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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Ming Lei <ming.lei@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.14 047/183] dm mpath: return DM_MAPIO_REQUEUE on blk-mq rq allocation failure
Date:   Wed, 25 Apr 2018 12:34:27 +0200
Message-Id: <20180425103244.450168940@linuxfoundation.org>
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4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/md/dm-mpath.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -502,8 +502,20 @@ static int multipath_clone_and_map(struc
 		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;