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

[Konstantin Khorenko <khorenko@virtuozzo.com>]

On 10/6/26 17:56, Denis V. Lunev wrote:
> 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.

Well, i agree here, so please disregard my comment about dirty bit handling.

On the other hand i would like to note that the current design does not allow kernel to distinguish 2 situations:

1. the image is CLEAN (it might be checked / fixed after the crash, does not matter),
ploop utility marks it as DIRTY when sending to the kernel - intended behavior.

2. the image had DIRTY bit because the Node crashed, so the image was NOT fixed, but
someone issued "dmsetup create" manually, not via ploop util.


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