1 /* 2 * Functions related to segment and merge handling 3 */ 4 #include <linux/kernel.h> 5 #include <linux/module.h> 6 #include <linux/bio.h> 7 #include <linux/blkdev.h> 8 #include <linux/scatterlist.h> 9 10 #include "blk.h" 11 12 static struct bio *blk_bio_discard_split(struct request_queue *q, 13 struct bio *bio, 14 struct bio_set *bs, 15 unsigned *nsegs) 16 { 17 unsigned int max_discard_sectors, granularity; 18 int alignment; 19 sector_t tmp; 20 unsigned split_sectors; 21 22 *nsegs = 1; 23 24 /* Zero-sector (unknown) and one-sector granularities are the same. */ 25 granularity = max(q->limits.discard_granularity >> 9, 1U); 26 27 max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9); 28 max_discard_sectors -= max_discard_sectors % granularity; 29 30 if (unlikely(!max_discard_sectors)) { 31 /* XXX: warn */ 32 return NULL; 33 } 34 35 if (bio_sectors(bio) <= max_discard_sectors) 36 return NULL; 37 38 split_sectors = max_discard_sectors; 39 40 /* 41 * If the next starting sector would be misaligned, stop the discard at 42 * the previous aligned sector. 43 */ 44 alignment = (q->limits.discard_alignment >> 9) % granularity; 45 46 tmp = bio->bi_iter.bi_sector + split_sectors - alignment; 47 tmp = sector_div(tmp, granularity); 48 49 if (split_sectors > tmp) 50 split_sectors -= tmp; 51 52 return bio_split(bio, split_sectors, GFP_NOIO, bs); 53 } 54 55 static struct bio *blk_bio_write_same_split(struct request_queue *q, 56 struct bio *bio, 57 struct bio_set *bs, 58 unsigned *nsegs) 59 { 60 *nsegs = 1; 61 62 if (!q->limits.max_write_same_sectors) 63 return NULL; 64 65 if (bio_sectors(bio) <= q->limits.max_write_same_sectors) 66 return NULL; 67 68 return bio_split(bio, q->limits.max_write_same_sectors, GFP_NOIO, bs); 69 } 70 71 static struct bio *blk_bio_segment_split(struct request_queue *q, 72 struct bio *bio, 73 struct bio_set *bs, 74 unsigned *segs) 75 { 76 struct bio_vec bv, bvprv, *bvprvp = NULL; 77 struct bvec_iter iter; 78 unsigned seg_size = 0, nsegs = 0, sectors = 0; 79 unsigned front_seg_size = bio->bi_seg_front_size; 80 bool do_split = true; 81 struct bio *new = NULL; 82 83 bio_for_each_segment(bv, bio, iter) { 84 if (sectors + (bv.bv_len >> 9) > queue_max_sectors(q)) 85 goto split; 86 87 /* 88 * If the queue doesn't support SG gaps and adding this 89 * offset would create a gap, disallow it. 90 */ 91 if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset)) 92 goto split; 93 94 if (bvprvp && blk_queue_cluster(q)) { 95 if (seg_size + bv.bv_len > queue_max_segment_size(q)) 96 goto new_segment; 97 if (!BIOVEC_PHYS_MERGEABLE(bvprvp, &bv)) 98 goto new_segment; 99 if (!BIOVEC_SEG_BOUNDARY(q, bvprvp, &bv)) 100 goto new_segment; 101 102 seg_size += bv.bv_len; 103 bvprv = bv; 104 bvprvp = &bvprv; 105 sectors += bv.bv_len >> 9; 106 continue; 107 } 108 new_segment: 109 if (nsegs == queue_max_segments(q)) 110 goto split; 111 112 nsegs++; 113 bvprv = bv; 114 bvprvp = &bvprv; 115 seg_size = bv.bv_len; 116 sectors += bv.bv_len >> 9; 117 118 if (nsegs == 1 && seg_size > front_seg_size) 119 front_seg_size = seg_size; 120 } 121 122 do_split = false; 123 split: 124 *segs = nsegs; 125 126 if (do_split) { 127 new = bio_split(bio, sectors, GFP_NOIO, bs); 128 if (new) 129 bio = new; 130 } 131 132 bio->bi_seg_front_size = front_seg_size; 133 if (seg_size > bio->bi_seg_back_size) 134 bio->bi_seg_back_size = seg_size; 135 136 return do_split ? new : NULL; 137 } 138 139 void blk_queue_split(struct request_queue *q, struct bio **bio, 140 struct bio_set *bs) 141 { 142 struct bio *split, *res; 143 unsigned nsegs; 144 145 if ((*bio)->bi_rw & REQ_DISCARD) 146 split = blk_bio_discard_split(q, *bio, bs, &nsegs); 147 else if ((*bio)->bi_rw & REQ_WRITE_SAME) 148 split = blk_bio_write_same_split(q, *bio, bs, &nsegs); 149 else 150 split = blk_bio_segment_split(q, *bio, q->bio_split, &nsegs); 151 152 /* physical segments can be figured out during splitting */ 153 res = split ? split : *bio; 154 res->bi_phys_segments = nsegs; 155 bio_set_flag(res, BIO_SEG_VALID); 156 157 if (split) { 158 /* there isn't chance to merge the splitted bio */ 159 split->bi_rw |= REQ_NOMERGE; 160 161 bio_chain(split, *bio); 162 generic_make_request(*bio); 163 *bio = split; 164 } 165 } 166 EXPORT_SYMBOL(blk_queue_split); 167 168 static unsigned int __blk_recalc_rq_segments(struct request_queue *q, 169 struct bio *bio, 170 bool no_sg_merge) 171 { 172 struct bio_vec bv, bvprv = { NULL }; 173 int cluster, prev = 0; 174 unsigned int seg_size, nr_phys_segs; 175 struct bio *fbio, *bbio; 176 struct bvec_iter iter; 177 178 if (!bio) 179 return 0; 180 181 /* 182 * This should probably be returning 0, but blk_add_request_payload() 183 * (Christoph!!!!) 184 */ 185 if (bio->bi_rw & REQ_DISCARD) 186 return 1; 187 188 if (bio->bi_rw & REQ_WRITE_SAME) 189 return 1; 190 191 fbio = bio; 192 cluster = blk_queue_cluster(q); 193 seg_size = 0; 194 nr_phys_segs = 0; 195 for_each_bio(bio) { 196 bio_for_each_segment(bv, bio, iter) { 197 /* 198 * If SG merging is disabled, each bio vector is 199 * a segment 200 */ 201 if (no_sg_merge) 202 goto new_segment; 203 204 if (prev && cluster) { 205 if (seg_size + bv.bv_len 206 > queue_max_segment_size(q)) 207 goto new_segment; 208 if (!BIOVEC_PHYS_MERGEABLE(&bvprv, &bv)) 209 goto new_segment; 210 if (!BIOVEC_SEG_BOUNDARY(q, &bvprv, &bv)) 211 goto new_segment; 212 213 seg_size += bv.bv_len; 214 bvprv = bv; 215 continue; 216 } 217 new_segment: 218 if (nr_phys_segs == 1 && seg_size > 219 fbio->bi_seg_front_size) 220 fbio->bi_seg_front_size = seg_size; 221 222 nr_phys_segs++; 223 bvprv = bv; 224 prev = 1; 225 seg_size = bv.bv_len; 226 } 227 bbio = bio; 228 } 229 230 if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size) 231 fbio->bi_seg_front_size = seg_size; 232 if (seg_size > bbio->bi_seg_back_size) 233 bbio->bi_seg_back_size = seg_size; 234 235 return nr_phys_segs; 236 } 237 238 void blk_recalc_rq_segments(struct request *rq) 239 { 240 bool no_sg_merge = !!test_bit(QUEUE_FLAG_NO_SG_MERGE, 241 &rq->q->queue_flags); 242 243 rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio, 244 no_sg_merge); 245 } 246 247 void blk_recount_segments(struct request_queue *q, struct bio *bio) 248 { 249 unsigned short seg_cnt; 250 251 /* estimate segment number by bi_vcnt for non-cloned bio */ 252 if (bio_flagged(bio, BIO_CLONED)) 253 seg_cnt = bio_segments(bio); 254 else 255 seg_cnt = bio->bi_vcnt; 256 257 if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) && 258 (seg_cnt < queue_max_segments(q))) 259 bio->bi_phys_segments = seg_cnt; 260 else { 261 struct bio *nxt = bio->bi_next; 262 263 bio->bi_next = NULL; 264 bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, false); 265 bio->bi_next = nxt; 266 } 267 268 bio_set_flag(bio, BIO_SEG_VALID); 269 } 270 EXPORT_SYMBOL(blk_recount_segments); 271 272 static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio, 273 struct bio *nxt) 274 { 275 struct bio_vec end_bv = { NULL }, nxt_bv; 276 struct bvec_iter iter; 277 278 if (!blk_queue_cluster(q)) 279 return 0; 280 281 if (bio->bi_seg_back_size + nxt->bi_seg_front_size > 282 queue_max_segment_size(q)) 283 return 0; 284 285 if (!bio_has_data(bio)) 286 return 1; 287 288 bio_for_each_segment(end_bv, bio, iter) 289 if (end_bv.bv_len == iter.bi_size) 290 break; 291 292 nxt_bv = bio_iovec(nxt); 293 294 if (!BIOVEC_PHYS_MERGEABLE(&end_bv, &nxt_bv)) 295 return 0; 296 297 /* 298 * bio and nxt are contiguous in memory; check if the queue allows 299 * these two to be merged into one 300 */ 301 if (BIOVEC_SEG_BOUNDARY(q, &end_bv, &nxt_bv)) 302 return 1; 303 304 return 0; 305 } 306 307 static inline void 308 __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec, 309 struct scatterlist *sglist, struct bio_vec *bvprv, 310 struct scatterlist **sg, int *nsegs, int *cluster) 311 { 312 313 int nbytes = bvec->bv_len; 314 315 if (*sg && *cluster) { 316 if ((*sg)->length + nbytes > queue_max_segment_size(q)) 317 goto new_segment; 318 319 if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec)) 320 goto new_segment; 321 if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec)) 322 goto new_segment; 323 324 (*sg)->length += nbytes; 325 } else { 326 new_segment: 327 if (!*sg) 328 *sg = sglist; 329 else { 330 /* 331 * If the driver previously mapped a shorter 332 * list, we could see a termination bit 333 * prematurely unless it fully inits the sg 334 * table on each mapping. We KNOW that there 335 * must be more entries here or the driver 336 * would be buggy, so force clear the 337 * termination bit to avoid doing a full 338 * sg_init_table() in drivers for each command. 339 */ 340 sg_unmark_end(*sg); 341 *sg = sg_next(*sg); 342 } 343 344 sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset); 345 (*nsegs)++; 346 } 347 *bvprv = *bvec; 348 } 349 350 static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio, 351 struct scatterlist *sglist, 352 struct scatterlist **sg) 353 { 354 struct bio_vec bvec, bvprv = { NULL }; 355 struct bvec_iter iter; 356 int nsegs, cluster; 357 358 nsegs = 0; 359 cluster = blk_queue_cluster(q); 360 361 if (bio->bi_rw & REQ_DISCARD) { 362 /* 363 * This is a hack - drivers should be neither modifying the 364 * biovec, nor relying on bi_vcnt - but because of 365 * blk_add_request_payload(), a discard bio may or may not have 366 * a payload we need to set up here (thank you Christoph) and 367 * bi_vcnt is really the only way of telling if we need to. 368 */ 369 370 if (bio->bi_vcnt) 371 goto single_segment; 372 373 return 0; 374 } 375 376 if (bio->bi_rw & REQ_WRITE_SAME) { 377 single_segment: 378 *sg = sglist; 379 bvec = bio_iovec(bio); 380 sg_set_page(*sg, bvec.bv_page, bvec.bv_len, bvec.bv_offset); 381 return 1; 382 } 383 384 for_each_bio(bio) 385 bio_for_each_segment(bvec, bio, iter) 386 __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg, 387 &nsegs, &cluster); 388 389 return nsegs; 390 } 391 392 /* 393 * map a request to scatterlist, return number of sg entries setup. Caller 394 * must make sure sg can hold rq->nr_phys_segments entries 395 */ 396 int blk_rq_map_sg(struct request_queue *q, struct request *rq, 397 struct scatterlist *sglist) 398 { 399 struct scatterlist *sg = NULL; 400 int nsegs = 0; 401 402 if (rq->bio) 403 nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg); 404 405 if (unlikely(rq->cmd_flags & REQ_COPY_USER) && 406 (blk_rq_bytes(rq) & q->dma_pad_mask)) { 407 unsigned int pad_len = 408 (q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1; 409 410 sg->length += pad_len; 411 rq->extra_len += pad_len; 412 } 413 414 if (q->dma_drain_size && q->dma_drain_needed(rq)) { 415 if (rq->cmd_flags & REQ_WRITE) 416 memset(q->dma_drain_buffer, 0, q->dma_drain_size); 417 418 sg_unmark_end(sg); 419 sg = sg_next(sg); 420 sg_set_page(sg, virt_to_page(q->dma_drain_buffer), 421 q->dma_drain_size, 422 ((unsigned long)q->dma_drain_buffer) & 423 (PAGE_SIZE - 1)); 424 nsegs++; 425 rq->extra_len += q->dma_drain_size; 426 } 427 428 if (sg) 429 sg_mark_end(sg); 430 431 /* 432 * Something must have been wrong if the figured number of 433 * segment is bigger than number of req's physical segments 434 */ 435 WARN_ON(nsegs > rq->nr_phys_segments); 436 437 return nsegs; 438 } 439 EXPORT_SYMBOL(blk_rq_map_sg); 440 441 static inline int ll_new_hw_segment(struct request_queue *q, 442 struct request *req, 443 struct bio *bio) 444 { 445 int nr_phys_segs = bio_phys_segments(q, bio); 446 447 if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(q)) 448 goto no_merge; 449 450 if (blk_integrity_merge_bio(q, req, bio) == false) 451 goto no_merge; 452 453 /* 454 * This will form the start of a new hw segment. Bump both 455 * counters. 456 */ 457 req->nr_phys_segments += nr_phys_segs; 458 return 1; 459 460 no_merge: 461 req->cmd_flags |= REQ_NOMERGE; 462 if (req == q->last_merge) 463 q->last_merge = NULL; 464 return 0; 465 } 466 467 int ll_back_merge_fn(struct request_queue *q, struct request *req, 468 struct bio *bio) 469 { 470 if (req_gap_back_merge(req, bio)) 471 return 0; 472 if (blk_integrity_rq(req) && 473 integrity_req_gap_back_merge(req, bio)) 474 return 0; 475 if (blk_rq_sectors(req) + bio_sectors(bio) > 476 blk_rq_get_max_sectors(req)) { 477 req->cmd_flags |= REQ_NOMERGE; 478 if (req == q->last_merge) 479 q->last_merge = NULL; 480 return 0; 481 } 482 if (!bio_flagged(req->biotail, BIO_SEG_VALID)) 483 blk_recount_segments(q, req->biotail); 484 if (!bio_flagged(bio, BIO_SEG_VALID)) 485 blk_recount_segments(q, bio); 486 487 return ll_new_hw_segment(q, req, bio); 488 } 489 490 int ll_front_merge_fn(struct request_queue *q, struct request *req, 491 struct bio *bio) 492 { 493 494 if (req_gap_front_merge(req, bio)) 495 return 0; 496 if (blk_integrity_rq(req) && 497 integrity_req_gap_front_merge(req, bio)) 498 return 0; 499 if (blk_rq_sectors(req) + bio_sectors(bio) > 500 blk_rq_get_max_sectors(req)) { 501 req->cmd_flags |= REQ_NOMERGE; 502 if (req == q->last_merge) 503 q->last_merge = NULL; 504 return 0; 505 } 506 if (!bio_flagged(bio, BIO_SEG_VALID)) 507 blk_recount_segments(q, bio); 508 if (!bio_flagged(req->bio, BIO_SEG_VALID)) 509 blk_recount_segments(q, req->bio); 510 511 return ll_new_hw_segment(q, req, bio); 512 } 513 514 /* 515 * blk-mq uses req->special to carry normal driver per-request payload, it 516 * does not indicate a prepared command that we cannot merge with. 517 */ 518 static bool req_no_special_merge(struct request *req) 519 { 520 struct request_queue *q = req->q; 521 522 return !q->mq_ops && req->special; 523 } 524 525 static int ll_merge_requests_fn(struct request_queue *q, struct request *req, 526 struct request *next) 527 { 528 int total_phys_segments; 529 unsigned int seg_size = 530 req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size; 531 532 /* 533 * First check if the either of the requests are re-queued 534 * requests. Can't merge them if they are. 535 */ 536 if (req_no_special_merge(req) || req_no_special_merge(next)) 537 return 0; 538 539 if (req_gap_back_merge(req, next->bio)) 540 return 0; 541 542 /* 543 * Will it become too large? 544 */ 545 if ((blk_rq_sectors(req) + blk_rq_sectors(next)) > 546 blk_rq_get_max_sectors(req)) 547 return 0; 548 549 total_phys_segments = req->nr_phys_segments + next->nr_phys_segments; 550 if (blk_phys_contig_segment(q, req->biotail, next->bio)) { 551 if (req->nr_phys_segments == 1) 552 req->bio->bi_seg_front_size = seg_size; 553 if (next->nr_phys_segments == 1) 554 next->biotail->bi_seg_back_size = seg_size; 555 total_phys_segments--; 556 } 557 558 if (total_phys_segments > queue_max_segments(q)) 559 return 0; 560 561 if (blk_integrity_merge_rq(q, req, next) == false) 562 return 0; 563 564 /* Merge is OK... */ 565 req->nr_phys_segments = total_phys_segments; 566 return 1; 567 } 568 569 /** 570 * blk_rq_set_mixed_merge - mark a request as mixed merge 571 * @rq: request to mark as mixed merge 572 * 573 * Description: 574 * @rq is about to be mixed merged. Make sure the attributes 575 * which can be mixed are set in each bio and mark @rq as mixed 576 * merged. 577 */ 578 void blk_rq_set_mixed_merge(struct request *rq) 579 { 580 unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK; 581 struct bio *bio; 582 583 if (rq->cmd_flags & REQ_MIXED_MERGE) 584 return; 585 586 /* 587 * @rq will no longer represent mixable attributes for all the 588 * contained bios. It will just track those of the first one. 589 * Distributes the attributs to each bio. 590 */ 591 for (bio = rq->bio; bio; bio = bio->bi_next) { 592 WARN_ON_ONCE((bio->bi_rw & REQ_FAILFAST_MASK) && 593 (bio->bi_rw & REQ_FAILFAST_MASK) != ff); 594 bio->bi_rw |= ff; 595 } 596 rq->cmd_flags |= REQ_MIXED_MERGE; 597 } 598 599 static void blk_account_io_merge(struct request *req) 600 { 601 if (blk_do_io_stat(req)) { 602 struct hd_struct *part; 603 int cpu; 604 605 cpu = part_stat_lock(); 606 part = req->part; 607 608 part_round_stats(cpu, part); 609 part_dec_in_flight(part, rq_data_dir(req)); 610 611 hd_struct_put(part); 612 part_stat_unlock(); 613 } 614 } 615 616 /* 617 * Has to be called with the request spinlock acquired 618 */ 619 static int attempt_merge(struct request_queue *q, struct request *req, 620 struct request *next) 621 { 622 if (!rq_mergeable(req) || !rq_mergeable(next)) 623 return 0; 624 625 if (!blk_check_merge_flags(req->cmd_flags, next->cmd_flags)) 626 return 0; 627 628 /* 629 * not contiguous 630 */ 631 if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next)) 632 return 0; 633 634 if (rq_data_dir(req) != rq_data_dir(next) 635 || req->rq_disk != next->rq_disk 636 || req_no_special_merge(next)) 637 return 0; 638 639 if (req->cmd_flags & REQ_WRITE_SAME && 640 !blk_write_same_mergeable(req->bio, next->bio)) 641 return 0; 642 643 /* 644 * If we are allowed to merge, then append bio list 645 * from next to rq and release next. merge_requests_fn 646 * will have updated segment counts, update sector 647 * counts here. 648 */ 649 if (!ll_merge_requests_fn(q, req, next)) 650 return 0; 651 652 /* 653 * If failfast settings disagree or any of the two is already 654 * a mixed merge, mark both as mixed before proceeding. This 655 * makes sure that all involved bios have mixable attributes 656 * set properly. 657 */ 658 if ((req->cmd_flags | next->cmd_flags) & REQ_MIXED_MERGE || 659 (req->cmd_flags & REQ_FAILFAST_MASK) != 660 (next->cmd_flags & REQ_FAILFAST_MASK)) { 661 blk_rq_set_mixed_merge(req); 662 blk_rq_set_mixed_merge(next); 663 } 664 665 /* 666 * At this point we have either done a back merge 667 * or front merge. We need the smaller start_time of 668 * the merged requests to be the current request 669 * for accounting purposes. 670 */ 671 if (time_after(req->start_time, next->start_time)) 672 req->start_time = next->start_time; 673 674 req->biotail->bi_next = next->bio; 675 req->biotail = next->biotail; 676 677 req->__data_len += blk_rq_bytes(next); 678 679 elv_merge_requests(q, req, next); 680 681 /* 682 * 'next' is going away, so update stats accordingly 683 */ 684 blk_account_io_merge(next); 685 686 req->ioprio = ioprio_best(req->ioprio, next->ioprio); 687 if (blk_rq_cpu_valid(next)) 688 req->cpu = next->cpu; 689 690 /* owner-ship of bio passed from next to req */ 691 next->bio = NULL; 692 __blk_put_request(q, next); 693 return 1; 694 } 695 696 int attempt_back_merge(struct request_queue *q, struct request *rq) 697 { 698 struct request *next = elv_latter_request(q, rq); 699 700 if (next) 701 return attempt_merge(q, rq, next); 702 703 return 0; 704 } 705 706 int attempt_front_merge(struct request_queue *q, struct request *rq) 707 { 708 struct request *prev = elv_former_request(q, rq); 709 710 if (prev) 711 return attempt_merge(q, prev, rq); 712 713 return 0; 714 } 715 716 int blk_attempt_req_merge(struct request_queue *q, struct request *rq, 717 struct request *next) 718 { 719 return attempt_merge(q, rq, next); 720 } 721 722 bool blk_rq_merge_ok(struct request *rq, struct bio *bio) 723 { 724 if (!rq_mergeable(rq) || !bio_mergeable(bio)) 725 return false; 726 727 if (!blk_check_merge_flags(rq->cmd_flags, bio->bi_rw)) 728 return false; 729 730 /* different data direction or already started, don't merge */ 731 if (bio_data_dir(bio) != rq_data_dir(rq)) 732 return false; 733 734 /* must be same device and not a special request */ 735 if (rq->rq_disk != bio->bi_bdev->bd_disk || req_no_special_merge(rq)) 736 return false; 737 738 /* only merge integrity protected bio into ditto rq */ 739 if (blk_integrity_merge_bio(rq->q, rq, bio) == false) 740 return false; 741 742 /* must be using the same buffer */ 743 if (rq->cmd_flags & REQ_WRITE_SAME && 744 !blk_write_same_mergeable(rq->bio, bio)) 745 return false; 746 747 return true; 748 } 749 750 int blk_try_merge(struct request *rq, struct bio *bio) 751 { 752 if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector) 753 return ELEVATOR_BACK_MERGE; 754 else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector) 755 return ELEVATOR_FRONT_MERGE; 756 return ELEVATOR_NO_MERGE; 757 } 758