Re: [Qemu-devel] [PATCH v3 1/5] block: Allow replacement of a BDS by its overlay

[Fam Zheng <famz@redhat.com>]

On Fri, 06/10 20:57, Max Reitz wrote:
> change_parent_backing_link() asserts that the BDS to be replaced is not
> used as a backing file. However, we may want to replace a BDS by its
> overlay in which case that very link should not be redirected.
> 
> For instance, when doing a sync=none drive-mirror operation, we may have
> the following BDS/BB forest before block job completion:
> 
>   target
> 
>   base <- source <- BlockBackend
> 
> During job completion, we want to establish the source BDS as the
> target's backing node:
> 
>           target
>             |
>             v
>   base <- source <- BlockBackend
> 
> This makes the target a valid replacement for the source:
> 
>           target <- BlockBackend
>             |
>             v
>   base <- source
> 
> Without this modification to change_parent_backing_link() we have to
> inject the target into the graph before the source is its backing node,
> thus temporarily creating a wrong graph:
> 
>   target <- BlockBackend
> 
>   base <- source
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/block.c b/block.c
> index f54bc25..dc76c159 100644
> --- a/block.c
> +++ b/block.c
> @@ -2224,9 +2224,23 @@ void bdrv_close_all(void)
>  static void change_parent_backing_link(BlockDriverState *from,
>                                         BlockDriverState *to)
>  {
> -    BdrvChild *c, *next;
> +    BdrvChild *c, *next, *to_c;
>  
>      QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
> +        if (c->role == &child_backing) {
> +            /* @from is generally not allowed to be a backing file, except for
> +             * when @to is the overlay. In that case, @from may not be replaced
> +             * by @to as @to's backing node. */
> +            QLIST_FOREACH(to_c, &to->children, next) {
> +                if (to_c == c) {
> +                    break;
> +                }
> +            }

Why not just test "c->bs == to" here, why is it necessary to iterate through
to->children list?

Fam

> +            if (to_c) {
> +                continue;
> +            }
> +        }
> +
>          assert(c->role != &child_backing);
>          bdrv_ref(to);
>          bdrv_replace_child(c, to);
> -- 
> 2.8.3
>