block: relax check on sg gap (729204ef) · Commits · 戴 / test

If the last bvec of the 1st bio and the 1st bvec of the next bio are physically contigious, and the latter can be merged to last segment of the 1st bio, we should think they don't violate sg gap(or virt boundary) limit. Both Vitaly and Dexuan reported lots of unmergeable small bios are observed when running mkfs on Hyper-V virtual storage, and performance becomes quite low. This patch fixes that performance issue. The same issue should exist on NVMe, since it sets virt boundary too. Reported-by:

Vitaly Kuznetsov <vkuznets@redhat.com> Reported-by:

Dexuan Cui <decui@microsoft.com> Tested-by:

Dexuan Cui <decui@microsoft.com> Cc: Keith Busch <keith.busch@intel.com> Signed-off-by:

Ming Lei <ming.lei@canonical.com> Signed-off-by:

Jens Axboe <axboe@fb.com>

include/linux/blkdev.h

+21 −1

Original line number	Diff line number	Diff line
		@@ -1607,6 +1607,25 @@ static inline bool bvec_gap_to_prev(struct request_queue *q,
		return __bvec_gap_to_prev(q, bprv, offset);
		}

		/*
		* Check if the two bvecs from two bios can be merged to one segment.
		* If yes, no need to check gap between the two bios since the 1st bio
		* and the 1st bvec in the 2nd bio can be handled in one segment.
		*/
		static inline bool bios_segs_mergeable(struct request_queue *q,
		struct bio prev, struct bio_vec prev_last_bv,
		struct bio_vec *next_first_bv)
		{
		if (!BIOVEC_PHYS_MERGEABLE(prev_last_bv, next_first_bv))
		return false;
		if (!BIOVEC_SEG_BOUNDARY(q, prev_last_bv, next_first_bv))
		return false;
		if (prev->bi_seg_back_size + next_first_bv->bv_len >
		queue_max_segment_size(q))
		return false;
		return true;
		}

		static inline bool bio_will_gap(struct request_queue q, struct bio prev,
		struct bio *next)
		{
		@@ -1616,6 +1635,7 @@ static inline bool bio_will_gap(struct request_queue q, struct bio prev,
		bio_get_last_bvec(prev, &pb);
		bio_get_first_bvec(next, &nb);

		if (!bios_segs_mergeable(q, prev, &pb, &nb))
		return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
		}

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