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

[Hanna Czenczek]

On 25.09.23 13:40, Jean-Louis Dupond wrote:
> On 15/09/2023 13:21, Hanna Czenczek wrote:
> > 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.
> But it's still touching the discard code in the zeroize code path.
> Cause we modify the way zeroize does its discard (when 
> BDRV_REQ_MAY_UNMAP)

I find there’s a difference between discard code handling discards from 
the guest, and code handling zero-writes from the guest that internally 
issues discards.  I see your POV, but the documentation isn’t clear that 
not unref'ing on discards not only affects discards issued by the guest, 
but also internal discards that have been generated upon write-zero from 
the guest.

> > 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.
> I agree. How would you name the option then? Cause it still involves 
> discard-only code.

I wouldn’t change the name, just the definition (description).

Hanna

> Next to that, the option was already added to libvirt also (so this 
> needs to be fixed afterwards also).
> > Hanna