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Linux/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         unsigned long           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 long 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 reusable entry with the given key
159  * @cache: cache where we should search
160  * @key: key to look for
161  *
162  * Search in @cache for a reusable entry with key @key. Grabs reference to the
163  * first reusable 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 reusable entry with the same key
174  * @cache: cache where we should search
175  * @entry: entry to start search from
176  *
177  * Finds next reusable 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 with the
179  * search), finds the first reusable entry in the hash chain. The function drops
180  * 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 long nr_to_scan)
278 {
279         struct mb_cache_entry *entry;
280         struct hlist_bl_head *head;
281         unsigned long 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(&entry->e_list, &cache->c_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         struct mb_cache *cache = container_of(shrink, struct mb_cache,
320                                               c_shrink);
321         return mb_cache_shrink(cache, sc->nr_to_scan);
322 }
323 
324 /* We shrink 1/X of the cache when we have too many entries in it */
325 #define SHRINK_DIVISOR 16
326 
327 static void mb_cache_shrink_worker(struct work_struct *work)
328 {
329         struct mb_cache *cache = container_of(work, struct mb_cache,
330                                               c_shrink_work);
331         mb_cache_shrink(cache, cache->c_max_entries / SHRINK_DIVISOR);
332 }
333 
334 /*
335  * mb_cache_create - create cache
336  * @bucket_bits: log2 of the hash table size
337  *
338  * Create cache for keys with 2^bucket_bits hash entries.
339  */
340 struct mb_cache *mb_cache_create(int bucket_bits)
341 {
342         struct mb_cache *cache;
343         unsigned long bucket_count = 1UL << bucket_bits;
344         unsigned long i;
345 
346         cache = kzalloc(sizeof(struct mb_cache), GFP_KERNEL);
347         if (!cache)
348                 goto err_out;
349         cache->c_bucket_bits = bucket_bits;
350         cache->c_max_entries = bucket_count << 4;
351         INIT_LIST_HEAD(&cache->c_list);
352         spin_lock_init(&cache->c_list_lock);
353         cache->c_hash = kmalloc(bucket_count * sizeof(struct hlist_bl_head),
354                                 GFP_KERNEL);
355         if (!cache->c_hash) {
356                 kfree(cache);
357                 goto err_out;
358         }
359         for (i = 0; i < bucket_count; i++)
360                 INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
361 
362         cache->c_shrink.count_objects = mb_cache_count;
363         cache->c_shrink.scan_objects = mb_cache_scan;
364         cache->c_shrink.seeks = DEFAULT_SEEKS;
365         if (register_shrinker(&cache->c_shrink)) {
366                 kfree(cache->c_hash);
367                 kfree(cache);
368                 goto err_out;
369         }
370 
371         INIT_WORK(&cache->c_shrink_work, mb_cache_shrink_worker);
372 
373         return cache;
374 
375 err_out:
376         return NULL;
377 }
378 EXPORT_SYMBOL(mb_cache_create);
379 
380 /*
381  * mb_cache_destroy - destroy cache
382  * @cache: the cache to destroy
383  *
384  * Free all entries in cache and cache itself. Caller must make sure nobody
385  * (except shrinker) can reach @cache when calling this.
386  */
387 void mb_cache_destroy(struct mb_cache *cache)
388 {
389         struct mb_cache_entry *entry, *next;
390 
391         unregister_shrinker(&cache->c_shrink);
392 
393         /*
394          * We don't bother with any locking. Cache must not be used at this
395          * point.
396          */
397         list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
398                 if (!hlist_bl_unhashed(&entry->e_hash_list)) {
399                         hlist_bl_del_init(&entry->e_hash_list);
400                         atomic_dec(&entry->e_refcnt);
401                 } else
402                         WARN_ON(1);
403                 list_del(&entry->e_list);
404                 WARN_ON(atomic_read(&entry->e_refcnt) != 1);
405                 mb_cache_entry_put(cache, entry);
406         }
407         kfree(cache->c_hash);
408         kfree(cache);
409 }
410 EXPORT_SYMBOL(mb_cache_destroy);
411 
412 static int __init mbcache_init(void)
413 {
414         mb_entry_cache = kmem_cache_create("mbcache",
415                                 sizeof(struct mb_cache_entry), 0,
416                                 SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
417         if (!mb_entry_cache)
418                 return -ENOMEM;
419         return 0;
420 }
421 
422 static void __exit mbcache_exit(void)
423 {
424         kmem_cache_destroy(mb_entry_cache);
425 }
426 
427 module_init(mbcache_init)
428 module_exit(mbcache_exit)
429 
430 MODULE_AUTHOR("Jan Kara <jack@suse.cz>");
431 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
432 MODULE_LICENSE("GPL");
433 

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