linux-kernel - [PATCH 3.2 045/221] dm io: reject unsupported DISCARD requests with EOPNOTSUPP

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Date:	Tue, 05 May 2015 02:16:39 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	"Mike Snitzer" <snitzer@...hat.com>
Subject: [PATCH 3.2 045/221] dm io: reject unsupported DISCARD requests
 with EOPNOTSUPP

3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

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

From: "Darrick J. Wong" <darrick.wong@...cle.com>

commit 37527b869207ad4c208b1e13967d69b8bba1fbf9 upstream.

I created a dm-raid1 device backed by a device that supports DISCARD
and another device that does NOT support DISCARD with the following
dm configuration:

 #  echo '0 2048 mirror core 1 512 2 /dev/sda 0 /dev/sdb 0' | dmsetup create moo
 # lsblk -D
 NAME         DISC-ALN DISC-GRAN DISC-MAX DISC-ZERO
 sda                 0        4K       1G         0
 `-moo (dm-0)        0        4K       1G         0
 sdb                 0        0B       0B         0
 `-moo (dm-0)        0        4K       1G         0

Notice that the mirror device /dev/mapper/moo advertises DISCARD
support even though one of the mirror halves doesn't.

If I issue a DISCARD request (via fstrim, mount -o discard, or ioctl
BLKDISCARD) through the mirror, kmirrord gets stuck in an infinite
loop in do_region() when it tries to issue a DISCARD request to sdb.
The problem is that when we call do_region() against sdb, num_sectors
is set to zero because q->limits.max_discard_sectors is zero.
Therefore, "remaining" never decreases and the loop never terminates.

To fix this: before entering the loop, check for the combination of
REQ_DISCARD and no discard and return -EOPNOTSUPP to avoid hanging up
the mirror device.

This bug was found by the unfortunate coincidence of pvmove and a
discard operation in the RHEL 6.5 kernel; upstream is also affected.

Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
Acked-by: "Martin K. Petersen" <martin.petersen@...cle.com>
Signed-off-by: Mike Snitzer <snitzer@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/md/dm-io.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -300,6 +300,12 @@ static void do_region(int rw, unsigned r
 	struct request_queue *q = bdev_get_queue(where->bdev);
 	sector_t discard_sectors;

+	/* Reject unsupported discard requests */
+	if ((rw & REQ_DISCARD) && !blk_queue_discard(q)) {
+		dec_count(io, region, -EOPNOTSUPP);
+		return;
+	}
+
 	/*
 	 * where->count may be zero if rw holds a flush and we need to
 	 * send a zero-sized flush.

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