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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     NeilBrown <neilb@suse.com>, Shaohua Li <shli@fb.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.4 072/115] md/raid6: Fix anomily when recovering
 a single device in RAID6.
Thread-Topic: [PATCH AUTOSEL for 4.4 072/115] md/raid6: Fix anomily when
 recovering a single device in RAID6.
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Date:   Sat, 3 Mar 2018 22:31:31 +0000
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From: NeilBrown <neilb@suse.com>

[ Upstream commit 7471fb77ce4dc4cb81291189947fcdf621a97987 ]

When recoverying a single missing/failed device in a RAID6,
those stripes where the Q block is on the missing device are
handled a bit differently.  In these cases it is easy to
check that the P block is correct, so we do.  This results
in the P block be destroy.  Consequently the P block needs
to be read a second time in order to compute Q.  This causes
lots of seeks and hurts performance.

It shouldn't be necessary to re-read P as it can be computed
from the DATA.  But we only compute blocks on missing
devices, since c337869d9501 ("md: do not compute parity
unless it is on a failed drive").

So relax the change made in that commit to allow computing
of the P block in a RAID6 which it is the only missing that
block.

This makes RAID6 recovery run much faster as the disk just
"before" the recovering device is no longer seeking
back-and-forth.

Reported-by-tested-by: Brad Campbell <lists2009@fnarfbargle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 drivers/md/raid5.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 86ab6d14d782..ca968c3f25c7 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3372,9 +3372,20 @@ static int fetch_block(struct stripe_head *sh, struc=
t stripe_head_state *s,
 		BUG_ON(test_bit(R5_Wantcompute, &dev->flags));
 		BUG_ON(test_bit(R5_Wantread, &dev->flags));
 		BUG_ON(sh->batch_head);
+
+		/*
+		 * In the raid6 case if the only non-uptodate disk is P
+		 * then we already trusted P to compute the other failed
+		 * drives. It is safe to compute rather than re-read P.
+		 * In other cases we only compute blocks from failed
+		 * devices, otherwise check/repair might fail to detect
+		 * a real inconsistency.
+		 */
+
 		if ((s->uptodate =3D=3D disks - 1) &&
+		    ((sh->qd_idx >=3D 0 && sh->pd_idx =3D=3D disk_idx) ||
 		    (s->failed && (disk_idx =3D=3D s->failed_num[0] ||
-				   disk_idx =3D=3D s->failed_num[1]))) {
+				   disk_idx =3D=3D s->failed_num[1])))) {
 			/* have disk failed, and we're requested to fetch it;
 			 * do compute it
 			 */
--=20
2.14.1