aboutsummaryrefslogtreecommitdiffstats
path: root/block/elevator.c
blob: 78805c74ea8a4b42d21134db8399cdcbdfc368a1 (plain)
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
// SPDX-License-Identifier: GPL-2.0
/*
 *  Block device elevator/IO-scheduler.
 *
 *  Copyright (C) 2000 Andrea Arcangeli <andrea@suse.de> SuSE
 *
 * 30042000 Jens Axboe <axboe@kernel.dk> :
 *
 * Split the elevator a bit so that it is possible to choose a different
 * one or even write a new "plug in". There are three pieces:
 * - elevator_fn, inserts a new request in the queue list
 * - elevator_merge_fn, decides whether a new buffer can be merged with
 *   an existing request
 * - elevator_dequeue_fn, called when a request is taken off the active list
 *
 * 20082000 Dave Jones <davej@suse.de> :
 * Removed tests for max-bomb-segments, which was breaking elvtune
 *  when run without -bN
 *
 * Jens:
 * - Rework again to work with bio instead of buffer_heads
 * - loose bi_dev comparisons, partition handling is right now
 * - completely modularize elevator setup and teardown
 *
 */
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/bio.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/compiler.h>
#include <linux/blktrace_api.h>
#include <linux/hash.h>
#include <linux/uaccess.h>
#include <linux/pm_runtime.h>
#include <linux/blk-cgroup.h>

#include <trace/events/block.h>

#include "blk.h"
#include "blk-mq-sched.h"
#include "blk-pm.h"
#include "blk-wbt.h"

static DEFINE_SPINLOCK(elv_list_lock);
static LIST_HEAD(elv_list);

/*
 * Merge hash stuff.
 */
#define rq_hash_key(rq)		(blk_rq_pos(rq) + blk_rq_sectors(rq))

/*
 * Query io scheduler to see if the current process issuing bio may be
 * merged with rq.
 */
static int elv_iosched_allow_bio_merge(struct request *rq, struct bio *bio)
{
	struct request_queue *q = rq->q;
	struct elevator_queue *e = q->elevator;

	if (e->type->ops.allow_merge)
		return e->type->ops.allow_merge(q, rq, bio);

	return 1;
}

/*
 * can we safely merge with this request?
 */
bool elv_bio_merge_ok(struct request *rq, struct bio *bio)
{
	if (!blk_rq_merge_ok(rq, bio))
		return false;

	if (!elv_iosched_allow_bio_merge(rq, bio))
		return false;

	return true;
}
EXPORT_SYMBOL(elv_bio_merge_ok);

static inline bool elv_support_features(unsigned int elv_features,
					unsigned int required_features)
{
	return (required_features & elv_features) == required_features;
}

/**
 * elevator_match - Test an elevator name and features
 * @e: Scheduler to test
 * @name: Elevator name to test
 * @required_features: Features that the elevator must provide
 *
 * Return true is the elevator @e name matches @name and if @e provides all the
 * the feratures spcified by @required_features.
 */
static bool elevator_match(const struct elevator_type *e, const char *name,
			   unsigned int required_features)
{
	if (!elv_support_features(e->elevator_features, required_features))
		return false;
	if (!strcmp(e->elevator_name, name))
		return true;
	if (e->elevator_alias && !strcmp(e->elevator_alias, name))
		return true;

	return false;
}

/**
 * elevator_find - Find an elevator
 * @name: Name of the elevator to find
 * @required_features: Features that the elevator must provide
 *
 * Return the first registered scheduler with name @name and supporting the
 * features @required_features and NULL otherwise.
 */
static struct elevator_type *elevator_find(const char *name,
					   unsigned int required_features)
{
	struct elevator_type *e;

	list_for_each_entry(e, &elv_list, list) {
		if (elevator_match(e, name, required_features))
			return e;
	}

	return NULL;
}

static void elevator_put(struct elevator_type *e)
{
	module_put(e->elevator_owner);
}

static struct elevator_type *elevator_get(struct request_queue *q,
					  const char *name, bool try_loading)
{
	struct elevator_type *e;

	spin_lock(&elv_list_lock);

	e = elevator_find(name, q->required_elevator_features);
	if (!e && try_loading) {
		spin_unlock(&elv_list_lock);
		request_module("%s-iosched", name);
		spin_lock(&elv_list_lock);
		e = elevator_find(name, q->required_elevator_features);
	}

	if (e && !try_module_get(e->elevator_owner))
		e = NULL;

	spin_unlock(&elv_list_lock);
	return e;
}

static struct kobj_type elv_ktype;

struct elevator_queue *elevator_alloc(struct request_queue *q,
				  struct elevator_type *e)
{
	struct elevator_queue *eq;

	eq = kzalloc_node(sizeof(*eq), GFP_KERNEL, q->node);
	if (unlikely(!eq))
		return NULL;

	eq->type = e;
	kobject_init(&eq->kobj, &elv_ktype);
	mutex_init(&eq->sysfs_lock);
	hash_init(eq->hash);

	return eq;
}
EXPORT_SYMBOL(elevator_alloc);

static void elevator_release(struct kobject *kobj)
{
	struct elevator_queue *e;

	e = container_of(kobj, struct elevator_queue, kobj);
	elevator_put(e->type);
	kfree(e);
}

void __elevator_exit(struct request_queue *q, struct elevator_queue *e)
{
	mutex_lock(&e->sysfs_lock);
	if (e->type->ops.exit_sched)
		blk_mq_exit_sched(q, e);
	mutex_unlock(&e->sysfs_lock);

	kobject_put(&e->kobj);
}

static inline void __elv_rqhash_del(struct request *rq)
{
	hash_del(&rq->hash);
	rq->rq_flags &= ~RQF_HASHED;
}

void elv_rqhash_del(struct request_queue *q, struct request *rq)
{
	if (ELV_ON_HASH(rq))
		__elv_rqhash_del(rq);
}
EXPORT_SYMBOL_GPL(elv_rqhash_del);

void elv_rqhash_add(struct request_queue *q, struct request *rq)
{
	struct elevator_queue *e = q->elevator;

	BUG_ON(ELV_ON_HASH(rq));
	hash_add(e->hash, &rq->hash, rq_hash_key(rq));
	rq->rq_flags |= RQF_HASHED;
}
EXPORT_SYMBOL_GPL(elv_rqhash_add);

void elv_rqhash_reposition(struct request_queue *q, struct request *rq)
{
	__elv_rqhash_del(rq);
	elv_rqhash_add(q, rq);
}

struct request *elv_rqhash_find(struct request_queue *q, sector_t offset)
{
	struct elevator_queue *e = q->elevator;
	struct hlist_node *next;
	struct request *rq;

	hash_for_each_possible_safe(e->hash, rq, next, hash, offset) {
		BUG_ON(!ELV_ON_HASH(rq));

		if (unlikely(!rq_mergeable(rq))) {
			__elv_rqhash_del(rq);
			continue;
		}

		if (rq_hash_key(rq) == offset)
			return rq;
	}

	return NULL;
}

/*
 * RB-tree support functions for inserting/lookup/removal of requests
 * in a sorted RB tree.
 */
void elv_rb_add(struct rb_root *root, struct request *rq)
{
	struct rb_node **p = &root->rb_node;
	struct rb_node *parent = NULL;
	struct request *__rq;

	while (*p) {
		parent = *p;
		__rq = rb_entry(parent, struct request, rb_node);

		if (blk_rq_pos(rq) < blk_rq_pos(__rq))
			p = &(*p)->rb_left;
		else if (blk_rq_pos(rq) >= blk_rq_pos(__rq))
			p = &(*p)->rb_right;
	}

	rb_link_node(&rq->rb_node, parent, p);
	rb_insert_color(&rq->rb_node, root);
}
EXPORT_SYMBOL(elv_rb_add);

void elv_rb_del(struct rb_root *root, struct request *rq)
{
	BUG_ON(RB_EMPTY_NODE(&rq->rb_node));
	rb_erase(&rq->rb_node, root);
	RB_CLEAR_NODE(&rq->rb_node);
}
EXPORT_SYMBOL(elv_rb_del);

struct request *elv_rb_find(struct rb_root *root, sector_t sector)
{
	struct rb_node *n = root->rb_node;
	struct request *rq;

	while (n) {
		rq = rb_entry(n, struct request, rb_node);

		if (sector < blk_rq_pos(rq))
			n = n->rb_left;
		else if (sector > blk_rq_pos(rq))
			n = n->rb_right;
		else
			return rq;
	}

	return NULL;
}
EXPORT_SYMBOL(elv_rb_find);

enum elv_merge elv_merge(struct request_queue *q, struct request **req,
		struct bio *bio)
{
	struct elevator_queue *e = q->elevator;
	struct request *__rq;

	/*
	 * Levels of merges:
	 * 	nomerges:  No merges at all attempted
	 * 	noxmerges: Only simple one-hit cache try
	 * 	merges:	   All merge tries attempted
	 */
	if (blk_queue_nomerges(q) || !bio_mergeable(bio))
		return ELEVATOR_NO_MERGE;

	/*
	 * First try one-hit cache.
	 */
	if (q->last_merge && elv_bio_merge_ok(q->last_merge, bio)) {
		enum elv_merge ret = blk_try_merge(q->last_merge, bio);

		if (ret != ELEVATOR_NO_MERGE) {
			*req = q->last_merge;
			return ret;
		}
	}

	if (blk_queue_noxmerges(q))
		return ELEVATOR_NO_MERGE;

	/*
	 * See if our hash lookup can find a potential backmerge.
	 */
	__rq = elv_rqhash_find(q, bio->bi_iter.bi_sector);
	if (__rq && elv_bio_merge_ok(__rq, bio)) {
		*req = __rq;
		return ELEVATOR_BACK_MERGE;
	}

	if (e->type->ops.request_merge)
		return e->type->ops.request_merge(q, req, bio);

	return ELEVATOR_NO_MERGE;
}

/*
 * Attempt to do an insertion back merge. Only check for the case where
 * we can append 'rq' to an existing request, so we can throw 'rq' away
 * afterwards.
 *
 * Returns true if we merged, false otherwise
 */
bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
{
	struct request *__rq;
	bool ret;

	if (blk_queue_nomerges(q))
		return false;

	/*
	 * First try one-hit cache.
	 */
	if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
		return true;

	if (blk_queue_noxmerges(q))
		return false;

	ret = false;
	/*
	 * See if our hash lookup can find a potential backmerge.
	 */
	while (1) {
		__rq = elv_rqhash_find(q, blk_rq_pos(rq));
		if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
			break;

		/* The merged request could be merged with others, try again */
		ret = true;
		rq = __rq;
	}

	return ret;
}

void elv_merged_request(struct request_queue *q, struct request *rq,
		enum elv_merge type)
{
	struct elevator_queue *e = q->elevator;

	if (e->type->ops.request_merged)
		e->type->ops.request_merged(q, rq, type);

	if (type == ELEVATOR_BACK_MERGE)
		elv_rqhash_reposition(q, rq);

	q->last_merge = rq;
}

void elv_merge_requests(struct request_queue *q, struct request *rq,
			     struct request *next)
{
	struct elevator_queue *e = q->elevator;

	if (e->type->ops.requests_merged)
		e->type->ops.requests_merged(q, rq, next);

	elv_rqhash_reposition(q, rq);
	q->last_merge = rq;
}

struct request *elv_latter_request(struct request_queue *q, struct request *rq)
{
	struct elevator_queue *e = q->elevator;

	if (e->type->ops.next_request)
		return e->type->ops.next_request(q, rq);

	return NULL;
}

struct request *elv_former_request(struct request_queue *q, struct request *rq)
{
	struct elevator_queue *e = q->elevator;

	if (e->type->ops.former_request)
		return e->type->ops.former_request(q, rq);

	return NULL;
}

#define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)

static ssize_t
elv_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
{
	struct elv_fs_entry *entry = to_elv(attr);
	struct elevator_queue *e;
	ssize_t error;

	if (!entry->show)
		return -EIO;

	e = container_of(kobj, struct elevator_queue, kobj);
	mutex_lock(&e->sysfs_lock);
	error = e->type ? entry->show(e, page) : -ENOENT;
	mutex_unlock(&e->sysfs_lock);
	return error;
}

static ssize_t
elv_attr_store(struct kobject *kobj, struct attribute *attr,
	       const char *page, size_t length)
{
	struct elv_fs_entry *entry = to_elv(attr);
	struct elevator_queue *e;
	ssize_t error;

	if (!entry->store)
		return -EIO;

	e = container_of(kobj, struct elevator_queue, kobj);
	mutex_lock(&e->sysfs_lock);
	error = e->type ? entry->store(e, page, length) : -ENOENT;
	mutex_unlock(&e->sysfs_lock);
	return error;
}

static const struct sysfs_ops elv_sysfs_ops = {
	.show	= elv_attr_show,
	.store	= elv_attr_store,
};

static struct kobj_type elv_ktype = {
	.sysfs_ops	= &elv_sysfs_ops,
	.release	= elevator_release,
};

/*
 * elv_register_queue is called from either blk_register_queue or
 * elevator_switch, elevator switch is prevented from being happen
 * in the two paths, so it is safe to not hold q->sysfs_lock.
 */
int elv_register_queue(struct request_queue *q, bool uevent)
{
	struct elevator_queue *e = q->elevator;
	int error;

	error = kobject_add(&e->kobj, &q->kobj, "%s", "iosched");
	if (!error) {
		struct elv_fs_entry *attr = e->type->elevator_attrs;
		if (attr) {
			while (attr->attr.name) {
				if (sysfs_create_file(&e->kobj, &attr->attr))
					break;
				attr++;
			}
		}
		if (uevent)
			kobject_uevent(&e->kobj, KOBJ_ADD);

		e->registered = 1;
	}
	return error;
}

/*
 * elv_unregister_queue is called from either blk_unregister_queue or
 * elevator_switch, elevator switch is prevented from being happen
 * in the two paths, so it is safe to not hold q->sysfs_lock.
 */
void elv_unregister_queue(struct request_queue *q)
{
	if (q) {
		struct elevator_queue *e = q->elevator;

		kobject_uevent(&e->kobj, KOBJ_REMOVE);
		kobject_del(&e->kobj);

		e->registered = 0;
	}
}

int elv_register(struct elevator_type *e)
{
	/* create icq_cache if requested */
	if (e->icq_size) {
		if (WARN_ON(e->icq_size < sizeof(struct io_cq)) ||
		    WARN_ON(e->icq_align < __alignof__(struct io_cq)))
			return -EINVAL;

		snprintf(e->icq_cache_name, sizeof(e->icq_cache_name),
			 "%s_io_cq", e->elevator_name);
		e->icq_cache = kmem_cache_create(e->icq_cache_name, e->icq_size,
						 e->icq_align, 0, NULL);
		if (!e->icq_cache)
			return -ENOMEM;
	}

	/* register, don't allow duplicate names */
	spin_lock(&elv_list_lock);
	if (elevator_find(e->elevator_name, 0)) {
		spin_unlock(&elv_list_lock);
		kmem_cache_destroy(e->icq_cache);
		return -EBUSY;
	}
	list_add_tail(&e->list, &elv_list);
	spin_unlock(&elv_list_lock);

	printk(KERN_INFO "io scheduler %s registered\n", e->elevator_name);

	return 0;
}
EXPORT_SYMBOL_GPL(elv_register);

void elv_unregister(struct elevator_type *e)
{
	/* unregister */
	spin_lock(&elv_list_lock);
	list_del_init(&e->list);
	spin_unlock(&elv_list_lock);

	/*
	 * Destroy icq_cache if it exists.  icq's are RCU managed.  Make
	 * sure all RCU operations are complete before proceeding.
	 */
	if (e->icq_cache) {
		rcu_barrier();
		kmem_cache_destroy(e->icq_cache);
		e->icq_cache = NULL;
	}
}
EXPORT_SYMBOL_GPL(elv_unregister);

int elevator_switch_mq(struct request_queue *q,
			      struct elevator_type *new_e)
{
	int ret;

	lockdep_assert_held(&q->sysfs_lock);

	if (q->elevator) {
		if (q->elevator->registered)
			elv_unregister_queue(q);

		ioc_clear_queue(q);
		elevator_exit(q, q->elevator);
	}

	ret = blk_mq_init_sched(q, new_e);
	if (ret)
		goto out;

	if (new_e) {
		ret = elv_register_queue(q, true);
		if (ret) {
			elevator_exit(q, q->elevator);
			goto out;
		}
	}

	if (new_e)
		blk_add_trace_msg(q, "elv switch: %s", new_e->elevator_name);
	else
		blk_add_trace_msg(q, "elv switch: none");

out:
	return ret;
}

static inline bool elv_support_iosched(struct request_queue *q)
{
	if (!q->mq_ops ||
	    (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED)))
		return false;
	return true;
}

/*
 * For single queue devices, default to using mq-deadline. If we have multiple
 * queues or mq-deadline is not available, default to "none".
 */
static struct elevator_type *elevator_get_default(struct request_queue *q)
{
	if (q->nr_hw_queues != 1)
		return NULL;

	return elevator_get(q, "mq-deadline", false);
}

/*
 * Get the first elevator providing the features required by the request queue.
 * Default to "none" if no matching elevator is found.
 */
static struct elevator_type *elevator_get_by_features(struct request_queue *q)
{
	struct elevator_type *e, *found = NULL;

	spin_lock(&elv_list_lock);

	list_for_each_entry(e, &elv_list, list) {
		if (elv_support_features(e->elevator_features,
					 q->required_elevator_features)) {
			found = e;
			break;
		}
	}

	if (found && !try_module_get(found->elevator_owner))
		found = NULL;

	spin_unlock(&elv_list_lock);
	return found;
}

/*
 * For a device queue that has no required features, use the default elevator
 * settings. Otherwise, use the first elevator available matching the required
 * features. If no suitable elevator is find or if the chosen elevator
 * initialization fails, fall back to the "none" elevator (no elevator).
 */
void elevator_init_mq(struct request_queue *q)
{
	struct elevator_type *e;
	int err;

	if (!elv_support_iosched(q))
		return;

	WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));

	if (unlikely(q->elevator))
		return;

	if (!q->required_elevator_features)
		e = elevator_get_default(q);
	else
		e = elevator_get_by_features(q);
	if (!e)
		return;

	blk_mq_freeze_queue(q);
	blk_mq_quiesce_queue(q);

	err = blk_mq_init_sched(q, e);

	blk_mq_unquiesce_queue(q);
	blk_mq_unfreeze_queue(q);

	if (err) {
		pr_warn("\"%s\" elevator initialization failed, "
			"falling back to \"none\"\n", e->elevator_name);
		elevator_put(e);
	}
}


/*
 * switch to new_e io scheduler. be careful not to introduce deadlocks -
 * we don't free the old io scheduler, before we have allocated what we
 * need for the new one. this way we have a chance of going back to the old
 * one, if the new one fails init for some reason.
 */
static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
{
	int err;

	lockdep_assert_held(&q->sysfs_lock);

	blk_mq_freeze_queue(q);
	blk_mq_quiesce_queue(q);

	err = elevator_switch_mq(q, new_e);

	blk_mq_unquiesce_queue(q);
	blk_mq_unfreeze_queue(q);

	return err;
}

/*
 * Switch this queue to the given IO scheduler.
 */
static int __elevator_change(struct request_queue *q, const char *name)
{
	char elevator_name[ELV_NAME_MAX];
	struct elevator_type *e;

	/* Make sure queue is not in the middle of being removed */
	if (!blk_queue_registered(q))
		return -ENOENT;

	/*
	 * Special case for mq, turn off scheduling
	 */
	if (!strncmp(name, "none", 4)) {
		if (!q->elevator)
			return 0;
		return elevator_switch(q, NULL);
	}

	strlcpy(elevator_name, name, sizeof(elevator_name));
	e = elevator_get(q, strstrip(elevator_name), true);
	if (!e)
		return -EINVAL;

	if (q->elevator &&
	    elevator_match(q->elevator->type, elevator_name, 0)) {
		elevator_put(e);
		return 0;
	}

	return elevator_switch(q, e);
}

ssize_t elv_iosched_store(struct request_queue *q, const char *name,
			  size_t count)
{
	int ret;

	if (!queue_is_mq(q) || !elv_support_iosched(q))
		return count;

	ret = __elevator_change(q, name);
	if (!ret)
		return count;

	return ret;
}

ssize_t elv_iosched_show(struct request_queue *q, char *name)
{
	struct elevator_queue *e = q->elevator;
	struct elevator_type *elv = NULL;
	struct elevator_type *__e;
	int len = 0;

	if (!queue_is_mq(q))
		return sprintf(name, "none\n");

	if (!q->elevator)
		len += sprintf(name+len, "[none] ");
	else
		elv = e->type;

	spin_lock(&elv_list_lock);
	list_for_each_entry(__e, &elv_list, list) {
		if (elv && elevator_match(elv, __e->elevator_name, 0)) {
			len += sprintf(name+len, "[%s] ", elv->elevator_name);
			continue;
		}
		if (elv_support_iosched(q) &&
		    elevator_match(__e, __e->elevator_name,
				   q->required_elevator_features))
			len += sprintf(name+len, "%s ", __e->elevator_name);
	}
	spin_unlock(&elv_list_lock);

	if (q->elevator)
		len += sprintf(name+len, "none");

	len += sprintf(len+name, "\n");
	return len;
}

struct request *elv_rb_former_request(struct request_queue *q,
				      struct request *rq)
{
	struct rb_node *rbprev = rb_prev(&rq->rb_node);

	if (rbprev)
		return rb_entry_rq(rbprev);

	return NULL;
}
EXPORT_SYMBOL(elv_rb_former_request);

struct request *elv_rb_latter_request(struct request_queue *q,
				      struct request *rq)
{
	struct rb_node *rbnext = rb_next(&rq->rb_node);

	if (rbnext)
		return rb_entry_rq(rbnext);

	return NULL;
}
EXPORT_SYMBOL(elv_rb_latter_request);