1 // SPDX-License-Identifier: GPL-2.0
3 * Functions related to segment and merge handling
5 #include <linux/kernel.h>
6 #include <linux/module.h>
8 #include <linux/blkdev.h>
9 #include <linux/scatterlist.h>
10 #include <linux/blk-cgroup.h>
12 #include <trace/events/block.h>
16 static inline bool bio_will_gap(struct request_queue *q,
17 struct request *prev_rq, struct bio *prev, struct bio *next)
19 struct bio_vec pb, nb;
21 if (!bio_has_data(prev) || !queue_virt_boundary(q))
25 * Don't merge if the 1st bio starts with non-zero offset, otherwise it
26 * is quite difficult to respect the sg gap limit. We work hard to
27 * merge a huge number of small single bios in case of mkfs.
30 bio_get_first_bvec(prev_rq->bio, &pb);
32 bio_get_first_bvec(prev, &pb);
33 if (pb.bv_offset & queue_virt_boundary(q))
37 * We don't need to worry about the situation that the merged segment
38 * ends in unaligned virt boundary:
40 * - if 'pb' ends aligned, the merged segment ends aligned
41 * - if 'pb' ends unaligned, the next bio must include
42 * one single bvec of 'nb', otherwise the 'nb' can't
45 bio_get_last_bvec(prev, &pb);
46 bio_get_first_bvec(next, &nb);
47 if (biovec_phys_mergeable(q, &pb, &nb))
49 return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
52 static inline bool req_gap_back_merge(struct request *req, struct bio *bio)
54 return bio_will_gap(req->q, req, req->biotail, bio);
57 static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
59 return bio_will_gap(req->q, NULL, bio, req->bio);
62 static struct bio *blk_bio_discard_split(struct request_queue *q,
67 unsigned int max_discard_sectors, granularity;
70 unsigned split_sectors;
74 /* Zero-sector (unknown) and one-sector granularities are the same. */
75 granularity = max(q->limits.discard_granularity >> 9, 1U);
77 max_discard_sectors = min(q->limits.max_discard_sectors,
78 bio_allowed_max_sectors(q));
79 max_discard_sectors -= max_discard_sectors % granularity;
81 if (unlikely(!max_discard_sectors)) {
86 if (bio_sectors(bio) <= max_discard_sectors)
89 split_sectors = max_discard_sectors;
92 * If the next starting sector would be misaligned, stop the discard at
93 * the previous aligned sector.
95 alignment = (q->limits.discard_alignment >> 9) % granularity;
97 tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
98 tmp = sector_div(tmp, granularity);
100 if (split_sectors > tmp)
101 split_sectors -= tmp;
103 return bio_split(bio, split_sectors, GFP_NOIO, bs);
106 static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
107 struct bio *bio, struct bio_set *bs, unsigned *nsegs)
111 if (!q->limits.max_write_zeroes_sectors)
114 if (bio_sectors(bio) <= q->limits.max_write_zeroes_sectors)
117 return bio_split(bio, q->limits.max_write_zeroes_sectors, GFP_NOIO, bs);
120 static struct bio *blk_bio_write_same_split(struct request_queue *q,
127 if (!q->limits.max_write_same_sectors)
130 if (bio_sectors(bio) <= q->limits.max_write_same_sectors)
133 return bio_split(bio, q->limits.max_write_same_sectors, GFP_NOIO, bs);
137 * Return the maximum number of sectors from the start of a bio that may be
138 * submitted as a single request to a block device. If enough sectors remain,
139 * align the end to the physical block size. Otherwise align the end to the
140 * logical block size. This approach minimizes the number of non-aligned
141 * requests that are submitted to a block device if the start of a bio is not
142 * aligned to a physical block boundary.
144 static inline unsigned get_max_io_size(struct request_queue *q,
147 unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
148 unsigned max_sectors = sectors;
149 unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
150 unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
151 unsigned start_offset = bio->bi_iter.bi_sector & (pbs - 1);
153 max_sectors += start_offset;
154 max_sectors &= ~(pbs - 1);
155 if (max_sectors > start_offset)
156 return max_sectors - start_offset;
158 return sectors & ~(lbs - 1);
161 static inline unsigned get_max_segment_size(const struct request_queue *q,
162 struct page *start_page,
163 unsigned long offset)
165 unsigned long mask = queue_segment_boundary(q);
167 offset = mask & (page_to_phys(start_page) + offset);
170 * overflow may be triggered in case of zero page physical address
171 * on 32bit arch, use queue's max segment size when that happens.
173 return min_not_zero(mask - offset + 1,
174 (unsigned long)queue_max_segment_size(q));
178 * bvec_split_segs - verify whether or not a bvec should be split in the middle
179 * @q: [in] request queue associated with the bio associated with @bv
180 * @bv: [in] bvec to examine
181 * @nsegs: [in,out] Number of segments in the bio being built. Incremented
182 * by the number of segments from @bv that may be appended to that
183 * bio without exceeding @max_segs
184 * @sectors: [in,out] Number of sectors in the bio being built. Incremented
185 * by the number of sectors from @bv that may be appended to that
186 * bio without exceeding @max_sectors
187 * @max_segs: [in] upper bound for *@nsegs
188 * @max_sectors: [in] upper bound for *@sectors
190 * When splitting a bio, it can happen that a bvec is encountered that is too
191 * big to fit in a single segment and hence that it has to be split in the
192 * middle. This function verifies whether or not that should happen. The value
193 * %true is returned if and only if appending the entire @bv to a bio with
194 * *@nsegs segments and *@sectors sectors would make that bio unacceptable for
197 static bool bvec_split_segs(const struct request_queue *q,
198 const struct bio_vec *bv, unsigned *nsegs,
199 unsigned *sectors, unsigned max_segs,
200 unsigned max_sectors)
202 unsigned max_len = (min(max_sectors, UINT_MAX >> 9) - *sectors) << 9;
203 unsigned len = min(bv->bv_len, max_len);
204 unsigned total_len = 0;
205 unsigned seg_size = 0;
207 while (len && *nsegs < max_segs) {
208 seg_size = get_max_segment_size(q, bv->bv_page,
209 bv->bv_offset + total_len);
210 seg_size = min(seg_size, len);
213 total_len += seg_size;
216 if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
220 *sectors += total_len >> 9;
222 /* tell the caller to split the bvec if it is too big to fit */
223 return len > 0 || bv->bv_len > max_len;
227 * blk_bio_segment_split - split a bio in two bios
228 * @q: [in] request queue pointer
229 * @bio: [in] bio to be split
230 * @bs: [in] bio set to allocate the clone from
231 * @segs: [out] number of segments in the bio with the first half of the sectors
233 * Clone @bio, update the bi_iter of the clone to represent the first sectors
234 * of @bio and update @bio->bi_iter to represent the remaining sectors. The
235 * following is guaranteed for the cloned bio:
236 * - That it has at most get_max_io_size(@q, @bio) sectors.
237 * - That it has at most queue_max_segments(@q) segments.
239 * Except for discard requests the cloned bio will point at the bi_io_vec of
240 * the original bio. It is the responsibility of the caller to ensure that the
241 * original bio is not freed before the cloned bio. The caller is also
242 * responsible for ensuring that @bs is only destroyed after processing of the
243 * split bio has finished.
245 static struct bio *blk_bio_segment_split(struct request_queue *q,
250 struct bio_vec bv, bvprv, *bvprvp = NULL;
251 struct bvec_iter iter;
252 unsigned nsegs = 0, sectors = 0;
253 const unsigned max_sectors = get_max_io_size(q, bio);
254 const unsigned max_segs = queue_max_segments(q);
256 bio_for_each_bvec(bv, bio, iter) {
258 * If the queue doesn't support SG gaps and adding this
259 * offset would create a gap, disallow it.
261 if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
264 if (nsegs < max_segs &&
265 sectors + (bv.bv_len >> 9) <= max_sectors &&
266 bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
268 sectors += bv.bv_len >> 9;
269 } else if (bvec_split_segs(q, &bv, &nsegs, §ors, max_segs,
282 return bio_split(bio, sectors, GFP_NOIO, bs);
286 * __blk_queue_split - split a bio and submit the second half
287 * @q: [in] request queue pointer
288 * @bio: [in, out] bio to be split
289 * @nr_segs: [out] number of segments in the first bio
291 * Split a bio into two bios, chain the two bios, submit the second half and
292 * store a pointer to the first half in *@bio. If the second bio is still too
293 * big it will be split by a recursive call to this function. Since this
294 * function may allocate a new bio from @q->bio_split, it is the responsibility
295 * of the caller to ensure that @q is only released after processing of the
296 * split bio has finished.
298 void __blk_queue_split(struct request_queue *q, struct bio **bio,
299 unsigned int *nr_segs)
303 switch (bio_op(*bio)) {
305 case REQ_OP_SECURE_ERASE:
306 split = blk_bio_discard_split(q, *bio, &q->bio_split, nr_segs);
308 case REQ_OP_WRITE_ZEROES:
309 split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split,
312 case REQ_OP_WRITE_SAME:
313 split = blk_bio_write_same_split(q, *bio, &q->bio_split,
317 split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
322 /* there isn't chance to merge the splitted bio */
323 split->bi_opf |= REQ_NOMERGE;
326 * Since we're recursing into make_request here, ensure
327 * that we mark this bio as already having entered the queue.
328 * If not, and the queue is going away, we can get stuck
329 * forever on waiting for the queue reference to drop. But
330 * that will never happen, as we're already holding a
333 bio_set_flag(*bio, BIO_QUEUE_ENTERED);
335 bio_chain(split, *bio);
336 trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
337 generic_make_request(*bio);
343 * blk_queue_split - split a bio and submit the second half
344 * @q: [in] request queue pointer
345 * @bio: [in, out] bio to be split
347 * Split a bio into two bios, chains the two bios, submit the second half and
348 * store a pointer to the first half in *@bio. Since this function may allocate
349 * a new bio from @q->bio_split, it is the responsibility of the caller to
350 * ensure that @q is only released after processing of the split bio has
353 void blk_queue_split(struct request_queue *q, struct bio **bio)
355 unsigned int nr_segs;
357 __blk_queue_split(q, bio, &nr_segs);
359 EXPORT_SYMBOL(blk_queue_split);
361 unsigned int blk_recalc_rq_segments(struct request *rq)
363 unsigned int nr_phys_segs = 0;
364 unsigned int nr_sectors = 0;
365 struct req_iterator iter;
371 switch (bio_op(rq->bio)) {
373 case REQ_OP_SECURE_ERASE:
374 if (queue_max_discard_segments(rq->q) > 1) {
375 struct bio *bio = rq->bio;
382 case REQ_OP_WRITE_ZEROES:
384 case REQ_OP_WRITE_SAME:
388 rq_for_each_bvec(bv, rq, iter)
389 bvec_split_segs(rq->q, &bv, &nr_phys_segs, &nr_sectors,
394 static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
395 struct scatterlist *sglist)
401 * If the driver previously mapped a shorter list, we could see a
402 * termination bit prematurely unless it fully inits the sg table
403 * on each mapping. We KNOW that there must be more entries here
404 * or the driver would be buggy, so force clear the termination bit
405 * to avoid doing a full sg_init_table() in drivers for each command.
411 static unsigned blk_bvec_map_sg(struct request_queue *q,
412 struct bio_vec *bvec, struct scatterlist *sglist,
413 struct scatterlist **sg)
415 unsigned nbytes = bvec->bv_len;
416 unsigned nsegs = 0, total = 0;
419 unsigned offset = bvec->bv_offset + total;
420 unsigned len = min(get_max_segment_size(q, bvec->bv_page,
422 struct page *page = bvec->bv_page;
425 * Unfortunately a fair number of drivers barf on scatterlists
426 * that have an offset larger than PAGE_SIZE, despite other
427 * subsystems dealing with that invariant just fine. For now
428 * stick to the legacy format where we never present those from
429 * the block layer, but the code below should be removed once
430 * these offenders (mostly MMC/SD drivers) are fixed.
432 page += (offset >> PAGE_SHIFT);
433 offset &= ~PAGE_MASK;
435 *sg = blk_next_sg(sg, sglist);
436 sg_set_page(*sg, page, len, offset);
446 static inline int __blk_bvec_map_sg(struct bio_vec bv,
447 struct scatterlist *sglist, struct scatterlist **sg)
449 *sg = blk_next_sg(sg, sglist);
450 sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
454 /* only try to merge bvecs into one sg if they are from two bios */
456 __blk_segment_map_sg_merge(struct request_queue *q, struct bio_vec *bvec,
457 struct bio_vec *bvprv, struct scatterlist **sg)
460 int nbytes = bvec->bv_len;
465 if ((*sg)->length + nbytes > queue_max_segment_size(q))
468 if (!biovec_phys_mergeable(q, bvprv, bvec))
471 (*sg)->length += nbytes;
476 static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
477 struct scatterlist *sglist,
478 struct scatterlist **sg)
480 struct bio_vec uninitialized_var(bvec), bvprv = { NULL };
481 struct bvec_iter iter;
483 bool new_bio = false;
486 bio_for_each_bvec(bvec, bio, iter) {
488 * Only try to merge bvecs from two bios given we
489 * have done bio internal merge when adding pages
493 __blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
496 if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE)
497 nsegs += __blk_bvec_map_sg(bvec, sglist, sg);
499 nsegs += blk_bvec_map_sg(q, &bvec, sglist, sg);
503 if (likely(bio->bi_iter.bi_size)) {
513 * map a request to scatterlist, return number of sg entries setup. Caller
514 * must make sure sg can hold rq->nr_phys_segments entries
516 int blk_rq_map_sg(struct request_queue *q, struct request *rq,
517 struct scatterlist *sglist)
519 struct scatterlist *sg = NULL;
522 if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
523 nsegs = __blk_bvec_map_sg(rq->special_vec, sglist, &sg);
524 else if (rq->bio && bio_op(rq->bio) == REQ_OP_WRITE_SAME)
525 nsegs = __blk_bvec_map_sg(bio_iovec(rq->bio), sglist, &sg);
527 nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
529 if (unlikely(rq->rq_flags & RQF_COPY_USER) &&
530 (blk_rq_bytes(rq) & q->dma_pad_mask)) {
531 unsigned int pad_len =
532 (q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1;
534 sg->length += pad_len;
535 rq->extra_len += pad_len;
538 if (q->dma_drain_size && q->dma_drain_needed(rq)) {
539 if (op_is_write(req_op(rq)))
540 memset(q->dma_drain_buffer, 0, q->dma_drain_size);
544 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
546 ((unsigned long)q->dma_drain_buffer) &
549 rq->extra_len += q->dma_drain_size;
556 * Something must have been wrong if the figured number of
557 * segment is bigger than number of req's physical segments
559 WARN_ON(nsegs > blk_rq_nr_phys_segments(rq));
563 EXPORT_SYMBOL(blk_rq_map_sg);
565 static inline unsigned int blk_rq_get_max_segments(struct request *rq)
567 if (req_op(rq) == REQ_OP_DISCARD)
568 return queue_max_discard_segments(rq->q);
569 return queue_max_segments(rq->q);
572 static inline int ll_new_hw_segment(struct request *req, struct bio *bio,
573 unsigned int nr_phys_segs)
575 if (!blk_cgroup_mergeable(req, bio))
578 if (blk_integrity_merge_bio(req->q, req, bio) == false)
581 /* discard request merge won't add new segment */
582 if (req_op(req) == REQ_OP_DISCARD)
585 if (req->nr_phys_segments + nr_phys_segs > blk_rq_get_max_segments(req))
589 * This will form the start of a new hw segment. Bump both
592 req->nr_phys_segments += nr_phys_segs;
596 req_set_nomerge(req->q, req);
600 int ll_back_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
602 if (req_gap_back_merge(req, bio))
604 if (blk_integrity_rq(req) &&
605 integrity_req_gap_back_merge(req, bio))
607 if (blk_rq_sectors(req) + bio_sectors(bio) >
608 blk_rq_get_max_sectors(req, blk_rq_pos(req))) {
609 req_set_nomerge(req->q, req);
613 return ll_new_hw_segment(req, bio, nr_segs);
616 int ll_front_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
618 if (req_gap_front_merge(req, bio))
620 if (blk_integrity_rq(req) &&
621 integrity_req_gap_front_merge(req, bio))
623 if (blk_rq_sectors(req) + bio_sectors(bio) >
624 blk_rq_get_max_sectors(req, bio->bi_iter.bi_sector)) {
625 req_set_nomerge(req->q, req);
629 return ll_new_hw_segment(req, bio, nr_segs);
632 static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
633 struct request *next)
635 unsigned short segments = blk_rq_nr_discard_segments(req);
637 if (segments >= queue_max_discard_segments(q))
639 if (blk_rq_sectors(req) + bio_sectors(next->bio) >
640 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
643 req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
646 req_set_nomerge(q, req);
650 static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
651 struct request *next)
653 int total_phys_segments;
655 if (req_gap_back_merge(req, next->bio))
659 * Will it become too large?
661 if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
662 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
665 total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
666 if (total_phys_segments > blk_rq_get_max_segments(req))
669 if (!blk_cgroup_mergeable(req, next->bio))
672 if (blk_integrity_merge_rq(q, req, next) == false)
676 req->nr_phys_segments = total_phys_segments;
681 * blk_rq_set_mixed_merge - mark a request as mixed merge
682 * @rq: request to mark as mixed merge
685 * @rq is about to be mixed merged. Make sure the attributes
686 * which can be mixed are set in each bio and mark @rq as mixed
689 void blk_rq_set_mixed_merge(struct request *rq)
691 unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
694 if (rq->rq_flags & RQF_MIXED_MERGE)
698 * @rq will no longer represent mixable attributes for all the
699 * contained bios. It will just track those of the first one.
700 * Distributes the attributs to each bio.
702 for (bio = rq->bio; bio; bio = bio->bi_next) {
703 WARN_ON_ONCE((bio->bi_opf & REQ_FAILFAST_MASK) &&
704 (bio->bi_opf & REQ_FAILFAST_MASK) != ff);
707 rq->rq_flags |= RQF_MIXED_MERGE;
710 static void blk_account_io_merge(struct request *req)
712 if (blk_do_io_stat(req)) {
713 struct hd_struct *part;
718 part_dec_in_flight(req->q, part, rq_data_dir(req));
725 static enum elv_merge blk_try_req_merge(struct request *req,
726 struct request *next)
728 if (blk_discard_mergable(req))
729 return ELEVATOR_DISCARD_MERGE;
730 else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
731 return ELEVATOR_BACK_MERGE;
733 return ELEVATOR_NO_MERGE;
737 * For non-mq, this has to be called with the request spinlock acquired.
738 * For mq with scheduling, the appropriate queue wide lock should be held.
740 static struct request *attempt_merge(struct request_queue *q,
741 struct request *req, struct request *next)
743 if (!rq_mergeable(req) || !rq_mergeable(next))
746 if (req_op(req) != req_op(next))
749 if (rq_data_dir(req) != rq_data_dir(next)
750 || req->rq_disk != next->rq_disk)
753 if (req_op(req) == REQ_OP_WRITE_SAME &&
754 !blk_write_same_mergeable(req->bio, next->bio))
758 * Don't allow merge of different write hints, or for a hint with
761 if (req->write_hint != next->write_hint)
764 if (req->ioprio != next->ioprio)
768 * If we are allowed to merge, then append bio list
769 * from next to rq and release next. merge_requests_fn
770 * will have updated segment counts, update sector
771 * counts here. Handle DISCARDs separately, as they
772 * have separate settings.
775 switch (blk_try_req_merge(req, next)) {
776 case ELEVATOR_DISCARD_MERGE:
777 if (!req_attempt_discard_merge(q, req, next))
780 case ELEVATOR_BACK_MERGE:
781 if (!ll_merge_requests_fn(q, req, next))
789 * If failfast settings disagree or any of the two is already
790 * a mixed merge, mark both as mixed before proceeding. This
791 * makes sure that all involved bios have mixable attributes
794 if (((req->rq_flags | next->rq_flags) & RQF_MIXED_MERGE) ||
795 (req->cmd_flags & REQ_FAILFAST_MASK) !=
796 (next->cmd_flags & REQ_FAILFAST_MASK)) {
797 blk_rq_set_mixed_merge(req);
798 blk_rq_set_mixed_merge(next);
802 * At this point we have either done a back merge or front merge. We
803 * need the smaller start_time_ns of the merged requests to be the
804 * current request for accounting purposes.
806 if (next->start_time_ns < req->start_time_ns)
807 req->start_time_ns = next->start_time_ns;
809 req->biotail->bi_next = next->bio;
810 req->biotail = next->biotail;
812 req->__data_len += blk_rq_bytes(next);
814 if (!blk_discard_mergable(req))
815 elv_merge_requests(q, req, next);
818 * 'next' is going away, so update stats accordingly
820 blk_account_io_merge(next);
823 * ownership of bio passed from next to req, return 'next' for
830 struct request *attempt_back_merge(struct request_queue *q, struct request *rq)
832 struct request *next = elv_latter_request(q, rq);
835 return attempt_merge(q, rq, next);
840 struct request *attempt_front_merge(struct request_queue *q, struct request *rq)
842 struct request *prev = elv_former_request(q, rq);
845 return attempt_merge(q, prev, rq);
850 int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
851 struct request *next)
853 struct request *free;
855 free = attempt_merge(q, rq, next);
857 blk_put_request(free);
864 bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
866 if (!rq_mergeable(rq) || !bio_mergeable(bio))
869 if (req_op(rq) != bio_op(bio))
872 /* different data direction or already started, don't merge */
873 if (bio_data_dir(bio) != rq_data_dir(rq))
876 /* must be same device */
877 if (rq->rq_disk != bio->bi_disk)
880 /* don't merge across cgroup boundaries */
881 if (!blk_cgroup_mergeable(rq, bio))
884 /* only merge integrity protected bio into ditto rq */
885 if (blk_integrity_merge_bio(rq->q, rq, bio) == false)
888 /* must be using the same buffer */
889 if (req_op(rq) == REQ_OP_WRITE_SAME &&
890 !blk_write_same_mergeable(rq->bio, bio))
894 * Don't allow merge of different write hints, or for a hint with
897 if (rq->write_hint != bio->bi_write_hint)
900 if (rq->ioprio != bio_prio(bio))
906 enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
908 if (blk_discard_mergable(rq))
909 return ELEVATOR_DISCARD_MERGE;
910 else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
911 return ELEVATOR_BACK_MERGE;
912 else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
913 return ELEVATOR_FRONT_MERGE;
914 return ELEVATOR_NO_MERGE;