[Devel] [PATCH hci-8.1 v3 2/5] dm-qcow2: fix backward merge of an inner image racing its reads

[no-reply at sashiko.virtuozzo.com]

> From: Denis V. Lunev <den at openvz.org>
> 
> set_backward_merge_in_process() sets the flag of an image other than the
> top one without stopping submission, as nothing writes to it. It is
> still read: a read of a cluster the top image does not map goes on to
> the lower image, and when that image is writable its worker tests the
> flag twice for the qio. When the merge starts between the two tests, the
> first one, in handle_metadata(), skips prepare_backward_merge() and
> leaves qio->ext NULL, the second one, in process_one_qio(), takes the
> merge path and submit_top_delta_read() dereferences qio->ext.
> 
> Pause submission and switch the inflight refs for every image. The
> pause flag is only tested on the top image, where all qios enter, so
> pause that one; clearing the flag of an inner image paused the inner
> image itself, which held nothing back.
> 
> Starting or stopping a merge of an inner image now pauses guest I/O to
> the top image for the inflight ref switch, as a merge of the top image
> always did.
> 
> Fixes: 8e522723a085 ("dm-qcow2: allow specifying depth to merge intermediate images")
> https://virtuozzo.atlassian.net/browse/VSTOR-153049
> Feature: dm-qcow2: block device over QCOW2 files driver
> Reviewed-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> Reviewed-by: Konstantin Khorenko <khorenko at virtuozzo.com>
> Signed-off-by: Denis V. Lunev <den at openvz.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261104180105.1807820-1-den@openvz.org?part=2