[Devel] [PATCH hci-8.1 0/3] dm-ploop: fix acknowledged writes lost when discard and allocation share a BAT writeback

[Denis V. Lunev]

The hci-ploop fstrim test caught dm-ploop losing an acknowledged write
(VSTOR-151979). Since "dm-ploop: fix and rework md updates", an
allocation and a discard on the same BAT page can share one pending
index writeback, and its completion applies the written page by the
writeback's type only. A first write that rides in a DISCARD writeback
is on disk but never reaches the local BAT: it reads back as zeroes,
the next write allocates the cluster again and the data is gone. A
discard that rides in an ALLOC writeback leaves the local BAT mapped
while disk has zero, so writes after it are lost on the next open.
fstrim or guest TRIM racing ordinary writes is enough.

Patch 3 is the fix: the completion applies the changes of the pios
attached to the writeback, and of the relocation done on grow, instead
of filtering the page by type. Patches 1 and 2 fix older bugs on the
same path that the old completion partly hid: a failed BAT writeback
freed the wrong clusters, and the online merge of the latest snapshot
left BAT levels above the top one.

Tested on dev-cl04, 6.12.0-211.52.1.22.9.vz10 with the module built
out of tree:
- a reproducer racing cluster discards with first writes on one BAT
  page, 512 clusters a round: without the series 3 of 15360 first
  writes read back as zeroes right after the write returned and stayed
  lost after remount; with a 200 us sleep before bat_lock in the
  allocation path 1801 of 2560; the reverse order, a sleep in the
  discard path, left data written after a discard unmapped on disk.
  With the series nothing is lost in either order;
- the ploop pytest suite, ploop and qcow2 formats, passes;
- nothing exercises a failing BAT writeback, that path is checked by
  reading only.

Denis V. Lunev (3):
  dm-ploop: fix freeing the wrong clusters when a BAT writeback fails
  dm-ploop: renumber BAT levels when the latest snapshot is merged
  dm-ploop: fix lost BAT updates when alloc and discard share a
    writeback

 drivers/md/dm-ploop-cmd.c |  22 ++++++
 drivers/md/dm-ploop-map.c | 139 +++++++++++++++++++-------------------
 drivers/md/dm-ploop.h     |   1 +
 3 files changed, 93 insertions(+), 69 deletions(-)


base-commit: 02401c5e378f282fbf11bdc65996253ff65dad9b
-- 
2.53.0