[dm-devel] [PATCH] dm-io: reject unsupported DISCARD/WRITE SAME requests with EOPNOTSUPP

[Darrick J. Wong]

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.

Before entering the loop, check for the combination of REQ_DISCARD and
no discard and return -EOPNOTSUPP to avoid hanging up the mirror
device.  Fix the same problem with WRITE_DISCARD while we're at it.

This bug was found by the unfortunate coincidence of pvmove and a
discard operation in the RHEL 6.5 kernel; 3.19 is also affected.

Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
Cc: "Martin K. Petersen" <martin.petersen at oracle.com>
Cc: Srinivas Eeda <srinivas.eeda at oracle.com>
---
 drivers/md/dm-io.c |    7 +++++++
 1 file changed, 7 insertions(+)


diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
index c09359d..32d330a 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c
@@ -290,6 +290,13 @@ static void do_region(int rw, unsigned region, struct dm_io_region *where,
 	unsigned short logical_block_size = queue_logical_block_size(q);
 	sector_t num_sectors;
 
+	/* Reject unsupported discard/write same requests */
+	if (((rw & REQ_DISCARD) && !blk_queue_discard(q)) ||
+	    ((rw & REQ_WRITE_SAME) && !bdev_write_same(where->bdev))) {
+		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.