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    Subject[PATCH 3.16.y-ckt 118/183] dm io: reject unsupported DISCARD requests with EOPNOTSUPP
    Date
    3.16.7-ckt8 -stable review patch.  If anyone has any objections, please let me know.

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

    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: Luis Henriques <luis.henriques@canonical.com>
    ---
    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 db404a0f7e2c..d2a8d64f8526 100644
    --- a/drivers/md/dm-io.c
    +++ b/drivers/md/dm-io.c
    @@ -292,6 +292,12 @@ 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 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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