dm-mirror has potential data corruption problem: while on-disk log shows
that all disk contents are in-sync, actual contents of the disks are not
synchronized. This problem occurs if initial recovery (synching) is
interrupted and resumed.
Attached patch fixes this problem.
Background:
rh_dec() changes the region state from RH_NOSYNC (out-of-sync) to RH_CLEAN
(in-sync), which results in the corresponding bit of clean_bits being set.
This is harmful if on-disk log is used and the map is removed/suspended
before the initial sync is completed. The clean_bits is written down to
the on-disk log at the map removal, and, upon resume, it's read and copied
to sync_bits. Since the recovery process refers to the sync_bits to find a
region to be recovered, the region whose state was changed from RH_NOSYNC
to RH_CLEAN is no longer recovered.
If you haven't applied dm-raid1-read-balancing.patch proposed in dm-devel
sometimes ago, the contents of the mirrored disk just corrupt silently. If
you have, balanced read may get bogus data from out-of-sync disks.
The patch keeps RH_NOSYNC state unchanged. It will be changed to
RH_RECOVERING when recovery starts and get reclaimed when the recovery
completes. So it doesn't leak the region hash entry.
Description:
Keep RH_NOSYNC state unchanged when I/O on the region completes.
rh_dec() changes the region state from RH_NOSYNC (out-of-sync) to RH_CLEAN
(in-sync), which results in the corresponding bit of clean_bits being set.
This is harmful if on-disk log is used and the map is removed/suspended
before the initial sync is completed. The clean_bits is written down to
the on-disk log at the map removal, and, upon resume, it's read and copied
to sync_bits. Since the recovery process refers to the sync_bits to find a
region to be recovered, the region whose state was changed from RH_NOSYNC
to RH_CLEAN is no longer recovered.
If you haven't applied dm-raid1-read-balancing.patch proposed in dm-devel
sometimes ago, the contents of the mirrored disk just corrupt silently. If
you have, balanced read may get bogus data from out-of-sync disks.
The RH_NOSYNC region will be changed to RH_RECOVERING when recovery starts
on the region and get reclaimed when the recovery completes. So it doesn't
leak the region hash entry.
Alasdair said:
I've analysed the relevant part of the state machine and I believe that
the patch is correct.
(Further work on this code is still needed - this patch has the
side-effect of holding onto memory unnecessarily for long periods of time
under certain workloads - but better that than corrupting data.)
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Acked-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Jun'ichi Nomura