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    Subject[PATCH AUTOSEL for 4.15 037/189] dm mpath: return DM_MAPIO_REQUEUE on blk-mq rq allocation failure
    Date
    From: Ming Lei <ming.lei@redhat.com>

    [ Upstream commit 050af08ffb1b62af69196d61c22a0755f9a3cdbd ]

    blk-mq will rerun queue via RESTART or dispatch wake after one request
    is completed, so not necessary to wait random time for requeuing, we
    should trust blk-mq to do it.

    More importantly, we need to return BLK_STS_RESOURCE to blk-mq so that
    dequeuing from the I/O scheduler can be stopped, this results in
    improved I/O merging.

    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    drivers/md/dm-mpath.c | 14 +++++++++++++-
    1 file changed, 13 insertions(+), 1 deletion(-)

    diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
    index 025e033ecb78..a3223c8fbaac 100644
    --- a/drivers/md/dm-mpath.c
    +++ b/drivers/md/dm-mpath.c
    @@ -534,8 +534,20 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
    if (queue_dying) {
    atomic_inc(&m->pg_init_in_progress);
    activate_or_offline_path(pgpath);
    + return DM_MAPIO_DELAY_REQUEUE;
    }
    - return DM_MAPIO_DELAY_REQUEUE;
    +
    + /*
    + * blk-mq's SCHED_RESTART can cover this requeue, so we
    + * needn't deal with it by DELAY_REQUEUE. More importantly,
    + * we have to return DM_MAPIO_REQUEUE so that blk-mq can
    + * get the queue busy feedback (via BLK_STS_RESOURCE),
    + * otherwise I/O merging can suffer.
    + */
    + if (q->mq_ops)
    + return DM_MAPIO_REQUEUE;
    + else
    + return DM_MAPIO_DELAY_REQUEUE;
    }
    clone->bio = clone->biotail = NULL;
    clone->rq_disk = bdev->bd_disk;
    --
    2.15.1
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