block/elevator.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *  Block device elevator/IO-scheduler.
   4  *
   5  *  Copyright (C) 2000 Andrea Arcangeli <andrea@suse.de> SuSE
   6  *
   7  * 30042000 Jens Axboe <axboe@kernel.dk> :
   8  *
   9  * Split the elevator a bit so that it is possible to choose a different
  10  * one or even write a new "plug in". There are three pieces:
  11  * - elevator_fn, inserts a new request in the queue list
  12  * - elevator_merge_fn, decides whether a new buffer can be merged with
  13  *   an existing request
  14  * - elevator_dequeue_fn, called when a request is taken off the active list
  15  *
  16  * 20082000 Dave Jones <davej@suse.de> :
  17  * Removed tests for max-bomb-segments, which was breaking elvtune
  18  *  when run without -bN
  19  *
  20  * Jens:
  21  * - Rework again to work with bio instead of buffer_heads
  22  * - loose bi_dev comparisons, partition handling is right now
  23  * - completely modularize elevator setup and teardown
  24  *
  25  */
  26 #include <linux/kernel.h>
  27 #include <linux/fs.h>
  28 #include <linux/blkdev.h>
  29 #include <linux/elevator.h>
  30 #include <linux/bio.h>
  31 #include <linux/module.h>
  32 #include <linux/slab.h>
  33 #include <linux/init.h>
  34 #include <linux/compiler.h>
  35 #include <linux/blktrace_api.h>
  36 #include <linux/hash.h>
  37 #include <linux/uaccess.h>
  38 #include <linux/pm_runtime.h>
  39 #include <linux/blk-cgroup.h>
  40
  41 #include <trace/events/block.h>
  42
  43 #include "blk.h"
  44 #include "blk-mq-sched.h"
  45 #include "blk-pm.h"
  46 #include "blk-wbt.h"
  47
  48 static DEFINE_SPINLOCK(elv_list_lock);
  49 static LIST_HEAD(elv_list);
  50
  51 /*
  52  * Merge hash stuff.
  53  */
  54 #define rq_hash_key(rq)         (blk_rq_pos(rq) + blk_rq_sectors(rq))
  55
  56 /*
  57  * Query io scheduler to see if the current process issuing bio may be
  58  * merged with rq.
  59  */
  60 static int elv_iosched_allow_bio_merge(struct request *rq, struct bio *bio)
  61 {
  62         struct request_queue *q = rq->q;
  63         struct elevator_queue *e = q->elevator;
  64
  65         if (e->type->ops.allow_merge)
  66                 return e->type->ops.allow_merge(q, rq, bio);
  67
  68         return 1;
  69 }
  70
  71 /*
  72  * can we safely merge with this request?
  73  */
  74 bool elv_bio_merge_ok(struct request *rq, struct bio *bio)
  75 {
  76         if (!blk_rq_merge_ok(rq, bio))
  77                 return false;
  78
  79         if (!elv_iosched_allow_bio_merge(rq, bio))
  80                 return false;
  81
  82         return true;
  83 }
  84 EXPORT_SYMBOL(elv_bio_merge_ok);
  85
  86 static bool elevator_match(const struct elevator_type *e, const char *name)
  87 {
  88         if (!strcmp(e->elevator_name, name))
  89                 return true;
  90         if (e->elevator_alias && !strcmp(e->elevator_alias, name))
  91                 return true;
  92
  93         return false;
  94 }
  95
  96 /*
  97  * Return scheduler with name 'name'
  98  */
  99 static struct elevator_type *elevator_find(const char *name)
 100 {
 101         struct elevator_type *e;
 102
 103         list_for_each_entry(e, &elv_list, list) {
 104                 if (elevator_match(e, name))
 105                         return e;
 106         }
 107
 108         return NULL;
 109 }
 110
 111 static void elevator_put(struct elevator_type *e)
 112 {
 113         module_put(e->elevator_owner);
 114 }
 115
 116 static struct elevator_type *elevator_get(struct request_queue *q,
 117                                           const char *name, bool try_loading)
 118 {
 119         struct elevator_type *e;
 120
 121         spin_lock(&elv_list_lock);
 122
 123         e = elevator_find(name);
 124         if (!e && try_loading) {
 125                 spin_unlock(&elv_list_lock);
 126                 request_module("%s-iosched", name);
 127                 spin_lock(&elv_list_lock);
 128                 e = elevator_find(name);
 129         }
 130
 131         if (e && !try_module_get(e->elevator_owner))
 132                 e = NULL;
 133
 134         spin_unlock(&elv_list_lock);
 135         return e;
 136 }
 137
 138 static char chosen_elevator[ELV_NAME_MAX];
 139
 140 static int __init elevator_setup(char *str)
 141 {
 142         /*
 143          * Be backwards-compatible with previous kernels, so users
 144          * won't get the wrong elevator.
 145          */
 146         strncpy(chosen_elevator, str, sizeof(chosen_elevator) - 1);
 147         return 1;
 148 }
 149
 150 __setup("elevator=", elevator_setup);
 151
 152 static struct kobj_type elv_ktype;
 153
 154 struct elevator_queue *elevator_alloc(struct request_queue *q,
 155                                   struct elevator_type *e)
 156 {
 157         struct elevator_queue *eq;
 158
 159         eq = kzalloc_node(sizeof(*eq), GFP_KERNEL, q->node);
 160         if (unlikely(!eq))
 161                 return NULL;
 162
 163         eq->type = e;
 164         kobject_init(&eq->kobj, &elv_ktype);
 165         mutex_init(&eq->sysfs_lock);
 166         hash_init(eq->hash);
 167
 168         return eq;
 169 }
 170 EXPORT_SYMBOL(elevator_alloc);
 171
 172 static void elevator_release(struct kobject *kobj)
 173 {
 174         struct elevator_queue *e;
 175
 176         e = container_of(kobj, struct elevator_queue, kobj);
 177         elevator_put(e->type);
 178         kfree(e);
 179 }
 180
 181 void __elevator_exit(struct request_queue *q, struct elevator_queue *e)
 182 {
 183         mutex_lock(&e->sysfs_lock);
 184         if (e->type->ops.exit_sched)
 185                 blk_mq_exit_sched(q, e);
 186         mutex_unlock(&e->sysfs_lock);
 187
 188         kobject_put(&e->kobj);
 189 }
 190
 191 static inline void __elv_rqhash_del(struct request *rq)
 192 {
 193         hash_del(&rq->hash);
 194         rq->rq_flags &= ~RQF_HASHED;
 195 }
 196
 197 void elv_rqhash_del(struct request_queue *q, struct request *rq)
 198 {
 199         if (ELV_ON_HASH(rq))
 200                 __elv_rqhash_del(rq);
 201 }
 202 EXPORT_SYMBOL_GPL(elv_rqhash_del);
 203
 204 void elv_rqhash_add(struct request_queue *q, struct request *rq)
 205 {
 206         struct elevator_queue *e = q->elevator;
 207
 208         BUG_ON(ELV_ON_HASH(rq));
 209         hash_add(e->hash, &rq->hash, rq_hash_key(rq));
 210         rq->rq_flags |= RQF_HASHED;
 211 }
 212 EXPORT_SYMBOL_GPL(elv_rqhash_add);
 213
 214 void elv_rqhash_reposition(struct request_queue *q, struct request *rq)
 215 {
 216         __elv_rqhash_del(rq);
 217         elv_rqhash_add(q, rq);
 218 }
 219
 220 struct request *elv_rqhash_find(struct request_queue *q, sector_t offset)
 221 {
 222         struct elevator_queue *e = q->elevator;
 223         struct hlist_node *next;
 224         struct request *rq;
 225
 226         hash_for_each_possible_safe(e->hash, rq, next, hash, offset) {
 227                 BUG_ON(!ELV_ON_HASH(rq));
 228
 229                 if (unlikely(!rq_mergeable(rq))) {
 230                         __elv_rqhash_del(rq);
 231                         continue;
 232                 }
 233
 234                 if (rq_hash_key(rq) == offset)
 235                         return rq;
 236         }
 237
 238         return NULL;
 239 }
 240
 241 /*
 242  * RB-tree support functions for inserting/lookup/removal of requests
 243  * in a sorted RB tree.
 244  */
 245 void elv_rb_add(struct rb_root *root, struct request *rq)
 246 {
 247         struct rb_node **p = &root->rb_node;
 248         struct rb_node *parent = NULL;
 249         struct request *__rq;
 250
 251         while (*p) {
 252                 parent = *p;
 253                 __rq = rb_entry(parent, struct request, rb_node);
 254
 255                 if (blk_rq_pos(rq) < blk_rq_pos(__rq))
 256                         p = &(*p)->rb_left;
 257                 else if (blk_rq_pos(rq) >= blk_rq_pos(__rq))
 258                         p = &(*p)->rb_right;
 259         }
 260
 261         rb_link_node(&rq->rb_node, parent, p);
 262         rb_insert_color(&rq->rb_node, root);
 263 }
 264 EXPORT_SYMBOL(elv_rb_add);
 265
 266 void elv_rb_del(struct rb_root *root, struct request *rq)
 267 {
 268         BUG_ON(RB_EMPTY_NODE(&rq->rb_node));
 269         rb_erase(&rq->rb_node, root);
 270         RB_CLEAR_NODE(&rq->rb_node);
 271 }
 272 EXPORT_SYMBOL(elv_rb_del);
 273
 274 struct request *elv_rb_find(struct rb_root *root, sector_t sector)
 275 {
 276         struct rb_node *n = root->rb_node;
 277         struct request *rq;
 278
 279         while (n) {
 280                 rq = rb_entry(n, struct request, rb_node);
 281
 282                 if (sector < blk_rq_pos(rq))
 283                         n = n->rb_left;
 284                 else if (sector > blk_rq_pos(rq))
 285                         n = n->rb_right;
 286                 else
 287                         return rq;
 288         }
 289
 290         return NULL;
 291 }
 292 EXPORT_SYMBOL(elv_rb_find);
 293
 294 enum elv_merge elv_merge(struct request_queue *q, struct request **req,
 295                 struct bio *bio)
 296 {
 297         struct elevator_queue *e = q->elevator;
 298         struct request *__rq;
 299
 300         /*
 301          * Levels of merges:
 302          *      nomerges:  No merges at all attempted
 303          *      noxmerges: Only simple one-hit cache try
 304          *      merges:    All merge tries attempted
 305          */
 306         if (blk_queue_nomerges(q) || !bio_mergeable(bio))
 307                 return ELEVATOR_NO_MERGE;
 308
 309         /*
 310          * First try one-hit cache.
 311          */
 312         if (q->last_merge && elv_bio_merge_ok(q->last_merge, bio)) {
 313                 enum elv_merge ret = blk_try_merge(q->last_merge, bio);
 314
 315                 if (ret != ELEVATOR_NO_MERGE) {
 316                         *req = q->last_merge;
 317                         return ret;
 318                 }
 319         }
 320
 321         if (blk_queue_noxmerges(q))
 322                 return ELEVATOR_NO_MERGE;
 323
 324         /*
 325          * See if our hash lookup can find a potential backmerge.
 326          */
 327         __rq = elv_rqhash_find(q, bio->bi_iter.bi_sector);
 328         if (__rq && elv_bio_merge_ok(__rq, bio)) {
 329                 *req = __rq;
 330                 return ELEVATOR_BACK_MERGE;
 331         }
 332
 333         if (e->type->ops.request_merge)
 334                 return e->type->ops.request_merge(q, req, bio);
 335
 336         return ELEVATOR_NO_MERGE;
 337 }
 338
 339 /*
 340  * Attempt to do an insertion back merge. Only check for the case where
 341  * we can append 'rq' to an existing request, so we can throw 'rq' away
 342  * afterwards.
 343  *
 344  * Returns true if we merged, false otherwise
 345  */
 346 bool elv_attempt_insert_merge(struct request_queue *q, struct request *rq)
 347 {
 348         struct request *__rq;
 349         bool ret;
 350
 351         if (blk_queue_nomerges(q))
 352                 return false;
 353
 354         /*
 355          * First try one-hit cache.
 356          */
 357         if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
 358                 return true;
 359
 360         if (blk_queue_noxmerges(q))
 361                 return false;
 362
 363         ret = false;
 364         /*
 365          * See if our hash lookup can find a potential backmerge.
 366          */
 367         while (1) {
 368                 __rq = elv_rqhash_find(q, blk_rq_pos(rq));
 369                 if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
 370                         break;
 371
 372                 /* The merged request could be merged with others, try again */
 373                 ret = true;
 374                 rq = __rq;
 375         }
 376
 377         return ret;
 378 }
 379
 380 void elv_merged_request(struct request_queue *q, struct request *rq,
 381                 enum elv_merge type)
 382 {
 383         struct elevator_queue *e = q->elevator;
 384
 385         if (e->type->ops.request_merged)
 386                 e->type->ops.request_merged(q, rq, type);
 387
 388         if (type == ELEVATOR_BACK_MERGE)
 389                 elv_rqhash_reposition(q, rq);
 390
 391         q->last_merge = rq;
 392 }
 393
 394 void elv_merge_requests(struct request_queue *q, struct request *rq,
 395                              struct request *next)
 396 {
 397         struct elevator_queue *e = q->elevator;
 398
 399         if (e->type->ops.requests_merged)
 400                 e->type->ops.requests_merged(q, rq, next);
 401
 402         elv_rqhash_reposition(q, rq);
 403         q->last_merge = rq;
 404 }
 405
 406 struct request *elv_latter_request(struct request_queue *q, struct request *rq)
 407 {
 408         struct elevator_queue *e = q->elevator;
 409
 410         if (e->type->ops.next_request)
 411                 return e->type->ops.next_request(q, rq);
 412
 413         return NULL;
 414 }
 415
 416 struct request *elv_former_request(struct request_queue *q, struct request *rq)
 417 {
 418         struct elevator_queue *e = q->elevator;
 419
 420         if (e->type->ops.former_request)
 421                 return e->type->ops.former_request(q, rq);
 422
 423         return NULL;
 424 }
 425
 426 #define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)
 427
 428 static ssize_t
 429 elv_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
 430 {
 431         struct elv_fs_entry *entry = to_elv(attr);
 432         struct elevator_queue *e;
 433         ssize_t error;
 434
 435         if (!entry->show)
 436                 return -EIO;
 437
 438         e = container_of(kobj, struct elevator_queue, kobj);
 439         mutex_lock(&e->sysfs_lock);
 440         error = e->type ? entry->show(e, page) : -ENOENT;
 441         mutex_unlock(&e->sysfs_lock);
 442         return error;
 443 }
 444
 445 static ssize_t
 446 elv_attr_store(struct kobject *kobj, struct attribute *attr,
 447                const char *page, size_t length)
 448 {
 449         struct elv_fs_entry *entry = to_elv(attr);
 450         struct elevator_queue *e;
 451         ssize_t error;
 452
 453         if (!entry->store)
 454                 return -EIO;
 455
 456         e = container_of(kobj, struct elevator_queue, kobj);
 457         mutex_lock(&e->sysfs_lock);
 458         error = e->type ? entry->store(e, page, length) : -ENOENT;
 459         mutex_unlock(&e->sysfs_lock);
 460         return error;
 461 }
 462
 463 static const struct sysfs_ops elv_sysfs_ops = {
 464         .show   = elv_attr_show,
 465         .store  = elv_attr_store,
 466 };
 467
 468 static struct kobj_type elv_ktype = {
 469         .sysfs_ops      = &elv_sysfs_ops,
 470         .release        = elevator_release,
 471 };
 472
 473 int elv_register_queue(struct request_queue *q)
 474 {
 475         struct elevator_queue *e = q->elevator;
 476         int error;
 477
 478         lockdep_assert_held(&q->sysfs_lock);
 479
 480         error = kobject_add(&e->kobj, &q->kobj, "%s", "iosched");
 481         if (!error) {
 482                 struct elv_fs_entry *attr = e->type->elevator_attrs;
 483                 if (attr) {
 484                         while (attr->attr.name) {
 485                                 if (sysfs_create_file(&e->kobj, &attr->attr))
 486                                         break;
 487                                 attr++;
 488                         }
 489                 }
 490                 kobject_uevent(&e->kobj, KOBJ_ADD);
 491                 e->registered = 1;
 492         }
 493         return error;
 494 }
 495
 496 void elv_unregister_queue(struct request_queue *q)
 497 {
 498         lockdep_assert_held(&q->sysfs_lock);
 499
 500         if (q) {
 501                 struct elevator_queue *e = q->elevator;
 502
 503                 kobject_uevent(&e->kobj, KOBJ_REMOVE);
 504                 kobject_del(&e->kobj);
 505                 e->registered = 0;
 506                 /* Re-enable throttling in case elevator disabled it */
 507                 wbt_enable_default(q);
 508         }
 509 }
 510
 511 int elv_register(struct elevator_type *e)
 512 {
 513         /* create icq_cache if requested */
 514         if (e->icq_size) {
 515                 if (WARN_ON(e->icq_size < sizeof(struct io_cq)) ||
 516                     WARN_ON(e->icq_align < __alignof__(struct io_cq)))
 517                         return -EINVAL;
 518
 519                 snprintf(e->icq_cache_name, sizeof(e->icq_cache_name),
 520                          "%s_io_cq", e->elevator_name);
 521                 e->icq_cache = kmem_cache_create(e->icq_cache_name, e->icq_size,
 522                                                  e->icq_align, 0, NULL);
 523                 if (!e->icq_cache)
 524                         return -ENOMEM;
 525         }
 526
 527         /* register, don't allow duplicate names */
 528         spin_lock(&elv_list_lock);
 529         if (elevator_find(e->elevator_name)) {
 530                 spin_unlock(&elv_list_lock);
 531                 kmem_cache_destroy(e->icq_cache);
 532                 return -EBUSY;
 533         }
 534         list_add_tail(&e->list, &elv_list);
 535         spin_unlock(&elv_list_lock);
 536
 537         printk(KERN_INFO "io scheduler %s registered\n", e->elevator_name);
 538
 539         return 0;
 540 }
 541 EXPORT_SYMBOL_GPL(elv_register);
 542
 543 void elv_unregister(struct elevator_type *e)
 544 {
 545         /* unregister */
 546         spin_lock(&elv_list_lock);
 547         list_del_init(&e->list);
 548         spin_unlock(&elv_list_lock);
 549
 550         /*
 551          * Destroy icq_cache if it exists.  icq's are RCU managed.  Make
 552          * sure all RCU operations are complete before proceeding.
 553          */
 554         if (e->icq_cache) {
 555                 rcu_barrier();
 556                 kmem_cache_destroy(e->icq_cache);
 557                 e->icq_cache = NULL;
 558         }
 559 }
 560 EXPORT_SYMBOL_GPL(elv_unregister);
 561
 562 int elevator_switch_mq(struct request_queue *q,
 563                               struct elevator_type *new_e)
 564 {
 565         int ret;
 566
 567         lockdep_assert_held(&q->sysfs_lock);
 568
 569         if (q->elevator) {
 570                 if (q->elevator->registered)
 571                         elv_unregister_queue(q);
 572                 ioc_clear_queue(q);
 573                 elevator_exit(q, q->elevator);
 574         }
 575
 576         ret = blk_mq_init_sched(q, new_e);
 577         if (ret)
 578                 goto out;
 579
 580         if (new_e) {
 581                 ret = elv_register_queue(q);
 582                 if (ret) {
 583                         elevator_exit(q, q->elevator);
 584                         goto out;
 585                 }
 586         }
 587
 588         if (new_e)
 589                 blk_add_trace_msg(q, "elv switch: %s", new_e->elevator_name);
 590         else
 591                 blk_add_trace_msg(q, "elv switch: none");
 592
 593 out:
 594         return ret;
 595 }
 596
 597 /*
 598  * For blk-mq devices, we default to using mq-deadline, if available, for single
 599  * queue devices.  If deadline isn't available OR we have multiple queues,
 600  * default to "none".
 601  */
 602 int elevator_init_mq(struct request_queue *q)
 603 {
 604         struct elevator_type *e;
 605         int err = 0;
 606
 607         if (q->nr_hw_queues != 1)
 608                 return 0;
 609
 610         /*
 611          * q->sysfs_lock must be held to provide mutual exclusion between
 612          * elevator_switch() and here.
 613          */
 614         mutex_lock(&q->sysfs_lock);
 615         if (unlikely(q->elevator))
 616                 goto out_unlock;
 617
 618         e = elevator_get(q, "mq-deadline", false);
 619         if (!e)
 620                 goto out_unlock;
 621
 622         err = blk_mq_init_sched(q, e);
 623         if (err)
 624                 elevator_put(e);
 625 out_unlock:
 626         mutex_unlock(&q->sysfs_lock);
 627         return err;
 628 }
 629
 630
 631 /*
 632  * switch to new_e io scheduler. be careful not to introduce deadlocks -
 633  * we don't free the old io scheduler, before we have allocated what we
 634  * need for the new one. this way we have a chance of going back to the old
 635  * one, if the new one fails init for some reason.
 636  */
 637 static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 638 {
 639         int err;
 640
 641         lockdep_assert_held(&q->sysfs_lock);
 642
 643         blk_mq_freeze_queue(q);
 644         blk_mq_quiesce_queue(q);
 645
 646         err = elevator_switch_mq(q, new_e);
 647
 648         blk_mq_unquiesce_queue(q);
 649         blk_mq_unfreeze_queue(q);
 650
 651         return err;
 652 }
 653
 654 /*
 655  * Switch this queue to the given IO scheduler.
 656  */
 657 static int __elevator_change(struct request_queue *q, const char *name)
 658 {
 659         char elevator_name[ELV_NAME_MAX];
 660         struct elevator_type *e;
 661
 662         /* Make sure queue is not in the middle of being removed */
 663         if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
 664                 return -ENOENT;
 665
 666         /*
 667          * Special case for mq, turn off scheduling
 668          */
 669         if (!strncmp(name, "none", 4)) {
 670                 if (!q->elevator)
 671                         return 0;
 672                 return elevator_switch(q, NULL);
 673         }
 674
 675         strlcpy(elevator_name, name, sizeof(elevator_name));
 676         e = elevator_get(q, strstrip(elevator_name), true);
 677         if (!e)
 678                 return -EINVAL;
 679
 680         if (q->elevator && elevator_match(q->elevator->type, elevator_name)) {
 681                 elevator_put(e);
 682                 return 0;
 683         }
 684
 685         return elevator_switch(q, e);
 686 }
 687
 688 static inline bool elv_support_iosched(struct request_queue *q)
 689 {
 690         if (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED))
 691                 return false;
 692         return true;
 693 }
 694
 695 ssize_t elv_iosched_store(struct request_queue *q, const char *name,
 696                           size_t count)
 697 {
 698         int ret;
 699
 700         if (!queue_is_mq(q) || !elv_support_iosched(q))
 701                 return count;
 702
 703         ret = __elevator_change(q, name);
 704         if (!ret)
 705                 return count;
 706
 707         return ret;
 708 }
 709
 710 ssize_t elv_iosched_show(struct request_queue *q, char *name)
 711 {
 712         struct elevator_queue *e = q->elevator;
 713         struct elevator_type *elv = NULL;
 714         struct elevator_type *__e;
 715         int len = 0;
 716
 717         if (!queue_is_mq(q))
 718                 return sprintf(name, "none\n");
 719
 720         if (!q->elevator)
 721                 len += sprintf(name+len, "[none] ");
 722         else
 723                 elv = e->type;
 724
 725         spin_lock(&elv_list_lock);
 726         list_for_each_entry(__e, &elv_list, list) {
 727                 if (elv && elevator_match(elv, __e->elevator_name)) {
 728                         len += sprintf(name+len, "[%s] ", elv->elevator_name);
 729                         continue;
 730                 }
 731                 if (elv_support_iosched(q))
 732                         len += sprintf(name+len, "%s ", __e->elevator_name);
 733         }
 734         spin_unlock(&elv_list_lock);
 735
 736         if (q->elevator)
 737                 len += sprintf(name+len, "none");
 738
 739         len += sprintf(len+name, "\n");
 740         return len;
 741 }
 742
 743 struct request *elv_rb_former_request(struct request_queue *q,
 744                                       struct request *rq)
 745 {
 746         struct rb_node *rbprev = rb_prev(&rq->rb_node);
 747
 748         if (rbprev)
 749                 return rb_entry_rq(rbprev);
 750
 751         return NULL;
 752 }
 753 EXPORT_SYMBOL(elv_rb_former_request);
 754
 755 struct request *elv_rb_latter_request(struct request_queue *q,
 756                                       struct request *rq)
 757 {
 758         struct rb_node *rbnext = rb_next(&rq->rb_node);
 759
 760         if (rbnext)
 761                 return rb_entry_rq(rbnext);
 762
 763         return NULL;
 764 }
 765 EXPORT_SYMBOL(elv_rb_latter_request);