aboutsummaryrefslogtreecommitdiffstats
path: root/fs/mbcache.c
blob: bf166e388f0d4cb8b8826698df51fe759aa6f99f (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
/*
 * linux/fs/mbcache.c
 * (C) 2001-2002 Andreas Gruenbacher, <a.gruenbacher@computer.org>
 */

/*
 * Filesystem Meta Information Block Cache (mbcache)
 *
 * The mbcache caches blocks of block devices that need to be located
 * by their device/block number, as well as by other criteria (such
 * as the block's contents).
 *
 * There can only be one cache entry in a cache per device and block number.
 * Additional indexes need not be unique in this sense. The number of
 * additional indexes (=other criteria) can be hardwired at compile time
 * or specified at cache create time.
 *
 * Each cache entry is of fixed size. An entry may be `valid' or `invalid'
 * in the cache. A valid entry is in the main hash tables of the cache,
 * and may also be in the lru list. An invalid entry is not in any hashes
 * or lists.
 *
 * A valid cache entry is only in the lru list if no handles refer to it.
 * Invalid cache entries will be freed when the last handle to the cache
 * entry is released. Entries that cannot be freed immediately are put
 * back on the lru list.
 */

/*
 * Lock descriptions and usage:
 *
 * Each hash chain of both the block and index hash tables now contains
 * a built-in lock used to serialize accesses to the hash chain.
 *
 * Accesses to global data structures mb_cache_list and mb_cache_lru_list
 * are serialized via the global spinlock mb_cache_spinlock.
 *
 * Each mb_cache_entry contains a spinlock, e_entry_lock, to serialize
 * accesses to its local data, such as e_used and e_queued.
 *
 * Lock ordering:
 *
 * Each block hash chain's lock has the highest lock order, followed by an
 * index hash chain's lock, mb_cache_bg_lock (used to implement mb_cache_entry's
 * lock), and mb_cach_spinlock, with the lowest order.  While holding
 * either a block or index hash chain lock, a thread can acquire an
 * mc_cache_bg_lock, which in turn can also acquire mb_cache_spinlock.
 *
 * Synchronization:
 *
 * Since both mb_cache_entry_get and mb_cache_entry_find scan the block and
 * index hash chian, it needs to lock the corresponding hash chain.  For each
 * mb_cache_entry within the chain, it needs to lock the mb_cache_entry to
 * prevent either any simultaneous release or free on the entry and also
 * to serialize accesses to either the e_used or e_queued member of the entry.
 *
 * To avoid having a dangling reference to an already freed
 * mb_cache_entry, an mb_cache_entry is only freed when it is not on a
 * block hash chain and also no longer being referenced, both e_used,
 * and e_queued are 0's.  When an mb_cache_entry is explicitly freed it is
 * first removed from a block hash chain.
 */

#include <linux/kernel.h>
#include <linux/module.h>

#include <linux/hash.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/list_bl.h>
#include <linux/mbcache.h>
#include <linux/init.h>
#include <linux/blockgroup_lock.h>

#ifdef MB_CACHE_DEBUG
# define mb_debug(f...) do { \
		printk(KERN_DEBUG f); \
		printk("\n"); \
	} while (0)
#define mb_assert(c) do { if (!(c)) \
		printk(KERN_ERR "assertion " #c " failed\n"); \
	} while(0)
#else
# define mb_debug(f...) do { } while(0)
# define mb_assert(c) do { } while(0)
#endif
#define mb_error(f...) do { \
		printk(KERN_ERR f); \
		printk("\n"); \
	} while(0)

#define MB_CACHE_WRITER ((unsigned short)~0U >> 1)

#define MB_CACHE_ENTRY_LOCK_BITS	__builtin_log2(NR_BG_LOCKS)
#define	MB_CACHE_ENTRY_LOCK_INDEX(ce)			\
	(hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))

static DECLARE_WAIT_QUEUE_HEAD(mb_cache_queue);
static struct blockgroup_lock *mb_cache_bg_lock;
static struct kmem_cache *mb_cache_kmem_cache;

MODULE_AUTHOR("Andreas Gruenbacher <a.gruenbacher@computer.org>");
MODULE_DESCRIPTION("Meta block cache (for extended attributes)");
MODULE_LICENSE("GPL");

EXPORT_SYMBOL(mb_cache_create);
EXPORT_SYMBOL(mb_cache_shrink);
EXPORT_SYMBOL(mb_cache_destroy);
EXPORT_SYMBOL(mb_cache_entry_alloc);
EXPORT_SYMBOL(mb_cache_entry_insert);
EXPORT_SYMBOL(mb_cache_entry_release);
EXPORT_SYMBOL(mb_cache_entry_free);
EXPORT_SYMBOL(mb_cache_entry_get);
#if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)
EXPORT_SYMBOL(mb_cache_entry_find_first);
EXPORT_SYMBOL(mb_cache_entry_find_next);
#endif

/*
 * Global data: list of all mbcache's, lru list, and a spinlock for
 * accessing cache data structures on SMP machines. The lru list is
 * global across all mbcaches.
 */

static LIST_HEAD(mb_cache_list);
static LIST_HEAD(mb_cache_lru_list);
static DEFINE_SPINLOCK(mb_cache_spinlock);

static inline void
__spin_lock_mb_cache_entry(struct mb_cache_entry *ce)
{
	spin_lock(bgl_lock_ptr(mb_cache_bg_lock,
		MB_CACHE_ENTRY_LOCK_INDEX(ce)));
}

static inline void
__spin_unlock_mb_cache_entry(struct mb_cache_entry *ce)
{
	spin_unlock(bgl_lock_ptr(mb_cache_bg_lock,
		MB_CACHE_ENTRY_LOCK_INDEX(ce)));
}

static inline int
__mb_cache_entry_is_block_hashed(struct mb_cache_entry *ce)
{
	return !hlist_bl_unhashed(&ce->e_block_list);
}


static inline void
__mb_cache_entry_unhash_block(struct mb_cache_entry *ce)
{
	if (__mb_cache_entry_is_block_hashed(ce))
		hlist_bl_del_init(&ce->e_block_list);
}

static inline int
__mb_cache_entry_is_index_hashed(struct mb_cache_entry *ce)
{
	return !hlist_bl_unhashed(&ce->e_index.o_list);
}

static inline void
__mb_cache_entry_unhash_index(struct mb_cache_entry *ce)
{
	if (__mb_cache_entry_is_index_hashed(ce))
		hlist_bl_del_init(&ce->e_index.o_list);
}

/*
 * __mb_cache_entry_unhash_unlock()
 *
 * This function is called to unhash both the block and index hash
 * chain.
 * It assumes both the block and index hash chain is locked upon entry.
 * It also unlock both hash chains both exit
 */
static inline void
__mb_cache_entry_unhash_unlock(struct mb_cache_entry *ce)
{
	__mb_cache_entry_unhash_index(ce);
	hlist_bl_unlock(ce->e_index_hash_p);
	__mb_cache_entry_unhash_block(ce);
	hlist_bl_unlock(ce->e_block_hash_p);
}

static void
__mb_cache_entry_forget(struct mb_cache_entry *ce, gfp_t gfp_mask)
{
	struct mb_cache *cache = ce->e_cache;

	mb_assert(!(ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt)));
	kmem_cache_free(cache->c_entry_cache, ce);
	atomic_dec(&cache->c_entry_count);
}

static void
__mb_cache_entry_release(struct mb_cache_entry *ce)
{
	/* First lock the entry to serialize access to its local data. */
	__spin_lock_mb_cache_entry(ce);
	/* Wake up all processes queuing for this cache entry. */
	if (ce->e_queued)
		wake_up_all(&mb_cache_queue);
	if (ce->e_used >= MB_CACHE_WRITER)
		ce->e_used -= MB_CACHE_WRITER;
	/*
	 * Make sure that all cache entries on lru_list have
	 * both e_used and e_qued of 0s.
	 */
	ce->e_used--;
	if (!(ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt))) {
		if (!__mb_cache_entry_is_block_hashed(ce)) {
			__spin_unlock_mb_cache_entry(ce);
			goto forget;
		}
		/*
		 * Need access to lru list, first drop entry lock,
		 * then reacquire the lock in the proper order.
		 */
		spin_lock(&mb_cache_spinlock);
		if (list_empty(&ce->e_lru_list))
			list_add_tail(&ce->e_lru_list, &mb_cache_lru_list);
		spin_unlock(&mb_cache_spinlock);
	}
	__spin_unlock_mb_cache_entry(ce);
	return;
forget:
	mb_assert(list_empty(&ce->e_lru_list));
	__mb_cache_entry_forget(ce, GFP_KERNEL);
}

/*
 * mb_cache_shrink_scan()  memory pressure callback
 *
 * This function is called by the kernel memory management when memory
 * gets low.
 *
 * @shrink: (ignored)
 * @sc: shrink_control passed from reclaim
 *
 * Returns the number of objects freed.
 */
static unsigned long
mb_cache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
{
	LIST_HEAD(free_list);
	struct mb_cache_entry *entry, *tmp;
	int nr_to_scan = sc->nr_to_scan;
	gfp_t gfp_mask = sc->gfp_mask;
	unsigned long freed = 0;

	mb_debug("trying to free %d entries", nr_to_scan);
	spin_lock(&mb_cache_spinlock);
	while ((nr_to_scan-- > 0) && !list_empty(&mb_cache_lru_list)) {
		struct mb_cache_entry *ce =
			list_entry(mb_cache_lru_list.next,
				struct mb_cache_entry, e_lru_list);
		list_del_init(&ce->e_lru_list);
		if (ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt))
			continue;
		spin_unlock(&mb_cache_spinlock);
		/* Prevent any find or get operation on the entry */
		hlist_bl_lock(ce->e_block_hash_p);
		hlist_bl_lock(ce->e_index_hash_p);
		/* Ignore if it is touched by a find/get */
		if (ce->e_used || ce->e_queued || atomic_read(&ce->e_refcnt) ||
			!list_empty(&ce->e_lru_list)) {
			hlist_bl_unlock(ce->e_index_hash_p);
			hlist_bl_unlock(ce->e_block_hash_p);
			spin_lock(&mb_cache_spinlock);
			continue;
		}
		__mb_cache_entry_unhash_unlock(ce);
		list_add_tail(&ce->e_lru_list, &free_list);
		spin_lock(&mb_cache_spinlock);
	}
	spin_unlock(&mb_cache_spinlock);

	list_for_each_entry_safe(entry, tmp, &free_list, e_lru_list) {
		__mb_cache_entry_forget(entry, gfp_mask);
		freed++;
	}
	return freed;
}

static unsigned long
mb_cache_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
{
	struct mb_cache *cache;
	unsigned long count = 0;

	spin_lock(&mb_cache_spinlock);
	list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
		mb_debug("cache %s (%d)", cache->c_name,
			  atomic_read(&cache->c_entry_count));
		count += atomic_read(&cache->c_entry_count);
	}
	spin_unlock(&mb_cache_spinlock);

	return vfs_pressure_ratio(count);
}

static struct shrinker mb_cache_shrinker = {
	.count_objects = mb_cache_shrink_count,
	.scan_objects = mb_cache_shrink_scan,
	.seeks = DEFAULT_SEEKS,
};

/*
 * mb_cache_create()  create a new cache
 *
 * All entries in one cache are equal size. Cache entries may be from
 * multiple devices. If this is the first mbcache created, registers
 * the cache with kernel memory management. Returns NULL if no more
 * memory was available.
 *
 * @name: name of the cache (informal)
 * @bucket_bits: log2(number of hash buckets)
 */
struct mb_cache *
mb_cache_create(const char *name, int bucket_bits)
{
	int n, bucket_count = 1 << bucket_bits;
	struct mb_cache *cache = NULL;

	if (!mb_cache_bg_lock) {
		mb_cache_bg_lock = kmalloc(sizeof(struct blockgroup_lock),
			GFP_KERNEL);
		if (!mb_cache_bg_lock)
			return NULL;
		bgl_lock_init(mb_cache_bg_lock);
	}

	cache = kmalloc(sizeof(struct mb_cache), GFP_KERNEL);
	if (!cache)
		return NULL;
	cache->c_name = name;
	atomic_set(&cache->c_entry_count, 0);
	cache->c_bucket_bits = bucket_bits;
	cache->c_block_hash = kmalloc(bucket_count *
		sizeof(struct hlist_bl_head), GFP_KERNEL);
	if (!cache->c_block_hash)
		goto fail;
	for (n=0; n<bucket_count; n++)
		INIT_HLIST_BL_HEAD(&cache->c_block_hash[n]);
	cache->c_index_hash = kmalloc(bucket_count *
		sizeof(struct hlist_bl_head), GFP_KERNEL);
	if (!cache->c_index_hash)
		goto fail;
	for (n=0; n<bucket_count; n++)
		INIT_HLIST_BL_HEAD(&cache->c_index_hash[n]);
	if (!mb_cache_kmem_cache) {
		mb_cache_kmem_cache = kmem_cache_create(name,
			sizeof(struct mb_cache_entry), 0,
			SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD, NULL);
		if (!mb_cache_kmem_cache)
			goto fail2;
	}
	cache->c_entry_cache = mb_cache_kmem_cache;

	/*
	 * Set an upper limit on the number of cache entries so that the hash
	 * chains won't grow too long.
	 */
	cache->c_max_entries = bucket_count << 4;

	spin_lock(&mb_cache_spinlock);
	list_add(&cache->c_cache_list, &mb_cache_list);
	spin_unlock(&mb_cache_spinlock);
	return cache;

fail2:
	kfree(cache->c_index_hash);

fail:
	kfree(cache->c_block_hash);
	kfree(cache);
	return NULL;
}


/*
 * mb_cache_shrink()
 *
 * Removes all cache entries of a device from the cache. All cache entries
 * currently in use cannot be freed, and thus remain in the cache. All others
 * are freed.
 *
 * @bdev: which device's cache entries to shrink
 */
void
mb_cache_shrink(struct block_device *bdev)
{
	LIST_HEAD(free_list);
	struct list_head *l;
	struct mb_cache_entry *ce, *tmp;

	l = &mb_cache_lru_list;
	spin_lock(&mb_cache_spinlock);
	while (!list_is_last(l, &mb_cache_lru_list)) {
		l = l->next;
		ce = list_entry(l, struct mb_cache_entry, e_lru_list);
		if (ce->e_bdev == bdev) {
			list_del_init(&ce->e_lru_list);
			if (ce->e_used || ce->e_queued ||
				atomic_read(&ce->e_refcnt))
				continue;
			spin_unlock(&mb_cache_spinlock);
			/*
			 * Prevent any find or get operation on the entry.
			 */
			hlist_bl_lock(ce->e_block_hash_p);
			hlist_bl_lock(ce->e_index_hash_p);
			/* Ignore if it is touched by a find/get */
			if (ce->e_used || ce->e_queued ||
				atomic_read(&ce->e_refcnt) ||
				!list_empty(&ce->e_lru_list)) {
				hlist_bl_unlock(ce->e_index_hash_p);
				hlist_bl_unlock(ce->e_block_hash_p);
				l = &mb_cache_lru_list;
				spin_lock(&mb_cache_spinlock);
				continue;
			}
			__mb_cache_entry_unhash_unlock(ce);
			mb_assert(!(ce->e_used || ce->e_queued ||
				atomic_read(&ce->e_refcnt)));
			list_add_tail(&ce->e_lru_list, &free_list);
			l = &mb_cache_lru_list;
			spin_lock(&mb_cache_spinlock);
		}
	}
	spin_unlock(&mb_cache_spinlock);

	list_for_each_entry_safe(ce, tmp, &free_list, e_lru_list) {
		__mb_cache_entry_forget(ce, GFP_KERNEL);
	}
}


/*
 * mb_cache_destroy()
 *
 * Shrinks the cache to its minimum possible size (hopefully 0 entries),
 * and then destroys it. If this was the last mbcache, un-registers the
 * mbcache from kernel memory management.
 */
void
mb_cache_destroy(struct mb_cache *cache)
{
	LIST_HEAD(free_list);
	struct mb_cache_entry *ce, *tmp;

	spin_lock(&mb_cache_spinlock);
	list_for_each_entry_safe(ce, tmp, &mb_cache_lru_list, e_lru_list) {
		if (ce->e_cache == cache)
			list_move_tail(&ce->e_lru_list, &free_list);
	}
	list_del(&cache->c_cache_list);
	spin_unlock(&mb_cache_spinlock);

	list_for_each_entry_safe(ce, tmp, &free_list, e_lru_list) {
		list_del_init(&ce->e_lru_list);
		/*
		 * Prevent any find or get operation on the entry.
		 */
		hlist_bl_lock(ce->e_block_hash_p);
		hlist_bl_lock(ce->e_index_hash_p);
		mb_assert(!(ce->e_used || ce->e_queued ||
			atomic_read(&ce->e_refcnt)));
		__mb_cache_entry_unhash_unlock(ce);
		__mb_cache_entry_forget(ce, GFP_KERNEL);
	}

	if (atomic_read(&cache->c_entry_count) > 0) {
		mb_error("cache %s: %d orphaned entries",
			  cache->c_name,
			  atomic_read(&cache->c_entry_count));
	}

	if (list_empty(&mb_cache_list)) {
		kmem_cache_destroy(mb_cache_kmem_cache);
		mb_cache_kmem_cache = NULL;
	}
	kfree(cache->c_index_hash);
	kfree(cache->c_block_hash);
	kfree(cache);
}

/*
 * mb_cache_entry_alloc()
 *
 * Allocates a new cache entry. The new entry will not be valid initially,
 * and thus cannot be looked up yet. It should be filled with data, and
 * then inserted into the cache using mb_cache_entry_insert(). Returns NULL
 * if no more memory was available.
 */
struct mb_cache_entry *
mb_cache_entry_alloc(struct mb_cache *cache, gfp_t gfp_flags)
{
	struct mb_cache_entry *ce;

	if (atomic_read(&cache->c_entry_count) >= cache->c_max_entries) {
		struct list_head *l;

		l = &mb_cache_lru_list;
		spin_lock(&mb_cache_spinlock);
		while (!list_is_last(l, &mb_cache_lru_list)) {
			l = l->next;
			ce = list_entry(l, struct mb_cache_entry, e_lru_list);
			if (ce->e_cache == cache) {
				list_del_init(&ce->e_lru_list);
				if (ce->e_used || ce->e_queued ||
					atomic_read(&ce->e_refcnt))
					continue;
				spin_unlock(&mb_cache_spinlock);
				/*
				 * Prevent any find or get operation on the
				 * entry.
				 */
				hlist_bl_lock(ce->e_block_hash_p);
				hlist_bl_lock(ce->e_index_hash_p);
				/* Ignore if it is touched by a find/get */
				if (ce->e_used || ce->e_queued ||
					atomic_read(&ce->e_refcnt) ||
					!list_empty(&ce->e_lru_list)) {
					hlist_bl_unlock(ce->e_index_hash_p);
					hlist_bl_unlock(ce->e_block_hash_p);
					l = &mb_cache_lru_list;
					spin_lock(&mb_cache_spinlock);
					continue;
				}
				mb_assert(list_empty(&ce->e_lru_list));
				mb_assert(!(ce->e_used || ce->e_queued ||
					atomic_read(&ce->e_refcnt)));
				__mb_cache_entry_unhash_unlock(ce);
				goto found;
			}
		}
		spin_unlock(&mb_cache_spinlock);
	}

	ce = kmem_cache_alloc(cache->c_entry_cache, gfp_flags);
	if (!ce)
		return NULL;
	atomic_inc(&cache->c_entry_count);
	INIT_LIST_HEAD(&ce->e_lru_list);
	INIT_HLIST_BL_NODE(&ce->e_block_list);
	INIT_HLIST_BL_NODE(&ce->e_index.o_list);
	ce->e_cache = cache;
	ce->e_queued = 0;
	atomic_set(&ce->e_refcnt, 0);
found:
	ce->e_block_hash_p = &cache->c_block_hash[0];
	ce->e_index_hash_p = &cache->c_index_hash[0];
	ce->e_used = 1 + MB_CACHE_WRITER;
	return ce;
}


/*
 * mb_cache_entry_insert()
 *
 * Inserts an entry that was allocated using mb_cache_entry_alloc() into
 * the cache. After this, the cache entry can be looked up, but is not yet
 * in the lru list as the caller still holds a handle to it. Returns 0 on
 * success, or -EBUSY if a cache entry for that device + inode exists
 * already (this may happen after a failed lookup, but when another process
 * has inserted the same cache entry in the meantime).
 *
 * @bdev: device the cache entry belongs to
 * @block: block number
 * @key: lookup key
 */
int
mb_cache_entry_insert(struct mb_cache_entry *ce, struct block_device *bdev,
		      sector_t block, unsigned int key)
{
	struct mb_cache *cache = ce->e_cache;
	unsigned int bucket;
	struct hlist_bl_node *l;
	struct hlist_bl_head *block_hash_p;
	struct hlist_bl_head *index_hash_p;
	struct mb_cache_entry *lce;

	mb_assert(ce);
	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff), 
			   cache->c_bucket_bits);
	block_hash_p = &cache->c_block_hash[bucket];
	hlist_bl_lock(block_hash_p);
	hlist_bl_for_each_entry(lce, l, block_hash_p, e_block_list) {
		if (lce->e_bdev == bdev && lce->e_block == block) {
			hlist_bl_unlock(block_hash_p);
			return -EBUSY;
		}
	}
	mb_assert(!__mb_cache_entry_is_block_hashed(ce));
	__mb_cache_entry_unhash_block(ce);
	__mb_cache_entry_unhash_index(ce);
	ce->e_bdev = bdev;
	ce->e_block = block;
	ce->e_block_hash_p = block_hash_p;
	ce->e_index.o_key = key;
	hlist_bl_add_head(&ce->e_block_list, block_hash_p);
	hlist_bl_unlock(block_hash_p);
	bucket = hash_long(key, cache->c_bucket_bits);
	index_hash_p = &cache->c_index_hash[bucket];
	hlist_bl_lock(index_hash_p);
	ce->e_index_hash_p = index_hash_p;
	hlist_bl_add_head(&ce->e_index.o_list, index_hash_p);
	hlist_bl_unlock(index_hash_p);
	return 0;
}


/*
 * mb_cache_entry_release()
 *
 * Release a handle to a cache entry. When the last handle to a cache entry
 * is released it is either freed (if it is invalid) or otherwise inserted
 * in to the lru list.
 */
void
mb_cache_entry_release(struct mb_cache_entry *ce)
{
	__mb_cache_entry_release(ce);
}


/*
 * mb_cache_entry_free()
 *
 */
void
mb_cache_entry_free(struct mb_cache_entry *ce)
{
	mb_assert(ce);
	mb_assert(list_empty(&ce->e_lru_list));
	hlist_bl_lock(ce->e_index_hash_p);
	__mb_cache_entry_unhash_index(ce);
	hlist_bl_unlock(ce->e_index_hash_p);
	hlist_bl_lock(ce->e_block_hash_p);
	__mb_cache_entry_unhash_block(ce);
	hlist_bl_unlock(ce->e_block_hash_p);
	__mb_cache_entry_release(ce);
}


/*
 * mb_cache_entry_get()
 *
 * Get a cache entry  by device / block number. (There can only be one entry
 * in the cache per device and block.) Returns NULL if no such cache entry
 * exists. The returned cache entry is locked for exclusive access ("single
 * writer").
 */
struct mb_cache_entry *
mb_cache_entry_get(struct mb_cache *cache, struct block_device *bdev,
		   sector_t block)
{
	unsigned int bucket;
	struct hlist_bl_node *l;
	struct mb_cache_entry *ce;
	struct hlist_bl_head *block_hash_p;

	bucket = hash_long((unsigned long)bdev + (block & 0xffffffff),
			   cache->c_bucket_bits);
	block_hash_p = &cache->c_block_hash[bucket];
	/* First serialize access to the block corresponding hash chain. */
	hlist_bl_lock(block_hash_p);
	hlist_bl_for_each_entry(ce, l, block_hash_p, e_block_list) {
		mb_assert(ce->e_block_hash_p == block_hash_p);
		if (ce->e_bdev == bdev && ce->e_block == block) {
			/*
			 * Prevent a free from removing the entry.
			 */
			atomic_inc(&ce->e_refcnt);
			hlist_bl_unlock(block_hash_p);
			__spin_lock_mb_cache_entry(ce);
			atomic_dec(&ce->e_refcnt);
			if (ce->e_used > 0) {
				DEFINE_WAIT(wait);
				while (ce->e_used > 0) {
					ce->e_queued++;
					prepare_to_wait(&mb_cache_queue, &wait,
							TASK_UNINTERRUPTIBLE);
					__spin_unlock_mb_cache_entry(ce);
					schedule();
					__spin_lock_mb_cache_entry(ce);
					ce->e_queued--;
				}
				finish_wait(&mb_cache_queue, &wait);
			}
			ce->e_used += 1 + MB_CACHE_WRITER;
			__spin_unlock_mb_cache_entry(ce);

			if (!list_empty(&ce->e_lru_list)) {
				spin_lock(&mb_cache_spinlock);
				list_del_init(&ce->e_lru_list);
				spin_unlock(&mb_cache_spinlock);
			}
			if (!__mb_cache_entry_is_block_hashed(ce)) {
				__mb_cache_entry_release(ce);
				return NULL;
			}
			return ce;
		}
	}
	hlist_bl_unlock(block_hash_p);
	return NULL;
}

#if !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0)

static struct mb_cache_entry *
__mb_cache_entry_find(struct hlist_bl_node *l, struct hlist_bl_head *head,
		      struct block_device *bdev, unsigned int key)
{

	/* The index hash chain is alredy acquire by caller. */
	while (l != NULL) {
		struct mb_cache_entry *ce =
			hlist_bl_entry(l, struct mb_cache_entry,
				e_index.o_list);
		mb_assert(ce->e_index_hash_p == head);
		if (ce->e_bdev == bdev && ce->e_index.o_key == key) {
			/*
			 * Prevent a free from removing the entry.
			 */
			atomic_inc(&ce->e_refcnt);
			hlist_bl_unlock(head);
			__spin_lock_mb_cache_entry(ce);
			atomic_dec(&ce->e_refcnt);
			ce->e_used++;
			/* Incrementing before holding the lock gives readers
			   priority over writers. */
			if (ce->e_used >= MB_CACHE_WRITER) {
				DEFINE_WAIT(wait);

				while (ce->e_used >= MB_CACHE_WRITER) {
					ce->e_queued++;
					prepare_to_wait(&mb_cache_queue, &wait,
							TASK_UNINTERRUPTIBLE);
					__spin_unlock_mb_cache_entry(ce);
					schedule();
					__spin_lock_mb_cache_entry(ce);
					ce->e_queued--;
				}
				finish_wait(&mb_cache_queue, &wait);
			}
			__spin_unlock_mb_cache_entry(ce);
			if (!list_empty(&ce->e_lru_list)) {
				spin_lock(&mb_cache_spinlock);
				list_del_init(&ce->e_lru_list);
				spin_unlock(&mb_cache_spinlock);
			}
			if (!__mb_cache_entry_is_block_hashed(ce)) {
				__mb_cache_entry_release(ce);
				return ERR_PTR(-EAGAIN);
			}
			return ce;
		}
		l = l->next;
	}
	hlist_bl_unlock(head);
	return NULL;
}


/*
 * mb_cache_entry_find_first()
 *
 * Find the first cache entry on a given device with a certain key in
 * an additional index. Additional matches can be found with
 * mb_cache_entry_find_next(). Returns NULL if no match was found. The
 * returned cache entry is locked for shared access ("multiple readers").
 *
 * @cache: the cache to search
 * @bdev: the device the cache entry should belong to
 * @key: the key in the index
 */
struct mb_cache_entry *
mb_cache_entry_find_first(struct mb_cache *cache, struct block_device *bdev,
			  unsigned int key)
{
	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
	struct hlist_bl_node *l;
	struct mb_cache_entry *ce = NULL;
	struct hlist_bl_head *index_hash_p;

	index_hash_p = &cache->c_index_hash[bucket];
	hlist_bl_lock(index_hash_p);
	if (!hlist_bl_empty(index_hash_p)) {
		l = hlist_bl_first(index_hash_p);
		ce = __mb_cache_entry_find(l, index_hash_p, bdev, key);
	} else
		hlist_bl_unlock(index_hash_p);
	return ce;
}


/*
 * mb_cache_entry_find_next()
 *
 * Find the next cache entry on a given device with a certain key in an
 * additional index. Returns NULL if no match could be found. The previous
 * entry is atomatically released, so that mb_cache_entry_find_next() can
 * be called like this:
 *
 * entry = mb_cache_entry_find_first();
 * while (entry) {
 * 	...
 *	entry = mb_cache_entry_find_next(entry, ...);
 * }
 *
 * @prev: The previous match
 * @bdev: the device the cache entry should belong to
 * @key: the key in the index
 */
struct mb_cache_entry *
mb_cache_entry_find_next(struct mb_cache_entry *prev,
			 struct block_device *bdev, unsigned int key)
{
	struct mb_cache *cache = prev->e_cache;
	unsigned int bucket = hash_long(key, cache->c_bucket_bits);
	struct hlist_bl_node *l;
	struct mb_cache_entry *ce;
	struct hlist_bl_head *index_hash_p;

	index_hash_p = &cache->c_index_hash[bucket];
	mb_assert(prev->e_index_hash_p == index_hash_p);
	hlist_bl_lock(index_hash_p);
	mb_assert(!hlist_bl_empty(index_hash_p));
	l = prev->e_index.o_list.next;
	ce = __mb_cache_entry_find(l, index_hash_p, bdev, key);
	__mb_cache_entry_release(prev);
	return ce;
}

#endif  /* !defined(MB_CACHE_INDEXES_COUNT) || (MB_CACHE_INDEXES_COUNT > 0) */

static int __init init_mbcache(void)
{
	register_shrinker(&mb_cache_shrinker);
	return 0;
}

static void __exit exit_mbcache(void)
{
	unregister_shrinker(&mb_cache_shrinker);
}

module_init(init_mbcache)
module_exit(exit_mbcache)