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MD: make bio mergeable

blk_queue_split marks bio unmergeable, which makes sense for normal bio.
But if dispatching the bio to underlayer disk, the blk_queue_split
checks are invalid, hence it's possible the bio becomes mergeable.

In the reported bug, this bug causes trim against raid0 performance slash
https://bugzilla.kernel.org/show_bug.cgi?id=117051

Reported-and-tested-by: Park Ju Hyung <qkrwngud825@gmail.com>
Fixes: 6ac45ae(block: avoid to merge splitted bio)
Cc: stable@vger.kernel.org (v4.3+)
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Neil Brown <neilb@suse.de>
Reviewed-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
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shligit committed Apr 25, 2016
1 parent b297874 commit 9c573de3283af007ea11c17bde1e4568d9417328
Showing with 2 additions and 0 deletions.
  1. +2 −0 drivers/md/md.c
@@ -284,6 +284,8 @@ static blk_qc_t md_make_request(struct request_queue *q, struct bio *bio)
* go away inside make_request
*/
sectors = bio_sectors(bio);
/* bio could be mergeable after passing to underlayer */
bio->bi_rw &= ~REQ_NOMERGE;
mddev->pers->make_request(mddev, bio);

cpu = part_stat_lock();

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