[Devel] [PATCH hci-8.1 v2 0/3] dm-qcow2: fix merges not stopping the users of the images they change

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

Checking dm-qcow2 for the same problem showed that a merge does not
stop everything using the images it changes (VSTOR-153049,
VSTOR-153050):

- a backward merge of an inner image pauses that image, while reads
  enter through the top one. A read reaching the inner image as the
  merge starts oopses on a NULL qio->ext (patch 2);

- SEEK_HOLE/SEEK_DATA walks the images with nothing keeping them
  alive, so merge_backward complete frees an image under the walk
  (patch 3).

Patch 1 (VSTOR-153051) is the counterpart of dm-ploop patch 4, an
error path leaving state behind: after a failed refcount table
relocation, allocating writes past the old refcount coverage fail with
EIO until the device is suspended and resumed.

The series does not depend on the dm-ploop one.

Tested on dev-cl04, stock 6.12.0-211.52.1.22.11.vz10, dm-qcow2 built
out of tree, with a test-only kprobe module widening the windows:
- on the stock module, with 4K clusters and refcount_bits=64 so the
  reftable relocates at 1G, a failed md page insert left every later
  write past 1G failing with EIO; a seek went on with an image
  qcow2_destroy() had freed; an intermediate merge oopsed in
  submit_read_sliced_clu(). None of them reproduce with the series;
- the ploop pytest suite, qcow2 format, passes.

Changes in v2, after Sashiko's review:
- 1/3: a note in the commit message on why the error path does not
  unlink the pages from the writeback lists;
- 3/3: guard the walk with a per-target rw_semaphore, taken for write
  by merge_backward complete, instead of an inflight ref held over the
  walk. With the ref every inflight ref switch waited for the whole
  scan, including the one starting a backward merge with the top image
  paused, so guest I/O stalled for as long as the scan ran.
The seek reproducer and the qcow2 pytest suite were rerun on v2: a
merge now starts while a seek is held, and merge_backward complete
waits for the seek to finish before destroying the image.

v1: http://lore.virtuozzo.com/kernel/20261103083708.131379-1-den@openvz.org/

Denis V. Lunev (3):
  dm-qcow2: fix a failed refcount table relocation leaking md pages
  dm-qcow2: fix backward merge of an inner image racing its reads
  dm-qcow2: fix SEEK_HOLE/SEEK_DATA using an image freed by a merge

 drivers/md/dm-qcow2-cmd.c    | 28 ++++++++++++----------------
 drivers/md/dm-qcow2-map.c    | 18 +++++++++++++-----
 drivers/md/dm-qcow2-target.c |  1 +
 drivers/md/dm-qcow2.h        |  2 ++
 4 files changed, 28 insertions(+), 21 deletions(-)


base-commit: 09eb9a82fe872048d082391415d1cdeda2ac6879
-- 
2.53.0

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