dm mpath: return DM_MAPIO_REQUEUE on blk-mq rq allocation failure

BugLink: http://bugs.launchpad.net/bugs/1774063
[ 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 025e033ecb78f39a91e1ff43df75b4590be4d0d5..a3223c8fbaacc290c8457b49a20ea2a5f10419f4 100644 (file)
--- 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;