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         /* Initial hash reference */
  94         atomic_set(&entry->e_refcnt, 1);
  95         entry->e_key = key;
  96         entry->e_value = value;
  97         entry->e_reusable = reusable;
  98         entry->e_referenced = 0;
  99         head = mb_cache_entry_head(cache, key);
 100         hlist_bl_lock(head);
 101         hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
 102                 if (dup->e_key == key && dup->e_value == value) {
 103                         hlist_bl_unlock(head);
 104                         kmem_cache_free(mb_entry_cache, entry);
 105                         return -EBUSY;
 106                 }
 107         }
 108         hlist_bl_add_head(&entry->e_hash_list, head);
 109         /*
 110          * Add entry to LRU list before it can be found by
 111          * mb_cache_entry_delete() to avoid races
 112          */
 113         spin_lock(&cache->c_list_lock);
 114         list_add_tail(&entry->e_list, &cache->c_list);
 115         cache->c_entry_count++;
 116         spin_unlock(&cache->c_list_lock);
 117         hlist_bl_unlock(head);
 118
 119         return 0;
 120 }
 121 EXPORT_SYMBOL(mb_cache_entry_create);
 122
 123 void __mb_cache_entry_free(struct mb_cache *cache, struct mb_cache_entry *entry)
 124 {
 125         struct hlist_bl_head *head;
 126
 127         head = mb_cache_entry_head(cache, entry->e_key);
 128         hlist_bl_lock(head);
 129         hlist_bl_del(&entry->e_hash_list);
 130         hlist_bl_unlock(head);
 131         kmem_cache_free(mb_entry_cache, entry);
 132 }
 133 EXPORT_SYMBOL(__mb_cache_entry_free);
 134
 135 /*
 136  * mb_cache_entry_wait_unused - wait to be the last user of the entry
 137  *
 138  * @entry - entry to work on
 139  *
 140  * Wait to be the last user of the entry.
 141  */
 142 void mb_cache_entry_wait_unused(struct mb_cache_entry *entry)
 143 {
 144         wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 2);
 145 }
 146 EXPORT_SYMBOL(mb_cache_entry_wait_unused);
 147
 148 static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 149                                            struct mb_cache_entry *entry,
 150                                            u32 key)
 151 {
 152         struct mb_cache_entry *old_entry = entry;
 153         struct hlist_bl_node *node;
 154         struct hlist_bl_head *head;
 155
 156         head = mb_cache_entry_head(cache, key);
 157         hlist_bl_lock(head);
 158         if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
 159                 node = entry->e_hash_list.next;
 160         else
 161                 node = hlist_bl_first(head);
 162         while (node) {
 163                 entry = hlist_bl_entry(node, struct mb_cache_entry,
 164                                        e_hash_list);
 165                 if (entry->e_key == key && entry->e_reusable &&
 166                     atomic_inc_not_zero(&entry->e_refcnt))
 167                         goto out;
 168                 node = node->next;
 169         }
 170         entry = NULL;
 171 out:
 172         hlist_bl_unlock(head);
 173         if (old_entry)
 174                 mb_cache_entry_put(cache, old_entry);
 175
 176         return entry;
 177 }
 178
 179 /*
 180  * mb_cache_entry_find_first - find the first reusable entry with the given key
 181  * @cache: cache where we should search
 182  * @key: key to look for
 183  *
 184  * Search in @cache for a reusable entry with key @key. Grabs reference to the
 185  * first reusable entry found and returns the entry.
 186  */
 187 struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
 188                                                  u32 key)
 189 {
 190         return __entry_find(cache, NULL, key);
 191 }
 192 EXPORT_SYMBOL(mb_cache_entry_find_first);
 193
 194 /*
 195  * mb_cache_entry_find_next - find next reusable entry with the same key
 196  * @cache: cache where we should search
 197  * @entry: entry to start search from
 198  *
 199  * Finds next reusable entry in the hash chain which has the same key as @entry.
 200  * If @entry is unhashed (which can happen when deletion of entry races with the
 201  * search), finds the first reusable entry in the hash chain. The function drops
 202  * reference to @entry and returns with a reference to the found entry.
 203  */
 204 struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
 205                                                 struct mb_cache_entry *entry)
 206 {
 207         return __entry_find(cache, entry, entry->e_key);
 208 }
 209 EXPORT_SYMBOL(mb_cache_entry_find_next);
 210
 211 /*
 212  * mb_cache_entry_get - get a cache entry by value (and key)
 213  * @cache - cache we work with
 214  * @key - key
 215  * @value - value
 216  */
 217 struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 218                                           u64 value)
 219 {
 220         struct hlist_bl_node *node;
 221         struct hlist_bl_head *head;
 222         struct mb_cache_entry *entry;
 223
 224         head = mb_cache_entry_head(cache, key);
 225         hlist_bl_lock(head);
 226         hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 227                 if (entry->e_key == key && entry->e_value == value &&
 228                     atomic_inc_not_zero(&entry->e_refcnt))
 229                         goto out;
 230         }
 231         entry = NULL;
 232 out:
 233         hlist_bl_unlock(head);
 234         return entry;
 235 }
 236 EXPORT_SYMBOL(mb_cache_entry_get);
 237
 238 /* mb_cache_entry_delete_or_get - remove a cache entry if it has no users
 239  * @cache - cache we work with
 240  * @key - key
 241  * @value - value
 242  *
 243  * Remove entry from cache @cache with key @key and value @value. The removal
 244  * happens only if the entry is unused. The function returns NULL in case the
 245  * entry was successfully removed or there's no entry in cache. Otherwise the
 246  * function grabs reference of the entry that we failed to delete because it
 247  * still has users and return it.
 248  */
 249 struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,
 250                                                     u32 key, u64 value)
 251 {
 252         struct mb_cache_entry *entry;
 253
 254         entry = mb_cache_entry_get(cache, key, value);
 255         if (!entry)
 256                 return NULL;
 257
 258         /*
 259          * Drop the ref we got from mb_cache_entry_get() and the initial hash
 260          * ref if we are the last user
 261          */
 262         if (atomic_cmpxchg(&entry->e_refcnt, 2, 0) != 2)
 263                 return entry;
 264
 265         spin_lock(&cache->c_list_lock);
 266         if (!list_empty(&entry->e_list))
 267                 list_del_init(&entry->e_list);
 268         cache->c_entry_count--;
 269         spin_unlock(&cache->c_list_lock);
 270         __mb_cache_entry_free(cache, entry);
 271         return NULL;
 272 }
 273 EXPORT_SYMBOL(mb_cache_entry_delete_or_get);
 274
 275 /* mb_cache_entry_touch - cache entry got used
 276  * @cache - cache the entry belongs to
 277  * @entry - entry that got used
 278  *
 279  * Marks entry as used to give hit higher chances of surviving in cache.
 280  */
 281 void mb_cache_entry_touch(struct mb_cache *cache,
 282                           struct mb_cache_entry *entry)
 283 {
 284         entry->e_referenced = 1;
 285 }
 286 EXPORT_SYMBOL(mb_cache_entry_touch);
 287
 288 static unsigned long mb_cache_count(struct shrinker *shrink,
 289                                     struct shrink_control *sc)
 290 {
 291         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 292                                               c_shrink);
 293
 294         return cache->c_entry_count;
 295 }
 296
 297 /* Shrink number of entries in cache */
 298 static unsigned long mb_cache_shrink(struct mb_cache *cache,
 299                                      unsigned long nr_to_scan)
 300 {
 301         struct mb_cache_entry *entry;
 302         unsigned long shrunk = 0;
 303
 304         spin_lock(&cache->c_list_lock);
 305         while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 306                 entry = list_first_entry(&cache->c_list,
 307                                          struct mb_cache_entry, e_list);
 308                 /* Drop initial hash reference if there is no user */
 309                 if (entry->e_referenced ||
 310                     atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
 311                         entry->e_referenced = 0;
 312                         list_move_tail(&entry->e_list, &cache->c_list);
 313                         continue;
 314                 }
 315                 list_del_init(&entry->e_list);
 316                 cache->c_entry_count--;
 317                 spin_unlock(&cache->c_list_lock);
 318                 __mb_cache_entry_free(cache, entry);
 319                 shrunk++;
 320                 cond_resched();
 321                 spin_lock(&cache->c_list_lock);
 322         }
 323         spin_unlock(&cache->c_list_lock);
 324
 325         return shrunk;
 326 }
 327
 328 static unsigned long mb_cache_scan(struct shrinker *shrink,
 329                                    struct shrink_control *sc)
 330 {
 331         struct mb_cache *cache = container_of(shrink, struct mb_cache,
 332                                               c_shrink);
 333         return mb_cache_shrink(cache, sc->nr_to_scan);
 334 }
 335
 336 /* We shrink 1/X of the cache when we have too many entries in it */
 337 #define SHRINK_DIVISOR 16
 338
 339 static void mb_cache_shrink_worker(struct work_struct *work)
 340 {
 341         struct mb_cache *cache = container_of(work, struct mb_cache,
 342                                               c_shrink_work);
 343         mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
 344 }
 345
 346 /*
 347  * mb_cache_create - create cache
 348  * @bucket_bits: log2 of the hash table size
 349  *
 350  * Create cache for keys with 2^bucket_bits hash entries.
 351  */
 352 struct mb_cache *mb_cache_create(int bucket_bits)
 353 {
 354         struct mb_cache *cache;
 355         unsigned long bucket_count = 1UL << bucket_bits;
 356         unsigned long i;
 357
 358         cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
 359         if (!cache)
 360                 goto err_out;
 361         cache->c_bucket_bits = bucket_bits;
 362         cache->c_max_entries = bucket_count << 4;
 363         INIT_LIST_HEAD(&cache->c_list);
 364         spin_lock_init(&cache->c_list_lock);
 365         cache->c_hash = kmalloc_array(bucket_count,
 366                                       sizeof(struct hlist_bl_head),
 367                                       GFP_KERNEL);
 368         if (!cache->c_hash) {
 369                 kfree(cache);
 370                 goto err_out;
 371         }
 372         for (i = 0; i < bucket_count; i++)
 373                 INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
 374
 375         cache->c_shrink.count_objects = mb_cache_count;
 376         cache->c_shrink.scan_objects = mb_cache_scan;
 377         cache->c_shrink.seeks = DEFAULT_SEEKS;
 378         if (register_shrinker(&cache->c_shrink, "mbcache-shrinker")) {
 379                 kfree(cache->c_hash);
 380                 kfree(cache);
 381                 goto err_out;
 382         }
 383
 384         INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
 385
 386         return cache;
 387
 388 err_out:
 389         return NULL;
 390 }
 391 EXPORT_SYMBOL(mb_cache_create);
 392
 393 /*
 394  * mb_cache_destroy - destroy cache
 395  * @cache: the cache to destroy
 396  *
 397  * Free all entries in cache and cache itself. Caller must make sure nobody
 398  * (except shrinker) can reach @cache when calling this.
 399  */
 400 void mb_cache_destroy(struct mb_cache *cache)
 401 {
 402         struct mb_cache_entry *entry, *next;
 403
 404         unregister_shrinker(&cache->c_shrink);
 405
 406         /*
 407          * We don't bother with any locking. Cache must not be used at this
 408          * point.
 409          */
 410         list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
 411                 list_del(&entry->e_list);
 412                 WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 413                 mb_cache_entry_put(cache, entry);
 414         }
 415         kfree(cache->c_hash);
 416         kfree(cache);
 417 }
 418 EXPORT_SYMBOL(mb_cache_destroy);
 419
 420 static int __init mbcache_init(void)
 421 {
 422         mb_entry_cache = kmem_cache_create("mbcache",
 423                                 sizeof(struct mb_cache_entry), 0,
 424                                 SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
 425         if (!mb_entry_cache)
 426                 return -ENOMEM;
 427         return 0;
 428 }
 429
 430 static void __exit mbcache_exit(void)
 431 {
 432         kmem_cache_destroy(mb_entry_cache);
 433 }
 434
 435 module_init(mbcache_init)
 436 module_exit(mbcache_exit)
 437
 438 MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
 439 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
 440 MODULE_LICENSE("GPL");