linux/fs/mbcache.c
<<
>>
Prefs
   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()).
  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
  28struct 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
  45static struct kmem_cache *mb_entry_cache;
  46
  47static unsigned long mb_cache_shrink(struct mb_cache *cache,
  48                                     unsigned long nr_to_scan);
  49
  50static 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 */
  74int 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        /* One ref for hash, one ref returned */
  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        hlist_bl_unlock(head);
 110
 111        spin_lock(&cache->c_list_lock);
 112        list_add_tail(&entry->e_list, &cache->c_list);
 113        /* Grab ref for LRU list */
 114        atomic_inc(&entry->e_refcnt);
 115        cache->c_entry_count++;
 116        spin_unlock(&cache->c_list_lock);
 117
 118        return 0;
 119}
 120EXPORT_SYMBOL(mb_cache_entry_create);
 121
 122void __mb_cache_entry_free(struct mb_cache_entry *entry)
 123{
 124        kmem_cache_free(mb_entry_cache, entry);
 125}
 126EXPORT_SYMBOL(__mb_cache_entry_free);
 127
 128static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 129                                           struct mb_cache_entry *entry,
 130                                           u32 key)
 131{
 132        struct mb_cache_entry *old_entry = entry;
 133        struct hlist_bl_node *node;
 134        struct hlist_bl_head *head;
 135
 136        head = mb_cache_entry_head(cache, key);
 137        hlist_bl_lock(head);
 138        if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
 139                node = entry->e_hash_list.next;
 140        else
 141                node = hlist_bl_first(head);
 142        while (node) {
 143                entry = hlist_bl_entry(node, struct mb_cache_entry,
 144                                       e_hash_list);
 145                if (entry->e_key == key && entry->e_reusable) {
 146                        atomic_inc(&entry->e_refcnt);
 147                        goto out;
 148                }
 149                node = node->next;
 150        }
 151        entry = NULL;
 152out:
 153        hlist_bl_unlock(head);
 154        if (old_entry)
 155                mb_cache_entry_put(cache, old_entry);
 156
 157        return entry;
 158}
 159
 160/*
 161 * mb_cache_entry_find_first - find the first reusable entry with the given key
 162 * @cache: cache where we should search
 163 * @key: key to look for
 164 *
 165 * Search in @cache for a reusable entry with key @key. Grabs reference to the
 166 * first reusable entry found and returns the entry.
 167 */
 168struct mb_cache_entry *mb_cache_entry_find_first(struct mb_cache *cache,
 169                                                 u32 key)
 170{
 171        return __entry_find(cache, NULL, key);
 172}
 173EXPORT_SYMBOL(mb_cache_entry_find_first);
 174
 175/*
 176 * mb_cache_entry_find_next - find next reusable entry with the same key
 177 * @cache: cache where we should search
 178 * @entry: entry to start search from
 179 *
 180 * Finds next reusable entry in the hash chain which has the same key as @entry.
 181 * If @entry is unhashed (which can happen when deletion of entry races with the
 182 * search), finds the first reusable entry in the hash chain. The function drops
 183 * reference to @entry and returns with a reference to the found entry.
 184 */
 185struct mb_cache_entry *mb_cache_entry_find_next(struct mb_cache *cache,
 186                                                struct mb_cache_entry *entry)
 187{
 188        return __entry_find(cache, entry, entry->e_key);
 189}
 190EXPORT_SYMBOL(mb_cache_entry_find_next);
 191
 192/*
 193 * mb_cache_entry_get - get a cache entry by value (and key)
 194 * @cache - cache we work with
 195 * @key - key
 196 * @value - value
 197 */
 198struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 199                                          u64 value)
 200{
 201        struct hlist_bl_node *node;
 202        struct hlist_bl_head *head;
 203        struct mb_cache_entry *entry;
 204
 205        head = mb_cache_entry_head(cache, key);
 206        hlist_bl_lock(head);
 207        hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 208                if (entry->e_key == key && entry->e_value == value) {
 209                        atomic_inc(&entry->e_refcnt);
 210                        goto out;
 211                }
 212        }
 213        entry = NULL;
 214out:
 215        hlist_bl_unlock(head);
 216        return entry;
 217}
 218EXPORT_SYMBOL(mb_cache_entry_get);
 219
 220/* mb_cache_entry_delete - remove a cache entry
 221 * @cache - cache we work with
 222 * @key - key
 223 * @value - value
 224 *
 225 * Remove entry from cache @cache with key @key and value @value.
 226 */
 227void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
 228{
 229        struct hlist_bl_node *node;
 230        struct hlist_bl_head *head;
 231        struct mb_cache_entry *entry;
 232
 233        head = mb_cache_entry_head(cache, key);
 234        hlist_bl_lock(head);
 235        hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 236                if (entry->e_key == key && entry->e_value == value) {
 237                        /* We keep hash list reference to keep entry alive */
 238                        hlist_bl_del_init(&entry->e_hash_list);
 239                        hlist_bl_unlock(head);
 240                        spin_lock(&cache->c_list_lock);
 241                        if (!list_empty(&entry->e_list)) {
 242                                list_del_init(&entry->e_list);
 243                                if (!WARN_ONCE(cache->c_entry_count == 0,
 244                "mbcache: attempt to decrement c_entry_count past zero"))
 245                                        cache->c_entry_count--;
 246                                atomic_dec(&entry->e_refcnt);
 247                        }
 248                        spin_unlock(&cache->c_list_lock);
 249                        mb_cache_entry_put(cache, entry);
 250                        return;
 251                }
 252        }
 253        hlist_bl_unlock(head);
 254}
 255EXPORT_SYMBOL(mb_cache_entry_delete);
 256
 257/* mb_cache_entry_touch - cache entry got used
 258 * @cache - cache the entry belongs to
 259 * @entry - entry that got used
 260 *
 261 * Marks entry as used to give hit higher chances of surviving in cache.
 262 */
 263void mb_cache_entry_touch(struct mb_cache *cache,
 264                          struct mb_cache_entry *entry)
 265{
 266        entry->e_referenced = 1;
 267}
 268EXPORT_SYMBOL(mb_cache_entry_touch);
 269
 270static unsigned long mb_cache_count(struct shrinker *shrink,
 271                                    struct shrink_control *sc)
 272{
 273        struct mb_cache *cache = container_of(shrink, struct mb_cache,
 274                                              c_shrink);
 275
 276        return cache->c_entry_count;
 277}
 278
 279/* Shrink number of entries in cache */
 280static unsigned long mb_cache_shrink(struct mb_cache *cache,
 281                                     unsigned long nr_to_scan)
 282{
 283        struct mb_cache_entry *entry;
 284        struct hlist_bl_head *head;
 285        unsigned long shrunk = 0;
 286
 287        spin_lock(&cache->c_list_lock);
 288        while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 289                entry = list_first_entry(&cache->c_list,
 290                                         struct mb_cache_entry, e_list);
 291                if (entry->e_referenced) {
 292                        entry->e_referenced = 0;
 293                        list_move_tail(&entry->e_list, &cache->c_list);
 294                        continue;
 295                }
 296                list_del_init(&entry->e_list);
 297                cache->c_entry_count--;
 298                /*
 299                 * We keep LRU list reference so that entry doesn't go away
 300                 * from under us.
 301                 */
 302                spin_unlock(&cache->c_list_lock);
 303                head = mb_cache_entry_head(cache, entry->e_key);
 304                hlist_bl_lock(head);
 305                if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 306                        hlist_bl_del_init(&entry->e_hash_list);
 307                        atomic_dec(&entry->e_refcnt);
 308                }
 309                hlist_bl_unlock(head);
 310                if (mb_cache_entry_put(cache, entry))
 311                        shrunk++;
 312                cond_resched();
 313                spin_lock(&cache->c_list_lock);
 314        }
 315        spin_unlock(&cache->c_list_lock);
 316
 317        return shrunk;
 318}
 319
 320static unsigned long mb_cache_scan(struct shrinker *shrink,
 321                                   struct shrink_control *sc)
 322{
 323        struct mb_cache *cache = container_of(shrink, struct mb_cache,
 324                                              c_shrink);
 325        return mb_cache_shrink(cache, sc->nr_to_scan);
 326}
 327
 328/* We shrink 1/X of the cache when we have too many entries in it */
 329#define SHRINK_DIVISOR 16
 330
 331static void mb_cache_shrink_worker(struct work_struct *work)
 332{
 333        struct mb_cache *cache = container_of(work, struct mb_cache,
 334                                              c_shrink_work);
 335        mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
 336}
 337
 338/*
 339 * mb_cache_create - create cache
 340 * @bucket_bits: log2 of the hash table size
 341 *
 342 * Create cache for keys with 2^bucket_bits hash entries.
 343 */
 344struct mb_cache *mb_cache_create(int bucket_bits)
 345{
 346        struct mb_cache *cache;
 347        unsigned long bucket_count = 1UL << bucket_bits;
 348        unsigned long i;
 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_array(bucket_count,
 358                                      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
 380err_out:
 381        return NULL;
 382}
 383EXPORT_SYMBOL(mb_cache_create);
 384
 385/*
 386 * mb_cache_destroy - destroy cache
 387 * @cache: the cache to destroy
 388 *
 389 * Free all entries in cache and cache itself. Caller must make sure nobody
 390 * (except shrinker) can reach @cache when calling this.
 391 */
 392void mb_cache_destroy(struct mb_cache *cache)
 393{
 394        struct mb_cache_entry *entry, *next;
 395
 396        unregister_shrinker(&cache->c_shrink);
 397
 398        /*
 399         * We don't bother with any locking. Cache must not be used at this
 400         * point.
 401         */
 402        list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
 403                if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 404                        hlist_bl_del_init(&entry->e_hash_list);
 405                        atomic_dec(&entry->e_refcnt);
 406                } else
 407                        WARN_ON(1);
 408                list_del(&entry->e_list);
 409                WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 410                mb_cache_entry_put(cache, entry);
 411        }
 412        kfree(cache->c_hash);
 413        kfree(cache);
 414}
 415EXPORT_SYMBOL(mb_cache_destroy);
 416
 417static int __init mbcache_init(void)
 418{
 419        mb_entry_cache = kmem_cache_create("mbcache",
 420                                sizeof(struct mb_cache_entry), 0,
 421                                SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
 422        if (!mb_entry_cache)
 423                return -ENOMEM;
 424        return 0;
 425}
 426
 427static void __exit mbcache_exit(void)
 428{
 429        kmem_cache_destroy(mb_entry_cache);
 430}
 431
 432module_init(mbcache_init)
 433module_exit(mbcache_exit)
 434
 435MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
 436MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
 437MODULE_LICENSE("GPL");
 438
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.