Re: [Devel] [PATCH hci-8.1 0/4] dm-ploop: fix the pio submission stop around cmds

[Konstantin Khorenko <khorenko@virtuozzo.com>]

I won't apply this version, please send v2 with the changes below.

On Tue, 03 Nov 2026 09:36:38 +0100, Denis V. Lunev wrote:
> Denis V. Lunev (4):
>   dm-ploop: fix pio submission racing with the suspend and resume of a
>     cmd
>   dm-ploop: fix a lost wakeup of the ploop_allocator() thread
>   dm-ploop: wait for the ploop_allocator() thread before changing
>     deltas[]
>   dm-ploop: fix a failed cluster copy on grow leaving the BAT page busy

What to change for v2:

1. Patch 1, commit message only, small: in the first race add
   notify_merged to "a merge, resize or delta index update". The code
   stays as is.

2. Patch 2, commit message only, small:
   - drop "until some later request wakes the thread", the stall does
     not end by itself;
   - add one sentence that the same reorder also fixes a lost
     kthread_stop() wakeup.
   The code stays as is.

3. Patch 3, code and commit message:
   - code: also wait for ploop_no_pending_prealloc() in
     ploop_flip_upper_deltas() before process_flip_upper_deltas(), keep
     the wait in ploop_postsuspend();
   - message: "merge and notify_merged_backward" becomes "merge and
     notify_merged", both directions move the top delta.

4. Patch 4: no changes, resend it as is.

5. One more old bug found, not part of this series: the forward
   notify_merged stores the new file_size into the wrong deltas[] slot
   (details below). Please file a Jira task for it and fix it with a
   separate patch, not in v2.

6. Cover letter of v2:
   - "cutting the live image to the preallocation size" becomes
     "cutting the live image";
   - reword "Patch 3 is the one losing data".

Details on each item are in the replies to the patches. Below are the
details on items 5 and 6.

Item 5, the old bug. In ploop_delta_clusters_merged() the forward case
stores file_size, file_preallocated_area_start and nr_be into
deltas[level + 1] after notify_delta_merged() has already shifted the
slots down, so the delta that took the clusters is at deltas[level] by
then. With level == top - 2 that overwrites the top delta's file_size,
and the next preallocation can truncate the live top image. It has been
there since the target was added.

> - the ploop_allocator() thread is not stopped at all. It truncates the
>   top delta while merge or flip moves it in deltas[], cutting the live
>   image to the preallocation size (patch 3). The thread can also miss a

Item 6. Only a merge cuts the image to the preallocation size. A flip
cuts the old top image to the size of the other delta plus the
preallocation.

> Patch 3 is the one losing data: a snapshot delete under write load is
> enough.

Item 6. Race A of patch 1 loses data too: a pio can read and write
through a delta that merge has already released. Patch 3 is the easiest
one to hit, not the only one losing data.

-- 
Konstantin Khorenko,
Virtuozzo Linux Kernel Team

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