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

[Andrey Shinkevich]

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?

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>