[Devel] [PATCH hci-8.1 0/2] dm-zero-rq: fix a crash on a flush after ploop release

["Denis V. Lunev" <den@openvz.org>]

ploop releases a device by reloading it to the error target, and
ploop_dmreload_zero() (VSTOR-146886) later reloads it to zero-rq while
the file system on top is still mounted. A request queued against the previous table waits out
the suspend and is dispatched to zero-rq on resume.

For a flush this is a host panic: the flush request has no bio and
zero_clone_and_map_rq() reads the operation from rq->bio. QA hit it in
the vzctl release-disks test on hci-8.1 (VSTOR-152097): sgdisk run by
"vzctl release-disks" called sync() just before the reload. The vmcore
shows the queue's own flush_rq (REQ_OP_FLUSH, bio NULL, end_io
flush_end_io) in zero-rq, and the original empty PREFLUSH bio from
ext4_sync_fs() still waiting on the flush queue.

Patch 1 fixes the crash: the operation is taken from the request, and a
flush is killed like any other request zero-rq does not handle. Patch 2
completes flushes, discards and write zeroes instead of failing them,
which is what the zero-rq reload is there for. It is split out because
it changes what the target returns, not only whether it crashes.

The error and multipath request-based targets do not look at rq->bio,
so mainline is not affected; dm-zero-rq is ours.

Tested on 6.12.0-211.52.1.22.10.vz10 with the module built out of tree,
by suspending a released device, starting a sync or a blkdiscard on it,
then running ploop_dmreload_zero():
- stock: the node panics with the oops QA got, every time;
- patch 1: no panic, the sync and the discard fail with EIO;
- patches 1+2: both complete, nothing in the log;
- the ploop release tests, ploop and qcow2 formats, pass. The new
  tests for ploop_dmreload_zero() and the reproducer above go to ploop
  separately.

Denis V. Lunev (2):
  dm-zero-rq: fix a NULL dereference mapping a flush request
  dm-zero-rq: drop flushes, discards and write zeroes instead of failing
    them

 drivers/md/dm-zero-rq.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)


base-commit: b499efa17aa9fb8f20512d1831d1cb61b22e60d3
-- 
2.53.0

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