[Devel] [PATCH hci-8.1] drivers/md/dm-qcow2: truncate unused tail of image on suspend

[Denis V. Lunev]

On 10/6/26 17:52, Konstantin Khorenko wrote:
>> drivers/md/dm-qcow2: truncate unused tail of image on suspend
> ...
>> Truncate the file to min(file_preallocated_area_start, used end).
>> Skip the scan if md writeback failed, as refcounts on disk are stale
>> then, and fall back to the old behaviour on scan error.
> There is one more case of stale refcounts on disk: an image with
> INCOMPATIBLE_FEATURES_DIRTY_BIT set. qcow2_check_convert_hdr() accepts it
> (the check is commented out), and with qemu lazy refcounts that bit means
> refcounts were not written for the last allocations, so they are too low.
>
> Today such an image loses data only when the allocator hands out a
> cluster whose refcount is zero but which L2 still references. After this
> patch the first suspend truncates live clusters at the tail without a
> single write to the device. The image is already invalid per spec, so
> this is not a regression in the strict sense, but either the commit
> message should say that a dirty image is not supported, or the dirty-bit
> check in qcow2_check_convert_hdr() should come back.

Image should go clean into the kernel on first pass.