[Devel] [PATCH hci-8.1 v2 0/2] dm-ploop, dm-qcow2: fix merge status shared by all devices

[Denis V. Lunev]

dm-ploop and dm-qcow2 collect the status of their merge service I/O in
a function-local static, so every device on the host shares one copy.

In dm-ploop it is never reset: after a single failed fake merge pio,
every later online merge on any ploop device stops after its first pio
and returns that old error until the module is reloaded, which on a
working node means a reboot.

dm-qcow2 resets it when each run starts, which makes it worse: a merge
or L1 COW break starting on one device wipes the error another device
has just recorded. That merge then reports success although a write
failed, userspace completes it and drops the top image, and the data
that never reached the lower image is gone.

The status goes on the stack. The pios/qios carry a pointer to it, so
both loops must not return while one is outstanding: every exit
already waits for the counter to drain, and a BUG_ON after the wait
stops the kernel rather than let a late pio/qio write into a dead
stack frame if that ever changes.

Tested on dev-cl04, 6.12.0-211.52.1.22.9.vz10 with the modules built
out of tree, using test-only module knobs to fail one service pio/qio:
- dm-ploop: after one injected failure, a merge on another device and a
  second merge on the same device both failed with EIO; with the fix
  both succeed;
- dm-qcow2: a backward merge with an injected qio error, held for 5 s
  while a merge on a second device started, returned success; with the
  fix it fails with "Can't break L1 COW";
- the ploop pytest suite, ploop and qcow2 formats, passes.

Changes v1 -> v2:
- dropped BUG_ON

Denis V. Lunev (2):
  dm-ploop: fix a failed merge failing every later merge on the host
  dm-qcow2: fix a merge reporting success after a failed write

 drivers/md/dm-ploop-cmd.c | 4 +++-
 drivers/md/dm-qcow2-cmd.c | 6 +++---
 2 files changed, 6 insertions(+), 4 deletions(-)


base-commit: 02401c5e378f282fbf11bdc65996253ff65dad9b
-- 
2.53.0