[PATCH 3.4 038/172] dm io: reject unsupported DISCARD requests with EOPNOTSUPP

From: lizf
Date: Tue Jun 16 2015 - 05:30:40 EST

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

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


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
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@xxxxxxxxxx>
Acked-by: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
drivers/md/dm-io.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
index ea5dd28..d3b1b54 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -299,6 +299,12 @@ static void do_region(int rw, unsigned region, struct dm_io_region *where,
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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