On Fri, Feb 13 2015 at 4:24am -0500, Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: > 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@xxxxxxxxxx> > Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx> > Cc: Srinivas Eeda <srinivas.eeda@xxxxxxxxxx> Your patch looks fine but it is laser focused on dm-io. Again, that is fine (fixes a real problem). But I'm wondering how other targets will respond in the face of partial discard support across the logical address space of the DM device. When I implemented dm_table_supports_discards() I consciously allowed a DM table to contain a mix of discard support. I'm now wondering where it is we benefit from that? Seems like more of a liability than anything -- so a bigger hammer approach to fixing this would be to require all targets and all devices in a DM table support discard. Which amounts to changing dm_table_supports_discards() to be like dm_table_supports_write_same(). BTW, given dm_table_supports_write_same(), your patch shouldn't need to worry about WRITE SAME. Did you experience issues with WRITE SAME too or were you just being proactive? Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
