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>
11 #include <trace/events/block.h>
15 static inline bool bio_will_gap(struct request_queue *q,
16 struct request *prev_rq, struct bio *prev, struct bio *next)
18 struct bio_vec pb, nb;
20 if (!bio_has_data(prev) || !queue_virt_boundary(q))
24 * Don't merge if the 1st bio starts with non-zero offset, otherwise it
25 * is quite difficult to respect the sg gap limit. We work hard to
26 * merge a huge number of small single bios in case of mkfs.
29 bio_get_first_bvec(prev_rq->bio, &pb);
31 bio_get_first_bvec(prev, &pb);
32 if (pb.bv_offset & queue_virt_boundary(q))
36 * We don't need to worry about the situation that the merged segment
37 * ends in unaligned virt boundary:
39 * - if 'pb' ends aligned, the merged segment ends aligned
40 * - if 'pb' ends unaligned, the next bio must include
41 * one single bvec of 'nb', otherwise the 'nb' can't
44 bio_get_last_bvec(prev, &pb);
45 bio_get_first_bvec(next, &nb);
46 if (biovec_phys_mergeable(q, &pb, &nb))
48 return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
51 static inline bool req_gap_back_merge(struct request *req, struct bio *bio)
53 return bio_will_gap(req->q, req, req->biotail, bio);
56 static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
58 return bio_will_gap(req->q, NULL, bio, req->bio);
61 static struct bio *blk_bio_discard_split(struct request_queue *q,
66 unsigned int max_discard_sectors, granularity;
69 unsigned split_sectors;
73 /* Zero-sector (unknown) and one-sector granularities are the same. */
74 granularity = max(q->limits.discard_granularity >> 9, 1U);
76 max_discard_sectors = min(q->limits.max_discard_sectors,
77 bio_allowed_max_sectors(q));
78 max_discard_sectors -= max_discard_sectors % granularity;
80 if (unlikely(!max_discard_sectors)) {
85 if (bio_sectors(bio) <= max_discard_sectors)
88 split_sectors = max_discard_sectors;
91 * If the next starting sector would be misaligned, stop the discard at
92 * the previous aligned sector.
94 alignment = (q->limits.discard_alignment >> 9) % granularity;
96 tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
97 tmp = sector_div(tmp, granularity);
99 if (split_sectors > tmp)
100 split_sectors -= tmp;
102 return bio_split(bio, split_sectors, GFP_NOIO, bs);
105 static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
106 struct bio *bio, struct bio_set *bs, unsigned *nsegs)
110 if (!q->limits.max_write_zeroes_sectors)
113 if (bio_sectors(bio) <= q->limits.max_write_zeroes_sectors)
116 return bio_split(bio, q->limits.max_write_zeroes_sectors, GFP_NOIO, bs);
119 static struct bio *blk_bio_write_same_split(struct request_queue *q,
126 if (!q->limits.max_write_same_sectors)
129 if (bio_sectors(bio) <= q->limits.max_write_same_sectors)
132 return bio_split(bio, q->limits.max_write_same_sectors, GFP_NOIO, bs);
136 * Return the maximum number of sectors from the start of a bio that may be
137 * submitted as a single request to a block device. If enough sectors remain,
138 * align the end to the physical block size. Otherwise align the end to the
139 * logical block size. This approach minimizes the number of non-aligned
140 * requests that are submitted to a block device if the start of a bio is not
141 * aligned to a physical block boundary.
143 static inline unsigned get_max_io_size(struct request_queue *q,
146 unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
147 unsigned max_sectors = sectors;
148 unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
149 unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
150 unsigned start_offset = bio->bi_iter.bi_sector & (pbs - 1);
152 max_sectors += start_offset;
153 max_sectors &= ~(pbs - 1);
154 if (max_sectors > start_offset)
155 return max_sectors - start_offset;
157 return sectors & (lbs - 1);
160 static inline unsigned get_max_segment_size(const struct request_queue *q,
161 struct page *start_page,
162 unsigned long offset)
164 unsigned long mask = queue_segment_boundary(q);
166 offset = mask & (page_to_phys(start_page) + offset);
167 return min_t(unsigned long, mask - offset + 1,
168 queue_max_segment_size(q));
172 * bvec_split_segs - verify whether or not a bvec should be split in the middle
173 * @q: [in] request queue associated with the bio associated with @bv
174 * @bv: [in] bvec to examine
175 * @nsegs: [in,out] Number of segments in the bio being built. Incremented
176 * by the number of segments from @bv that may be appended to that
177 * bio without exceeding @max_segs
178 * @sectors: [in,out] Number of sectors in the bio being built. Incremented
179 * by the number of sectors from @bv that may be appended to that
180 * bio without exceeding @max_sectors
181 * @max_segs: [in] upper bound for *@nsegs
182 * @max_sectors: [in] upper bound for *@sectors
184 * When splitting a bio, it can happen that a bvec is encountered that is too
185 * big to fit in a single segment and hence that it has to be split in the
186 * middle. This function verifies whether or not that should happen. The value
187 * %true is returned if and only if appending the entire @bv to a bio with
188 * *@nsegs segments and *@sectors sectors would make that bio unacceptable for
191 static bool bvec_split_segs(const struct request_queue *q,
192 const struct bio_vec *bv, unsigned *nsegs,
193 unsigned *sectors, unsigned max_segs,
194 unsigned max_sectors)
196 unsigned max_len = (min(max_sectors, UINT_MAX >> 9) - *sectors) << 9;
197 unsigned len = min(bv->bv_len, max_len);
198 unsigned total_len = 0;
199 unsigned seg_size = 0;
201 while (len && *nsegs < max_segs) {
202 seg_size = get_max_segment_size(q, bv->bv_page,
203 bv->bv_offset + total_len);
204 seg_size = min(seg_size, len);
207 total_len += seg_size;
210 if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
214 *sectors += total_len >> 9;
216 /* tell the caller to split the bvec if it is too big to fit */
217 return len > 0 || bv->bv_len > max_len;
221 * blk_bio_segment_split - split a bio in two bios
222 * @q: [in] request queue pointer
223 * @bio: [in] bio to be split
224 * @bs: [in] bio set to allocate the clone from
225 * @segs: [out] number of segments in the bio with the first half of the sectors
227 * Clone @bio, update the bi_iter of the clone to represent the first sectors
228 * of @bio and update @bio->bi_iter to represent the remaining sectors. The
229 * following is guaranteed for the cloned bio:
230 * - That it has at most get_max_io_size(@q, @bio) sectors.
231 * - That it has at most queue_max_segments(@q) segments.
233 * Except for discard requests the cloned bio will point at the bi_io_vec of
234 * the original bio. It is the responsibility of the caller to ensure that the
235 * original bio is not freed before the cloned bio. The caller is also
236 * responsible for ensuring that @bs is only destroyed after processing of the
237 * split bio has finished.
239 static struct bio *blk_bio_segment_split(struct request_queue *q,
244 struct bio_vec bv, bvprv, *bvprvp = NULL;
245 struct bvec_iter iter;
246 unsigned nsegs = 0, sectors = 0;
247 const unsigned max_sectors = get_max_io_size(q, bio);
248 const unsigned max_segs = queue_max_segments(q);
250 bio_for_each_bvec(bv, bio, iter) {
252 * If the queue doesn't support SG gaps and adding this
253 * offset would create a gap, disallow it.
255 if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
258 if (nsegs < max_segs &&
259 sectors + (bv.bv_len >> 9) <= max_sectors &&
260 bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
262 sectors += bv.bv_len >> 9;
263 } else if (bvec_split_segs(q, &bv, &nsegs, §ors, max_segs,
276 return bio_split(bio, sectors, GFP_NOIO, bs);
280 * __blk_queue_split - split a bio and submit the second half
281 * @q: [in] request queue pointer
282 * @bio: [in, out] bio to be split
283 * @nr_segs: [out] number of segments in the first bio
285 * Split a bio into two bios, chain the two bios, submit the second half and
286 * store a pointer to the first half in *@bio. If the second bio is still too
287 * big it will be split by a recursive call to this function. Since this
288 * function may allocate a new bio from @q->bio_split, it is the responsibility
289 * of the caller to ensure that @q is only released after processing of the
290 * split bio has finished.
292 void __blk_queue_split(struct request_queue *q, struct bio **bio,
293 unsigned int *nr_segs)
295 struct bio *split = NULL;
297 switch (bio_op(*bio)) {
299 case REQ_OP_SECURE_ERASE:
300 split = blk_bio_discard_split(q, *bio, &q->bio_split, nr_segs);
302 case REQ_OP_WRITE_ZEROES:
303 split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split,
306 case REQ_OP_WRITE_SAME:
307 split = blk_bio_write_same_split(q, *bio, &q->bio_split,
312 * All drivers must accept single-segments bios that are <=
313 * PAGE_SIZE. This is a quick and dirty check that relies on
314 * the fact that bi_io_vec[0] is always valid if a bio has data.
315 * The check might lead to occasional false negatives when bios
316 * are cloned, but compared to the performance impact of cloned
317 * bios themselves the loop below doesn't matter anyway.
319 if (!q->limits.chunk_sectors &&
320 (*bio)->bi_vcnt == 1 &&
321 ((*bio)->bi_io_vec[0].bv_len +
322 (*bio)->bi_io_vec[0].bv_offset) <= PAGE_SIZE) {
326 split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
331 /* there isn't chance to merge the splitted bio */
332 split->bi_opf |= REQ_NOMERGE;
335 * Since we're recursing into make_request here, ensure
336 * that we mark this bio as already having entered the queue.
337 * If not, and the queue is going away, we can get stuck
338 * forever on waiting for the queue reference to drop. But
339 * that will never happen, as we're already holding a
342 bio_set_flag(*bio, BIO_QUEUE_ENTERED);
344 bio_chain(split, *bio);
345 trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
346 generic_make_request(*bio);
352 * blk_queue_split - split a bio and submit the second half
353 * @q: [in] request queue pointer
354 * @bio: [in, out] bio to be split
356 * Split a bio into two bios, chains the two bios, submit the second half and
357 * store a pointer to the first half in *@bio. Since this function may allocate
358 * a new bio from @q->bio_split, it is the responsibility of the caller to
359 * ensure that @q is only released after processing of the split bio has
362 void blk_queue_split(struct request_queue *q, struct bio **bio)
364 unsigned int nr_segs;
366 __blk_queue_split(q, bio, &nr_segs);
368 EXPORT_SYMBOL(blk_queue_split);
370 unsigned int blk_recalc_rq_segments(struct request *rq)
372 unsigned int nr_phys_segs = 0;
373 unsigned int nr_sectors = 0;
374 struct req_iterator iter;
380 switch (bio_op(rq->bio)) {
382 case REQ_OP_SECURE_ERASE:
383 case REQ_OP_WRITE_ZEROES:
385 case REQ_OP_WRITE_SAME:
389 rq_for_each_bvec(bv, rq, iter)
390 bvec_split_segs(rq->q, &bv, &nr_phys_segs, &nr_sectors,
395 static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
396 struct scatterlist *sglist)
402 * If the driver previously mapped a shorter list, we could see a
403 * termination bit prematurely unless it fully inits the sg table
404 * on each mapping. We KNOW that there must be more entries here
405 * or the driver would be buggy, so force clear the termination bit
406 * to avoid doing a full sg_init_table() in drivers for each command.
412 static unsigned blk_bvec_map_sg(struct request_queue *q,
413 struct bio_vec *bvec, struct scatterlist *sglist,
414 struct scatterlist **sg)
416 unsigned nbytes = bvec->bv_len;
417 unsigned nsegs = 0, total = 0;
420 unsigned offset = bvec->bv_offset + total;
421 unsigned len = min(get_max_segment_size(q, bvec->bv_page,
423 struct page *page = bvec->bv_page;
426 * Unfortunately a fair number of drivers barf on scatterlists
427 * that have an offset larger than PAGE_SIZE, despite other
428 * subsystems dealing with that invariant just fine. For now
429 * stick to the legacy format where we never present those from
430 * the block layer, but the code below should be removed once
431 * these offenders (mostly MMC/SD drivers) are fixed.
433 page += (offset >> PAGE_SHIFT);
434 offset &= ~PAGE_MASK;
436 *sg = blk_next_sg(sg, sglist);
437 sg_set_page(*sg, page, len, offset);
447 static inline int __blk_bvec_map_sg(struct bio_vec bv,
448 struct scatterlist *sglist, struct scatterlist **sg)
450 *sg = blk_next_sg(sg, sglist);
451 sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
455 /* only try to merge bvecs into one sg if they are from two bios */
457 __blk_segment_map_sg_merge(struct request_queue *q, struct bio_vec *bvec,
458 struct bio_vec *bvprv, struct scatterlist **sg)
461 int nbytes = bvec->bv_len;
466 if ((*sg)->length + nbytes > queue_max_segment_size(q))
469 if (!biovec_phys_mergeable(q, bvprv, bvec))
472 (*sg)->length += nbytes;
477 static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
478 struct scatterlist *sglist,
479 struct scatterlist **sg)
481 struct bio_vec uninitialized_var(bvec), bvprv = { NULL };
482 struct bvec_iter iter;
484 bool new_bio = false;
487 bio_for_each_bvec(bvec, bio, iter) {
489 * Only try to merge bvecs from two bios given we
490 * have done bio internal merge when adding pages
494 __blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
497 if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE)
498 nsegs += __blk_bvec_map_sg(bvec, sglist, sg);
500 nsegs += blk_bvec_map_sg(q, &bvec, sglist, sg);
504 if (likely(bio->bi_iter.bi_size)) {
514 * map a request to scatterlist, return number of sg entries setup. Caller
515 * must make sure sg can hold rq->nr_phys_segments entries
517 int blk_rq_map_sg(struct request_queue *q, struct request *rq,
518 struct scatterlist *sglist)
520 struct scatterlist *sg = NULL;
523 if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
524 nsegs = __blk_bvec_map_sg(rq->special_vec, sglist, &sg);
525 else if (rq->bio && bio_op(rq->bio) == REQ_OP_WRITE_SAME)
526 nsegs = __blk_bvec_map_sg(bio_iovec(rq->bio), sglist, &sg);
528 nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
530 if (unlikely(rq->rq_flags & RQF_COPY_USER) &&
531 (blk_rq_bytes(rq) & q->dma_pad_mask)) {
532 unsigned int pad_len =
533 (q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1;
535 sg->length += pad_len;
536 rq->extra_len += pad_len;
539 if (q->dma_drain_size && q->dma_drain_needed(rq)) {
540 if (op_is_write(req_op(rq)))
541 memset(q->dma_drain_buffer, 0, q->dma_drain_size);
545 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
547 ((unsigned long)q->dma_drain_buffer) &
550 rq->extra_len += q->dma_drain_size;
557 * Something must have been wrong if the figured number of
558 * segment is bigger than number of req's physical segments
560 WARN_ON(nsegs > blk_rq_nr_phys_segments(rq));
564 EXPORT_SYMBOL(blk_rq_map_sg);
566 static inline int ll_new_hw_segment(struct request *req, struct bio *bio,
567 unsigned int nr_phys_segs)
569 if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(req->q))
572 if (blk_integrity_merge_bio(req->q, req, bio) == false)
576 * This will form the start of a new hw segment. Bump both
579 req->nr_phys_segments += nr_phys_segs;
583 req_set_nomerge(req->q, req);
587 int ll_back_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
589 if (req_gap_back_merge(req, bio))
591 if (blk_integrity_rq(req) &&
592 integrity_req_gap_back_merge(req, bio))
594 if (blk_rq_sectors(req) + bio_sectors(bio) >
595 blk_rq_get_max_sectors(req, blk_rq_pos(req))) {
596 req_set_nomerge(req->q, req);
600 return ll_new_hw_segment(req, bio, nr_segs);
603 int ll_front_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
605 if (req_gap_front_merge(req, bio))
607 if (blk_integrity_rq(req) &&
608 integrity_req_gap_front_merge(req, bio))
610 if (blk_rq_sectors(req) + bio_sectors(bio) >
611 blk_rq_get_max_sectors(req, bio->bi_iter.bi_sector)) {
612 req_set_nomerge(req->q, req);
616 return ll_new_hw_segment(req, bio, nr_segs);
619 static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
620 struct request *next)
622 unsigned short segments = blk_rq_nr_discard_segments(req);
624 if (segments >= queue_max_discard_segments(q))
626 if (blk_rq_sectors(req) + bio_sectors(next->bio) >
627 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
630 req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
633 req_set_nomerge(q, req);
637 static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
638 struct request *next)
640 int total_phys_segments;
642 if (req_gap_back_merge(req, next->bio))
646 * Will it become too large?
648 if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
649 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
652 total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
653 if (total_phys_segments > queue_max_segments(q))
656 if (blk_integrity_merge_rq(q, req, next) == false)
660 req->nr_phys_segments = total_phys_segments;
665 * blk_rq_set_mixed_merge - mark a request as mixed merge
666 * @rq: request to mark as mixed merge
669 * @rq is about to be mixed merged. Make sure the attributes
670 * which can be mixed are set in each bio and mark @rq as mixed
673 void blk_rq_set_mixed_merge(struct request *rq)
675 unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
678 if (rq->rq_flags & RQF_MIXED_MERGE)
682 * @rq will no longer represent mixable attributes for all the
683 * contained bios. It will just track those of the first one.
684 * Distributes the attributs to each bio.
686 for (bio = rq->bio; bio; bio = bio->bi_next) {
687 WARN_ON_ONCE((bio->bi_opf & REQ_FAILFAST_MASK) &&
688 (bio->bi_opf & REQ_FAILFAST_MASK) != ff);
691 rq->rq_flags |= RQF_MIXED_MERGE;
694 static void blk_account_io_merge(struct request *req)
696 if (blk_do_io_stat(req)) {
697 struct hd_struct *part;
702 part_dec_in_flight(req->q, part, rq_data_dir(req));
709 * Two cases of handling DISCARD merge:
710 * If max_discard_segments > 1, the driver takes every bio
711 * as a range and send them to controller together. The ranges
712 * needn't to be contiguous.
713 * Otherwise, the bios/requests will be handled as same as
714 * others which should be contiguous.
716 static inline bool blk_discard_mergable(struct request *req)
718 if (req_op(req) == REQ_OP_DISCARD &&
719 queue_max_discard_segments(req->q) > 1)
724 static enum elv_merge blk_try_req_merge(struct request *req,
725 struct request *next)
727 if (blk_discard_mergable(req))
728 return ELEVATOR_DISCARD_MERGE;
729 else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
730 return ELEVATOR_BACK_MERGE;
732 return ELEVATOR_NO_MERGE;
736 * For non-mq, this has to be called with the request spinlock acquired.
737 * For mq with scheduling, the appropriate queue wide lock should be held.
739 static struct request *attempt_merge(struct request_queue *q,
740 struct request *req, struct request *next)
742 if (!rq_mergeable(req) || !rq_mergeable(next))
745 if (req_op(req) != req_op(next))
748 if (rq_data_dir(req) != rq_data_dir(next)
749 || req->rq_disk != next->rq_disk)
752 if (req_op(req) == REQ_OP_WRITE_SAME &&
753 !blk_write_same_mergeable(req->bio, next->bio))
757 * Don't allow merge of different write hints, or for a hint with
760 if (req->write_hint != next->write_hint)
763 if (req->ioprio != next->ioprio)
767 * If we are allowed to merge, then append bio list
768 * from next to rq and release next. merge_requests_fn
769 * will have updated segment counts, update sector
770 * counts here. Handle DISCARDs separately, as they
771 * have separate settings.
774 switch (blk_try_req_merge(req, next)) {
775 case ELEVATOR_DISCARD_MERGE:
776 if (!req_attempt_discard_merge(q, req, next))
779 case ELEVATOR_BACK_MERGE:
780 if (!ll_merge_requests_fn(q, req, next))
788 * If failfast settings disagree or any of the two is already
789 * a mixed merge, mark both as mixed before proceeding. This
790 * makes sure that all involved bios have mixable attributes
793 if (((req->rq_flags | next->rq_flags) & RQF_MIXED_MERGE) ||
794 (req->cmd_flags & REQ_FAILFAST_MASK) !=
795 (next->cmd_flags & REQ_FAILFAST_MASK)) {
796 blk_rq_set_mixed_merge(req);
797 blk_rq_set_mixed_merge(next);
801 * At this point we have either done a back merge or front merge. We
802 * need the smaller start_time_ns of the merged requests to be the
803 * current request for accounting purposes.
805 if (next->start_time_ns < req->start_time_ns)
806 req->start_time_ns = next->start_time_ns;
808 req->biotail->bi_next = next->bio;
809 req->biotail = next->biotail;
811 req->__data_len += blk_rq_bytes(next);
813 if (!blk_discard_mergable(req))
814 elv_merge_requests(q, req, next);
817 * 'next' is going away, so update stats accordingly
819 blk_account_io_merge(next);
822 * ownership of bio passed from next to req, return 'next' for
829 struct request *attempt_back_merge(struct request_queue *q, struct request *rq)
831 struct request *next = elv_latter_request(q, rq);
834 return attempt_merge(q, rq, next);
839 struct request *attempt_front_merge(struct request_queue *q, struct request *rq)
841 struct request *prev = elv_former_request(q, rq);
844 return attempt_merge(q, prev, rq);
849 int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
850 struct request *next)
852 struct request *free;
854 free = attempt_merge(q, rq, next);
856 blk_put_request(free);
863 bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
865 if (!rq_mergeable(rq) || !bio_mergeable(bio))
868 if (req_op(rq) != bio_op(bio))
871 /* different data direction or already started, don't merge */
872 if (bio_data_dir(bio) != rq_data_dir(rq))
875 /* must be same device */
876 if (rq->rq_disk != bio->bi_disk)
879 /* only merge integrity protected bio into ditto rq */
880 if (blk_integrity_merge_bio(rq->q, rq, bio) == false)
883 /* must be using the same buffer */
884 if (req_op(rq) == REQ_OP_WRITE_SAME &&
885 !blk_write_same_mergeable(rq->bio, bio))
889 * Don't allow merge of different write hints, or for a hint with
892 if (rq->write_hint != bio->bi_write_hint)
895 if (rq->ioprio != bio_prio(bio))
901 enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
903 if (blk_discard_mergable(rq))
904 return ELEVATOR_DISCARD_MERGE;
905 else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
906 return ELEVATOR_BACK_MERGE;
907 else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
908 return ELEVATOR_FRONT_MERGE;
909 return ELEVATOR_NO_MERGE;