Re: [PATCH v2] qcow2: keep reference on zeroize with discard-no-unref enabled

[Hanna Czenczek]

On 05.09.23 15:08, Jean-Louis Dupond wrote:
> When the discard-no-unref flag is enabled, we keep the reference for
> normal discard requests.
> But when a discard is executed on a snapshot/qcow2 image with backing,
> the discards are saved as zero clusters in the snapshot image.
>
> When committing the snapshot to the backing file, not
> discard_in_l2_slice is called but zero_in_l2_slice. Which did not had
> any logic to keep the reference when discard-no-unref is enabled.
>
> Therefor we add logic in the zero_in_l2_slice call to keep the reference
> on commit.
>
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621
> Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
> ---
>   block/qcow2-cluster.c | 22 ++++++++++++++++++----
>   1 file changed, 18 insertions(+), 4 deletions(-)

The code looks OK, but the obvious problem I find is that this is not 
what the discard-no-unref option describes.  It talks about discards, 
but this now changes the zero-write path.

I’m fairly certain that you are the only one using this option for now, 
so we might as well change its definition to include zero writes for 
8.2, but we should do that.

Hanna