[PATCH 3.19 135/177] dm io: reject unsupported DISCARD requests with EOPNOTSUPP

Greg Kroah-Hartman
2015-03-16T14:17:13Z

From: "Darrick J. Wong" <darrick.wong@oracle.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@oracle.com>
Acked-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/dm-io.c | 6 ++++++
1 file changed, 6 insertions(+)

--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -290,6 +290,12 @@ static void do_region(int rw, unsigned r
unsigned short logical_block_size = queue_logical_block_size(q);
sector_t num_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.