Re: [PATCH v7 23/47] block/snapshot: Fix fallback

[Andrey Shinkevich]

On 16.07.2020 00:22, Andrey Shinkevich wrote:
> On 25.06.2020 18:21, Max Reitz wrote:
> > If the top node's driver does not provide snapshot functionality and we
> > want to fall back to a node down the chain, we need to snapshot all
> > non-COW children.  For simplicity's sake, just do not fall back if there
>
> I guess it comes to COW children like BDRV_CHILD_DATA | 
> BDRV_CHILD_METADATA  rather than non-COW ones, does it?

The BDRV_CHILD_COW is mutually exclusive with DATA, METADATA and 
FILTERED per definition.

Sorry about the question.

> Andrey
>
>
> > is more than one such child.  Furthermore, we really only can fall back
> > to bs->file and bs->backing, because bdrv_snapshot_goto() has to modify
> > the child link (notably, set it to NULL).
> >
> > Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > ---
> >   block/snapshot.c | 104 +++++++++++++++++++++++++++++++++++++----------
> >   1 file changed, 83 insertions(+), 21 deletions(-)
> >
> > diff --git a/block/snapshot.c b/block/snapshot.c
> ...
> > +    /*
> > +     * Check that there are no other children that would need to be
> > +     * snapshotted.  If there are, it is not safe to fall back to
> > +     * *fallback.
> > +     */
> > +    QLIST_FOREACH(child, &bs->children, next) {
> > +        if (child->role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
> > +                           BDRV_CHILD_FILTERED) &&
> > +            child != *fallback)
> > +        {
> > +            return NULL;
> > +        }
> > +    }
> > +
> > +    return fallback;
> > +}
>
> ...
>
> Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>