Re: [Qemu-devel] [Qemu-block] [PATCH v4 06/13] block: Generalize should_update_child() rule

[Alberto Garcia]

On Wed 11 Apr 2018 08:54:18 PM CEST, Max Reitz wrote:
> Currently, bdrv_replace_node() refuses to create loops from one BDS to
> itself if the BDS to be replaced is the backing node of the BDS to
> replace it: Say there is a node A and a node B.  Replacing B by A means
> making all references to B point to A.  If B is a child of A (i.e. A has
> a reference to B), that would mean we would have to make this reference
> point to A itself -- so we'd create a loop.
>
> bdrv_replace_node() (through should_update_child()) refuses to do so if
> B is the backing node of A.  There is no reason why we should create
> loops if B is not the backing node of A, though.  The BDS graph should
> never contain loops, so we should always refuse to create them.
>
> If B is a child of A and B is to be replaced by A, we should simply
> leave B in place there because it is the most sensible choice.
>
> A more specific argument would be: Putting filter drivers into the BDS
> graph is basically the same as appending an overlay to a backing chain.
> But the main child BDS of a filter driver is not "backing" but "file",
> so restricting the no-loop rule to backing nodes would fail here.
>
> Signed-off-by: Max Reitz <address@hidden>
> Reviewed-by: Fam Zheng <address@hidden>

Reviewed-by: Alberto Garcia <address@hidden>

Berto