block/elevator.c

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