) {
 380                 kfree(cache->c_hash);
 381                 kfree(cache);
 382                 goto err_out;
 383         }
 384
 385         cache->c_shrink->count_objects = mb_cache_count;
 386         cache->c_shrink->scan_objects = mb_cache_scan;
 387         cache->c_shrink->private_data = cache;
 388
 389         shrinker_register(cache->c_shrink);
 390
 391         INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
 392
 393         return cache;
 394
 395 err_out:
 396         return NULL;
 397 }
 398 EXPORT_SYMBOL(mb_cache_create);
 399
 400 /*
 401  * mb_cache_destroy - destroy cache
 402  * @cache: the cache to destroy
 403  *
 404  * Free all entries in cache and cache itself. Caller must make sure nobody
 405  * (except shrinker) can reach @cache when calling this.
 406  */
 407 void mb_cache_destroy(struct mb_cache *cache)
 408 {
 409         struct mb_cache_entry *entry, *next;
 410
 411         shrinker_free(cache->c_shrink);
 412
 413         /*
 414          * We don't bother with any locking. Cache must not be used at this
 415          * point.
 416          */
 417         list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
 418                 list_del(&entry->e_list);
 419                 WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 420                 mb_cache_entry_put(cache, entry);
 421         }
 422         kfree(cache->c_hash);
 423         kfree(cache);
 424 }
 425 EXPORT_SYMBOL(mb_cache_destroy);
 426
 427 static int __init mbcache_init(void)
 428 {
 429         mb_entry_cache = KMEM_CACHE(mb_cache_entry, SLAB_RECLAIM_ACCOUNT);
 430         if (!mb_entry_cache)
 431                 return -ENOMEM;
 432         return 0;
 433 }
 434
 435 static void __exit mbcache_exit(void)
 436 {
 437         kmem_cache_destroy(mb_entry_cache);
 438 }
 439
 440 module_init(mbcache_init)
 441 module_exit(mbcache_exit)
 442
 443 MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
 444 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
 445 MODULE_LICENSE("GPL");