Re: [Qemu-devel] [PATCH 3/9] block: Remove child references from bs->{options, explicit_options}

[Max Reitz <mreitz@redhat.com>]

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On 2018-08-26 16:09, Alberto Garcia wrote:
> Block drivers allow opening their children using a reference to an
> existing BlockDriverState. These references remain stored in the
> 'options' and 'explicit_options' QDicts, but we don't need to keep
> them once everything is open.
> 
> What is more important, these values can become wrong if the children
> change:
> 
>     $ qemu-img create -f qcow2 hd0.qcow2 10M
>     $ qemu-img create -f qcow2 hd1.qcow2 10M
>     $ qemu-img create -f qcow2 hd2.qcow2 10M
>     $ $QEMU -drive if=none,file=hd0.qcow2,node-name=hd0 \
>             -drive if=none,file=hd1.qcow2,node-name=hd1,backing=hd0 \
>             -drive file=hd2.qcow2,node-name=hd2,backing=hd1
> 
> After this hd2 has hd1 as its backing file. Now let's remove it using
> block_stream:
> 
>     (qemu) block_stream hd2 0 hd0.qcow2
> 
> Now hd0 is the backing file of hd2, but hd2's options QDicts still
> contain backing=hd1.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)

Also, the question is, why would we discard the child options, but keep
the references (which we do not add, so we only have the ones specified
by the user).

> diff --git a/block.c b/block.c
> index 0dbb1fcc7b..2f2484965d 100644
> --- a/block.c
> +++ b/block.c
> @@ -2763,12 +2763,15 @@ static BlockDriverState *bdrv_open_inherit(const char *filename,
>          }
>      }
>  
> -    /* Remove all children options from bs->options and bs->explicit_options */
> +    /* Remove all children options and references
> +     * from bs->options and bs->explicit_options */
>      QLIST_FOREACH(child, &bs->children, next) {
>          char *child_key_dot;
>          child_key_dot = g_strdup_printf("%s.", child->name);
>          qdict_extract_subqdict(bs->explicit_options, NULL, child_key_dot);
>          qdict_extract_subqdict(bs->options, NULL, child_key_dot);
> +        qdict_del(bs->explicit_options, child->name);
> +        qdict_del(bs->options, child->name);
>          g_free(child_key_dot);
>      }
>  
> @@ -3289,6 +3292,7 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
>  {
>      BlockDriver *drv;
>      BlockDriverState *bs;
> +    BdrvChild *child;
>      bool old_can_write, new_can_write;
>  
>      assert(reopen_state != NULL);
> @@ -3313,6 +3317,12 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
>      bs->open_flags         = reopen_state->flags;
>      bs->read_only = !(reopen_state->flags & BDRV_O_RDWR);
>  
> +    /* Remove child references from bs->options and bs->explicit_options */
> +    QLIST_FOREACH(child, &bs->children, next) {
> +        qdict_del(bs->explicit_options, child->name);
> +        qdict_del(bs->options, child->name);
> +    }
> +

Why don't you remove the child options as well?

Max

>      bdrv_refresh_limits(bs, NULL);
>  
>      bdrv_set_perm(reopen_state->bs, reopen_state->perm,
> 



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