[Devel] [PATCH hci-8.1 0/2] dm-ploop: fix writes held on ENOSPC never being retried

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

A write that fails with ENOSPC on the image is held and retried by
enospc_timer, so that I/O resumes once space is freed. The timer only
sets a flag that the dispatcher consumes, and nothing wakes the
dispatcher, so the held writes are retried only when other I/O reaches
the device. On a device whose only I/O is the held writes, the writers
stay in D state for good after the space is freed (patch 2,
VSTOR-154182).

Patch 1 makes the destructor shut the timer down first. Nothing stops
it there today, and with patch 2 the callback dereferences the
dispatcher thread as well as struct ploop.

Found in a vmcore: 16 writers in D state, submit_enospc set, all 16
pios on enospc_pios, the timer expired 12 s before the crash.

Tested on 6.12.0-211.52.1.22.19.vz10, dm-ploop built out of tree:
- 16 O_DIRECT writers held on a full host ext4, then the space freed
  with no other I/O on the device. Stock: still stuck after 60 s, one
  unrelated read releases them. With the series: done 8 s after the
  space is freed, on the next timer tick;
- ploop test suite, make -j48 test on dev-cl04, green for ploop and
  qcow2 with this series together with the VSTOR-154096 one.

Denis V. Lunev (2):
  dm-ploop: stop enospc_timer in ploop_destroy()
  dm-ploop: fix the ENOSPC retry timer not waking the dispatcher

 drivers/md/dm-ploop-map.c    | 9 +++++----
 drivers/md/dm-ploop-target.c | 1 +
 2 files changed, 6 insertions(+), 4 deletions(-)

-- 
2.53.0

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