Re: [Qemu-block] [PATCH v6 20/42] block/snapshot: Fix fallback

[Kevin Wolf]

Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
> 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
> is more than one such child.
> 
> bdrv_snapshot_goto() becomes a bit weird because we may have to redirect
> the actual child pointer, so it only works if the fallback child is
> bs->file or bs->backing (and then we have to find out which it is).
> 
> Suggested-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/snapshot.c | 100 +++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 79 insertions(+), 21 deletions(-)
> 
> diff --git a/block/snapshot.c b/block/snapshot.c
> index f2f48f926a..35403c167f 100644
> --- a/block/snapshot.c
> +++ b/block/snapshot.c
> @@ -146,6 +146,32 @@ bool bdrv_snapshot_find_by_id_and_name(BlockDriverState 
> *bs,
>      return ret;
>  }
>  
> +/**
> + * Return the child BDS to which we can fall back if the given BDS
> + * does not support snapshots.
> + * Return NULL if there is no BDS to (safely) fall back to.
> + */
> +static BlockDriverState *bdrv_snapshot_fallback(BlockDriverState *bs)
> +{
> +    BlockDriverState *child_bs = NULL;
> +    BdrvChild *child;
> +
> +    QLIST_FOREACH(child, &bs->children, next) {
> +        if (child == bdrv_filtered_cow_child(bs)) {
> +            /* Ignore: COW children need not be included in snapshots */
> +            continue;
> +        }
> +
> +        if (child_bs) {
> +            /* Cannot fall back to a single child if there are multiple */
> +            return NULL;
> +        }
> +        child_bs = child->bs;
> +    }
> +
> +    return child_bs;
> +}

Why do we return child->bs here when bdrv_snapshot_goto() then needs to
reconstruct what the associated BdrvChild was? Wouldn't it make more
sense to return BdrvChild** from here and maybe have a small wrapper for
the other functions that only need a BDS?

Kevin