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

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

Reviewed-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>

On 11/3/26 09:36, Denis V. Lunev wrote:
> A cmd stops pio submission and waits for the inflight pios before it
> rewrites the BAT levels or deltas[], and expects nothing to touch them
> until it resumes. Konstantin found three ways this does not hold while
> reviewing the VSTOR-151979 series (VSTOR-152985, VSTOR-152984,
> VSTOR-152981):
> 
> - a submitter racing the stop either runs without the cmd waiting for
>    it, or is parked after resume and never completes (patch 1);
> 
> - 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
>    wakeup, which stalls the device by itself and would hang the new wait,
>    so patch 2 fixes that first;
> 
> - a grow failing to copy a cluster resumes submission with that
>    cluster's BAT page still busy, and writes to it hang (patch 4).
> 
> Patch 3 is the one losing data: a snapshot delete under write load is
> enough.
> 
> Tested on dev-cl04, stock 6.12.0-211.52.1.22.11.vz10, dm-ploop built
> out of tree:
> - each bug reproduced on the stock module, the races with a test-only
>    kprobe module widening the window: a pio running past the suspend of
>    a cmd; a pio never completing after resume; the top image truncated
>    from 512M to 128M by a merge and to 256M by a flip; after a grow
>    failing with ENOSPC on a full host fs, a writer stuck in D state, an
>    unsuspendable device and a later cmd failing with ETIMEDOUT. None of
>    them reproduce with the series;
> - fio 4k random reads and writes, O_DIRECT, 1 to 48 jobs, on a fully
>    allocated image over brd: IOPS and system CPU stay within the
>    run-to-run spread of the stock module. This setup tops out at about
>    420K IOPS elsewhere in dm-ploop and would not show a lock taken on
>    submission either, so patch 1 keeps that path lockless;
> - the ploop pytest suite, ploop and qcow2 formats, passes. A test for
>    patch 4 goes to the ploop tests separately.
> 
> 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
> 
>   drivers/md/dm-ploop-cmd.c    | 18 ++++++++++++++++--
>   drivers/md/dm-ploop-map.c    | 22 +++++++++++++++-------
>   drivers/md/dm-ploop-target.c |  4 ++++
>   drivers/md/dm-ploop.h        |  1 +
>   4 files changed, 36 insertions(+), 9 deletions(-)
> 
> 
> base-commit: 09eb9a82fe872048d082391415d1cdeda2ac6879

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