[Devel] [PATCH hci-8.1 v2 0/3] dm-qcow2: fix merges not stopping the users of the images they change

[Andrey Zhadchenko]

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

On 11/3/26 10:08, Denis V. Lunev wrote:
> Checking dm-qcow2 for the same problem showed that a merge does not
> stop everything using the images it changes (VSTOR-153049,
> VSTOR-153050):
> 
> - a backward merge of an inner image pauses that image, while reads
>    enter through the top one. A read reaching the inner image as the
>    merge starts oopses on a NULL qio->ext (patch 2);
> 
> - SEEK_HOLE/SEEK_DATA walks the images with nothing keeping them
>    alive, so merge_backward complete frees an image under the walk
>    (patch 3).
> 
> Patch 1 (VSTOR-153051) is the counterpart of dm-ploop patch 4, an
> error path leaving state behind: after a failed refcount table
> relocation, allocating writes past the old refcount coverage fail with
> EIO until the device is suspended and resumed.
> 
> The series does not depend on the dm-ploop one.
> 
> Tested on dev-cl04, stock 6.12.0-211.52.1.22.11.vz10, dm-qcow2 built
> out of tree, with a test-only kprobe module widening the windows:
> - on the stock module, with 4K clusters and refcount_bits=64 so the
>    reftable relocates at 1G, a failed md page insert left every later
>    write past 1G failing with EIO; a seek went on with an image
>    qcow2_destroy() had freed; an intermediate merge oopsed in
>    submit_read_sliced_clu(). None of them reproduce with the series;
> - the ploop pytest suite, qcow2 format, passes.
> 
> Changes in v2, after Sashiko's review:
> - 1/3: a note in the commit message on why the error path does not
>    unlink the pages from the writeback lists;
> - 3/3: guard the walk with a per-target rw_semaphore, taken for write
>    by merge_backward complete, instead of an inflight ref held over the
>    walk. With the ref every inflight ref switch waited for the whole
>    scan, including the one starting a backward merge with the top image
>    paused, so guest I/O stalled for as long as the scan ran.
> The seek reproducer and the qcow2 pytest suite were rerun on v2: a
> merge now starts while a seek is held, and merge_backward complete
> waits for the seek to finish before destroying the image.
> 
> v1: http://lore.virtuozzo.com/kernel/20261103083708.131379-1-den@openvz.org/
> 
> Denis V. Lunev (3):
>    dm-qcow2: fix a failed refcount table relocation leaking md pages
>    dm-qcow2: fix backward merge of an inner image racing its reads
>    dm-qcow2: fix SEEK_HOLE/SEEK_DATA using an image freed by a merge
> 
>   drivers/md/dm-qcow2-cmd.c    | 28 ++++++++++++----------------
>   drivers/md/dm-qcow2-map.c    | 18 +++++++++++++-----
>   drivers/md/dm-qcow2-target.c |  1 +
>   drivers/md/dm-qcow2.h        |  2 ++
>   4 files changed, 28 insertions(+), 21 deletions(-)
> 
> 
> base-commit: 09eb9a82fe872048d082391415d1cdeda2ac6879