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

[Benoît Canet]

The Wednesday 12 Feb 2014 à 22:43:15 (+0100), Benoît Canet wrote :
> The Wednesday 12 Feb 2014 à 20:49:18 (+0100), Benoît Canet wrote :
> > 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 ?
> 
> I have a pending resping of this patch which tests for the two cases as a
> workaround.
> 
> Should I post it ?
> 
> Best regards
> 
> Benoît
> 
> > 
> > Best regards
> > 
> > Benoît
> > 
> 

I found that the used the command line init in a wrong way.
I will repost an updated version of the patch.

Quorum will need to be reposted since one commit message showed the wrong way of
doing the initialization.

Best regards

Benoît