xref: /openbmc/linux/fs/mbcache.c (revision f42b3800)
1 /*
2  * linux/fs/mbcache.c
3  * (C) 2001-2002 Andreas Gruenbacher, <a.gruenbacher@computer.org>
4  */
5 
6 /*
7  * Filesystem Meta Information Block Cache (mbcache)
8  *
9  * The mbcache caches blocks of block devices that need to be located
10  * by their device/block number, as well as by other criteria (such
11  * as the block's contents).
12  *
13  * There can only be one cache entry in a cache per device and block number.
14  * Additional indexes need not be unique in this sense. The number of
15  * additional indexes (=other criteria) can be hardwired at compile time
16  * or specified at cache create time.
17  *
18  * Each cache entry is of fixed size. An entry may be `valid' or `invalid'
19  * in the cache. A valid entry is in the main hash tables of the cache,
20  * and may also be in the lru list. An invalid entry is not in any hashes
21  * or lists.
22  *
23  * A valid cache entry is only in the lru list if no handles refer to it.
24  * Invalid cache entries will be freed when the last handle to the cache
25  * entry is released. Entries that cannot be freed immediately are put
26  * back on the lru list.
27  */
28 
29 #include <linux/kernel.h>
30 #include <linux/module.h>
31 
32 #include <linux/hash.h>
33 #include <linux/fs.h>
34 #include <linux/mm.h>
35 #include <linux/slab.h>
36 #include <linux/sched.h>
37 #include <linux/init.h>
38 #include <linux/mbcache.h>
39 
40 
41 #ifdef MB_CACHE_DEBUG
42 # define mb_debug(f...) do { \
43 		printk(KERN_DEBUG f); \
44 		printk("\n"); \
45 	} while (0)
46 #define mb_assert(c) do { if (!(c)) \
47 		printk(KERN_ERR "assertion " #c " failed\n"); \
48 	} while(0)
49 #else
50 # define mb_debug(f...) do { } while(0)
51 # define mb_assert(c) do { } while(0)
52 #endif
53 #define mb_error(f...) do { \
54 		printk(KERN_ERR f); \
55 		printk("\n"); \
56 	} while(0)
57 
58 #define MB_CACHE_WRITER ((unsigned short)~0U >> 1)
59 
60 static DECLARE_WAIT_QUEUE_HEAD(mb_cache_queue);
61 
62 MODULE_AUTHOR("Andreas Gruenbacher <a.gruenbacher@computer.org>");
63 MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
64 MODULE_LICENSE("GPL");
65 
66 EXPORT_SYMBOL(mb_cache_create);
67 EXPORT_SYMBOL(mb_cache_shrink);
68 EXPORT_SYMBOL(mb_cache_destroy);
69 EXPORT_SYMBOL(mb_cache_entry_alloc);
70 EXPORT_SYMBOL(mb_cache_entry_insert);
71 EXPORT_SYMBOL(mb_cache_entry_release);
72 EXPORT_SYMBOL(mb_cache_entry_free);
73 EXPORT_SYMBOL(mb_cache_entry_get);
74 #if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
75 EXPORT_SYMBOL(mb_cache_entry_find_first);
76 EXPORT_SYMBOL(mb_cache_entry_find_next);
77 #endif
78 
79 struct mb_cache {
80 	struct list_head		c_cache_list;
81 	const char			*c_name;
82 	struct mb_cache_op		c_op;
83 	atomic_t			c_entry_count;
84 	int				c_bucket_bits;
85 #ifndef MB_CACHE_INDEXES_COUNT
86 	int				c_indexes_count;
87 #endif
88 	struct kmem_cache			*c_entry_cache;
89 	struct list_head		*c_block_hash;
90 	struct list_head		*c_indexes_hash[0];
91 };
92 
93 
94 /*
95  * Global data: list of all mbcache's, lru list, and a spinlock for
96  * accessing cache data structures on SMP machines. The lru list is
97  * global across all mbcaches.
98  */
99 
100 static LIST_HEAD(mb_cache_list);
101 static LIST_HEAD(mb_cache_lru_list);
102 static DEFINE_SPINLOCK(mb_cache_spinlock);
103 
104 static inline int
105 mb_cache_indexes(struct mb_cache *cache)
106 {
107 #ifdef MB_CACHE_INDEXES_COUNT
108 	return MB_CACHE_INDEXES_COUNT;
109 #else
110 	return cache->c_indexes_count;
111 #endif
112 }
113 
114 /*
115  * What the mbcache registers as to get shrunk dynamically.
116  */
117 
118 static int mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask);
119 
120 static struct shrinker mb_cache_shrinker = {
121 	.shrink = mb_cache_shrink_fn,
122 	.seeks = DEFAULT_SEEKS,
123 };
124 
125 static inline int
126 __mb_cache_entry_is_hashed(struct mb_cache_entry *ce)
127 {
128 	return !list_empty(&ce->e_block_list);
129 }
130 
131 
132 static void
133 __mb_cache_entry_unhash(struct mb_cache_entry *ce)
134 {
135 	int n;
136 
137 	if (__mb_cache_entry_is_hashed(ce)) {
138 		list_del_init(&ce->e_block_list);
139 		for (n=0; n<mb_cache_indexes(ce->e_cache); n++)
140 			list_del(&ce->e_indexes[n].o_list);
141 	}
142 }
143 
144 
145 static void
146 __mb_cache_entry_forget(struct mb_cache_entry *ce, gfp_t gfp_mask)
147 {
148 	struct mb_cache *cache = ce->e_cache;
149 
150 	mb_assert(!(ce->e_used || ce->e_queued));
151 	if (cache->c_op.free && cache->c_op.free(ce, gfp_mask)) {
152 		/* free failed -- put back on the lru list
153 		   for freeing later. */
154 		spin_lock(&mb_cache_spinlock);
155 		list_add(&ce->e_lru_list, &mb_cache_lru_list);
156 		spin_unlock(&mb_cache_spinlock);
157 	} else {
158 		kmem_cache_free(cache->c_entry_cache, ce);
159 		atomic_dec(&cache->c_entry_count);
160 	}
161 }
162 
163 
164 static void
165 __mb_cache_entry_release_unlock(struct mb_cache_entry *ce)
166 	__releases(mb_cache_spinlock)
167 {
168 	/* Wake up all processes queuing for this cache entry. */
169 	if (ce->e_queued)
170 		wake_up_all(&mb_cache_queue);
171 	if (ce->e_used >= MB_CACHE_WRITER)
172 		ce->e_used -= MB_CACHE_WRITER;
173 	ce->e_used--;
174 	if (!(ce->e_used || ce->e_queued)) {
175 		if (!__mb_cache_entry_is_hashed(ce))
176 			goto forget;
177 		mb_assert(list_empty(&ce->e_lru_list));
178 		list_add_tail(&ce->e_lru_list, &mb_cache_lru_list);
179 	}
180 	spin_unlock(&mb_cache_spinlock);
181 	return;
182 forget:
183 	spin_unlock(&mb_cache_spinlock);
184 	__mb_cache_entry_forget(ce, GFP_KERNEL);
185 }
186 
187 
188 /*
189  * mb_cache_shrink_fn()  memory pressure callback
190  *
191  * This function is called by the kernel memory management when memory
192  * gets low.
193  *
194  * @nr_to_scan: Number of objects to scan
195  * @gfp_mask: (ignored)
196  *
197  * Returns the number of objects which are present in the cache.
198  */
199 static int
200 mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
201 {
202 	LIST_HEAD(free_list);
203 	struct list_head *l, *ltmp;
204 	int count = 0;
205 
206 	spin_lock(&mb_cache_spinlock);
207 	list_for_each(l, &mb_cache_list) {
208 		struct mb_cache *cache =
209 			list_entry(l, struct mb_cache, c_cache_list);
210 		mb_debug("cache %s (%d)", cache->c_name,
211 			  atomic_read(&cache->c_entry_count));
212 		count += atomic_read(&cache->c_entry_count);
213 	}
214 	mb_debug("trying to free %d entries", nr_to_scan);
215 	if (nr_to_scan == 0) {
216 		spin_unlock(&mb_cache_spinlock);
217 		goto out;
218 	}
219 	while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
220 		struct mb_cache_entry *ce =
221 			list_entry(mb_cache_lru_list.next,
222 				   struct mb_cache_entry, e_lru_list);
223 		list_move_tail(&ce->e_lru_list, &free_list);
224 		__mb_cache_entry_unhash(ce);
225 	}
226 	spin_unlock(&mb_cache_spinlock);
227 	list_for_each_safe(l, ltmp, &free_list) {
228 		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
229 						   e_lru_list), gfp_mask);
230 	}
231 out:
232 	return (count / 100) * sysctl_vfs_cache_pressure;
233 }
234 
235 
236 /*
237  * mb_cache_create()  create a new cache
238  *
239  * All entries in one cache are equal size. Cache entries may be from
240  * multiple devices. If this is the first mbcache created, registers
241  * the cache with kernel memory management. Returns NULL if no more
242  * memory was available.
243  *
244  * @name: name of the cache (informal)
245  * @cache_op: contains the callback called when freeing a cache entry
246  * @entry_size: The size of a cache entry, including
247  *              struct mb_cache_entry
248  * @indexes_count: number of additional indexes in the cache. Must equal
249  *                 MB_CACHE_INDEXES_COUNT if the number of indexes is
250  *                 hardwired.
251  * @bucket_bits: log2(number of hash buckets)
252  */
253 struct mb_cache *
254 mb_cache_create(const char *name, struct mb_cache_op *cache_op,
255 		size_t entry_size, int indexes_count, int bucket_bits)
256 {
257 	int m=0, n, bucket_count = 1 << bucket_bits;
258 	struct mb_cache *cache = NULL;
259 
260 	if(entry_size < sizeof(struct mb_cache_entry) +
261 	   indexes_count * sizeof(((struct mb_cache_entry *) 0)->e_indexes[0]))
262 		return NULL;
263 
264 	cache = kmalloc(sizeof(struct mb_cache) +
265 	                indexes_count * sizeof(struct list_head), GFP_KERNEL);
266 	if (!cache)
267 		goto fail;
268 	cache->c_name = name;
269 	cache->c_op.free = NULL;
270 	if (cache_op)
271 		cache->c_op.free = cache_op->free;
272 	atomic_set(&cache->c_entry_count, 0);
273 	cache->c_bucket_bits = bucket_bits;
274 #ifdef MB_CACHE_INDEXES_COUNT
275 	mb_assert(indexes_count == MB_CACHE_INDEXES_COUNT);
276 #else
277 	cache->c_indexes_count = indexes_count;
278 #endif
279 	cache->c_block_hash = kmalloc(bucket_count * sizeof(struct list_head),
280 	                              GFP_KERNEL);
281 	if (!cache->c_block_hash)
282 		goto fail;
283 	for (n=0; n<bucket_count; n++)
284 		INIT_LIST_HEAD(&cache->c_block_hash[n]);
285 	for (m=0; m<indexes_count; m++) {
286 		cache->c_indexes_hash[m] = kmalloc(bucket_count *
287 		                                 sizeof(struct list_head),
288 		                                 GFP_KERNEL);
289 		if (!cache->c_indexes_hash[m])
290 			goto fail;
291 		for (n=0; n<bucket_count; n++)
292 			INIT_LIST_HEAD(&cache->c_indexes_hash[m][n]);
293 	}
294 	cache->c_entry_cache = kmem_cache_create(name, entry_size, 0,
295 		SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
296 	if (!cache->c_entry_cache)
297 		goto fail;
298 
299 	spin_lock(&mb_cache_spinlock);
300 	list_add(&cache->c_cache_list, &mb_cache_list);
301 	spin_unlock(&mb_cache_spinlock);
302 	return cache;
303 
304 fail:
305 	if (cache) {
306 		while (--m >= 0)
307 			kfree(cache->c_indexes_hash[m]);
308 		kfree(cache->c_block_hash);
309 		kfree(cache);
310 	}
311 	return NULL;
312 }
313 
314 
315 /*
316  * mb_cache_shrink()
317  *
318  * Removes all cache entries of a device from the cache. All cache entries
319  * currently in use cannot be freed, and thus remain in the cache. All others
320  * are freed.
321  *
322  * @bdev: which device's cache entries to shrink
323  */
324 void
325 mb_cache_shrink(struct block_device *bdev)
326 {
327 	LIST_HEAD(free_list);
328 	struct list_head *l, *ltmp;
329 
330 	spin_lock(&mb_cache_spinlock);
331 	list_for_each_safe(l, ltmp, &mb_cache_lru_list) {
332 		struct mb_cache_entry *ce =
333 			list_entry(l, struct mb_cache_entry, e_lru_list);
334 		if (ce->e_bdev == bdev) {
335 			list_move_tail(&ce->e_lru_list, &free_list);
336 			__mb_cache_entry_unhash(ce);
337 		}
338 	}
339 	spin_unlock(&mb_cache_spinlock);
340 	list_for_each_safe(l, ltmp, &free_list) {
341 		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
342 						   e_lru_list), GFP_KERNEL);
343 	}
344 }
345 
346 
347 /*
348  * mb_cache_destroy()
349  *
350  * Shrinks the cache to its minimum possible size (hopefully 0 entries),
351  * and then destroys it. If this was the last mbcache, un-registers the
352  * mbcache from kernel memory management.
353  */
354 void
355 mb_cache_destroy(struct mb_cache *cache)
356 {
357 	LIST_HEAD(free_list);
358 	struct list_head *l, *ltmp;
359 	int n;
360 
361 	spin_lock(&mb_cache_spinlock);
362 	list_for_each_safe(l, ltmp, &mb_cache_lru_list) {
363 		struct mb_cache_entry *ce =
364 			list_entry(l, struct mb_cache_entry, e_lru_list);
365 		if (ce->e_cache == cache) {
366 			list_move_tail(&ce->e_lru_list, &free_list);
367 			__mb_cache_entry_unhash(ce);
368 		}
369 	}
370 	list_del(&cache->c_cache_list);
371 	spin_unlock(&mb_cache_spinlock);
372 
373 	list_for_each_safe(l, ltmp, &free_list) {
374 		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
375 						   e_lru_list), GFP_KERNEL);
376 	}
377 
378 	if (atomic_read(&cache->c_entry_count) > 0) {
379 		mb_error("cache %s: %d orphaned entries",
380 			  cache->c_name,
381 			  atomic_read(&cache->c_entry_count));
382 	}
383 
384 	kmem_cache_destroy(cache->c_entry_cache);
385 
386 	for (n=0; n < mb_cache_indexes(cache); n++)
387 		kfree(cache->c_indexes_hash[n]);
388 	kfree(cache->c_block_hash);
389 	kfree(cache);
390 }
391 
392 
393 /*
394  * mb_cache_entry_alloc()
395  *
396  * Allocates a new cache entry. The new entry will not be valid initially,
397  * and thus cannot be looked up yet. It should be filled with data, and
398  * then inserted into the cache using mb_cache_entry_insert(). Returns NULL
399  * if no more memory was available.
400  */
401 struct mb_cache_entry *
402 mb_cache_entry_alloc(struct mb_cache *cache, gfp_t gfp_flags)
403 {
404 	struct mb_cache_entry *ce;
405 
406 	ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
407 	if (ce) {
408 		atomic_inc(&cache->c_entry_count);
409 		INIT_LIST_HEAD(&ce->e_lru_list);
410 		INIT_LIST_HEAD(&ce->e_block_list);
411 		ce->e_cache = cache;
412 		ce->e_used = 1 + MB_CACHE_WRITER;
413 		ce->e_queued = 0;
414 	}
415 	return ce;
416 }
417 
418 
419 /*
420  * mb_cache_entry_insert()
421  *
422  * Inserts an entry that was allocated using mb_cache_entry_alloc() into
423  * the cache. After this, the cache entry can be looked up, but is not yet
424  * in the lru list as the caller still holds a handle to it. Returns 0 on
425  * success, or -EBUSY if a cache entry for that device + inode exists
426  * already (this may happen after a failed lookup, but when another process
427  * has inserted the same cache entry in the meantime).
428  *
429  * @bdev: device the cache entry belongs to
430  * @block: block number
431  * @keys: array of additional keys. There must be indexes_count entries
432  *        in the array (as specified when creating the cache).
433  */
434 int
435 mb_cache_entry_insert(struct mb_cache_entry *ce, struct block_device *bdev,
436 		      sector_t block, unsigned int keys[])
437 {
438 	struct mb_cache *cache = ce->e_cache;
439 	unsigned int bucket;
440 	struct list_head *l;
441 	int error = -EBUSY, n;
442 
443 	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff),
444 			   cache->c_bucket_bits);
445 	spin_lock(&mb_cache_spinlock);
446 	list_for_each_prev(l, &cache->c_block_hash[bucket]) {
447 		struct mb_cache_entry *ce =
448 			list_entry(l, struct mb_cache_entry, e_block_list);
449 		if (ce->e_bdev == bdev && ce->e_block == block)
450 			goto out;
451 	}
452 	__mb_cache_entry_unhash(ce);
453 	ce->e_bdev = bdev;
454 	ce->e_block = block;
455 	list_add(&ce->e_block_list, &cache->c_block_hash[bucket]);
456 	for (n=0; n<mb_cache_indexes(cache); n++) {
457 		ce->e_indexes[n].o_key = keys[n];
458 		bucket = hash_long(keys[n], cache->c_bucket_bits);
459 		list_add(&ce->e_indexes[n].o_list,
460 			 &cache->c_indexes_hash[n][bucket]);
461 	}
462 	error = 0;
463 out:
464 	spin_unlock(&mb_cache_spinlock);
465 	return error;
466 }
467 
468 
469 /*
470  * mb_cache_entry_release()
471  *
472  * Release a handle to a cache entry. When the last handle to a cache entry
473  * is released it is either freed (if it is invalid) or otherwise inserted
474  * in to the lru list.
475  */
476 void
477 mb_cache_entry_release(struct mb_cache_entry *ce)
478 {
479 	spin_lock(&mb_cache_spinlock);
480 	__mb_cache_entry_release_unlock(ce);
481 }
482 
483 
484 /*
485  * mb_cache_entry_free()
486  *
487  * This is equivalent to the sequence mb_cache_entry_takeout() --
488  * mb_cache_entry_release().
489  */
490 void
491 mb_cache_entry_free(struct mb_cache_entry *ce)
492 {
493 	spin_lock(&mb_cache_spinlock);
494 	mb_assert(list_empty(&ce->e_lru_list));
495 	__mb_cache_entry_unhash(ce);
496 	__mb_cache_entry_release_unlock(ce);
497 }
498 
499 
500 /*
501  * mb_cache_entry_get()
502  *
503  * Get a cache entry  by device / block number. (There can only be one entry
504  * in the cache per device and block.) Returns NULL if no such cache entry
505  * exists. The returned cache entry is locked for exclusive access ("single
506  * writer").
507  */
508 struct mb_cache_entry *
509 mb_cache_entry_get(struct mb_cache *cache, struct block_device *bdev,
510 		   sector_t block)
511 {
512 	unsigned int bucket;
513 	struct list_head *l;
514 	struct mb_cache_entry *ce;
515 
516 	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff),
517 			   cache->c_bucket_bits);
518 	spin_lock(&mb_cache_spinlock);
519 	list_for_each(l, &cache->c_block_hash[bucket]) {
520 		ce = list_entry(l, struct mb_cache_entry, e_block_list);
521 		if (ce->e_bdev == bdev && ce->e_block == block) {
522 			DEFINE_WAIT(wait);
523 
524 			if (!list_empty(&ce->e_lru_list))
525 				list_del_init(&ce->e_lru_list);
526 
527 			while (ce->e_used > 0) {
528 				ce->e_queued++;
529 				prepare_to_wait(&mb_cache_queue, &wait,
530 						TASK_UNINTERRUPTIBLE);
531 				spin_unlock(&mb_cache_spinlock);
532 				schedule();
533 				spin_lock(&mb_cache_spinlock);
534 				ce->e_queued--;
535 			}
536 			finish_wait(&mb_cache_queue, &wait);
537 			ce->e_used += 1 + MB_CACHE_WRITER;
538 
539 			if (!__mb_cache_entry_is_hashed(ce)) {
540 				__mb_cache_entry_release_unlock(ce);
541 				return NULL;
542 			}
543 			goto cleanup;
544 		}
545 	}
546 	ce = NULL;
547 
548 cleanup:
549 	spin_unlock(&mb_cache_spinlock);
550 	return ce;
551 }
552 
553 #if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
554 
555 static struct mb_cache_entry *
556 __mb_cache_entry_find(struct list_head *l, struct list_head *head,
557 		      int index, struct block_device *bdev, unsigned int key)
558 {
559 	while (l != head) {
560 		struct mb_cache_entry *ce =
561 			list_entry(l, struct mb_cache_entry,
562 			           e_indexes[index].o_list);
563 		if (ce->e_bdev == bdev && ce->e_indexes[index].o_key == key) {
564 			DEFINE_WAIT(wait);
565 
566 			if (!list_empty(&ce->e_lru_list))
567 				list_del_init(&ce->e_lru_list);
568 
569 			/* Incrementing before holding the lock gives readers
570 			   priority over writers. */
571 			ce->e_used++;
572 			while (ce->e_used >= MB_CACHE_WRITER) {
573 				ce->e_queued++;
574 				prepare_to_wait(&mb_cache_queue, &wait,
575 						TASK_UNINTERRUPTIBLE);
576 				spin_unlock(&mb_cache_spinlock);
577 				schedule();
578 				spin_lock(&mb_cache_spinlock);
579 				ce->e_queued--;
580 			}
581 			finish_wait(&mb_cache_queue, &wait);
582 
583 			if (!__mb_cache_entry_is_hashed(ce)) {
584 				__mb_cache_entry_release_unlock(ce);
585 				spin_lock(&mb_cache_spinlock);
586 				return ERR_PTR(-EAGAIN);
587 			}
588 			return ce;
589 		}
590 		l = l->next;
591 	}
592 	return NULL;
593 }
594 
595 
596 /*
597  * mb_cache_entry_find_first()
598  *
599  * Find the first cache entry on a given device with a certain key in
600  * an additional index. Additonal matches can be found with
601  * mb_cache_entry_find_next(). Returns NULL if no match was found. The
602  * returned cache entry is locked for shared access ("multiple readers").
603  *
604  * @cache: the cache to search
605  * @index: the number of the additonal index to search (0<=index<indexes_count)
606  * @bdev: the device the cache entry should belong to
607  * @key: the key in the index
608  */
609 struct mb_cache_entry *
610 mb_cache_entry_find_first(struct mb_cache *cache, int index,
611 			  struct block_device *bdev, unsigned int key)
612 {
613 	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
614 	struct list_head *l;
615 	struct mb_cache_entry *ce;
616 
617 	mb_assert(index < mb_cache_indexes(cache));
618 	spin_lock(&mb_cache_spinlock);
619 	l = cache->c_indexes_hash[index][bucket].next;
620 	ce = __mb_cache_entry_find(l, &cache->c_indexes_hash[index][bucket],
621 	                           index, bdev, key);
622 	spin_unlock(&mb_cache_spinlock);
623 	return ce;
624 }
625 
626 
627 /*
628  * mb_cache_entry_find_next()
629  *
630  * Find the next cache entry on a given device with a certain key in an
631  * additional index. Returns NULL if no match could be found. The previous
632  * entry is atomatically released, so that mb_cache_entry_find_next() can
633  * be called like this:
634  *
635  * entry = mb_cache_entry_find_first();
636  * while (entry) {
637  * 	...
638  *	entry = mb_cache_entry_find_next(entry, ...);
639  * }
640  *
641  * @prev: The previous match
642  * @index: the number of the additonal index to search (0<=index<indexes_count)
643  * @bdev: the device the cache entry should belong to
644  * @key: the key in the index
645  */
646 struct mb_cache_entry *
647 mb_cache_entry_find_next(struct mb_cache_entry *prev, int index,
648 			 struct block_device *bdev, unsigned int key)
649 {
650 	struct mb_cache *cache = prev->e_cache;
651 	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
652 	struct list_head *l;
653 	struct mb_cache_entry *ce;
654 
655 	mb_assert(index < mb_cache_indexes(cache));
656 	spin_lock(&mb_cache_spinlock);
657 	l = prev->e_indexes[index].o_list.next;
658 	ce = __mb_cache_entry_find(l, &cache->c_indexes_hash[index][bucket],
659 	                           index, bdev, key);
660 	__mb_cache_entry_release_unlock(prev);
661 	return ce;
662 }
663 
664 #endif  /* !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0) */
665 
666 static int __init init_mbcache(void)
667 {
668 	register_shrinker(&mb_cache_shrinker);
669 	return 0;
670 }
671 
672 static void __exit exit_mbcache(void)
673 {
674 	unregister_shrinker(&mb_cache_shrinker);
675 }
676 
677 module_init(init_mbcache)
678 module_exit(exit_mbcache)
679 
680