Re: [Qemu-devel] [FIX V2] block: Fix device snapshots broken by the block filter snapshots patchset.

[Benoît Canet]

The Tuesday 11 Feb 2014 à 16:12:17 (+0800), Fam Zheng wrote :
> On Mon, 02/10 22:49, Benoît Canet wrote:
> > Take into account the fact that a block filter like quorum will be in 
> > bs->file
> > while a regular block driver device is really on the top level.
> > 
> > Signed-off-by: Benoit Canet <address@hidden>
> > ---
> >  block.c | 14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> > 
> > diff --git a/block.c b/block.c
> > index 07ac50a..d04f535 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -5400,14 +5400,16 @@ bool bdrv_is_first_non_filter(BlockDriverState 
> > *candidate)
> >  
> >      /* walk down the bs forest recursively */
> >      QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> > -        bool perm;
> > -
> > -        if (!bs->file) {
> > -            continue;
> > +        bool perm = false;
> > +
> > +        if (bs->file &&
> > +            bs->file->drv &&
> > +            bs->file->drv->authorizations[BS_IS_A_FILTER]) {
> > +            perm = bdrv_recurse_is_first_non_filter(bs->file, candidate);
> > +        } else if (bs == candidate) {
> > +            perm = true;
> >          }
> >  
> > -        perm = bdrv_recurse_is_first_non_filter(bs->file, candidate);
> > -
> >          /* candidate is the first non filter */
> >          if (perm) {
> >              return true;
> 
> With this change, if the top level driver has ->file, its implementation of
> .bdrv_recurse_is_first_non_filter() is skipped and bs->file is the start 
> point.
> 
> So we have an implication that single child block drivers (that has ->file)
> doesn't need to, and shouldn't implement this operation, as commentted above
> bdrv_generic_is_first_non_filter.
> 
> Tested that this patch fixes the external snapshot problem, but didn't test
> the "quorum as bs->file case".
> 
> Thanks,
> 
> Reviewed-by: Fam Zheng <address@hidden>

After extensive testing of all type of quorum instanciations and snapshots I
discovered that we are not done yet with this issue.

When instantiating quorum from the command line the quorum driver is in
bs->file->drv.
When using QMP's blockdev_add at once or by using references the quorum driver
is in bs->drv.

As a result this patch work in half the cases: regular file and command line
quorum but not when quorum is instantiated via QMP.

I don't understand why the block layer have this irregular behavior regarding
quorum instantiation.

What would be the best way to fix this ?

Best regards

Benoît