Re: bitmap migration bug with -drive while block mirror runs

[Max Reitz <mreitz@redhat.com>]

[-- Attachment #1.1: Type: text/plain, Size: 2694 bytes --]

On 01.10.19 16:00, Vladimir Sementsov-Ogievskiy wrote:
> 01.10.2019 3:09, John Snow wrote:
>> Hi folks, I identified a problem with the migration code that Red Hat QE
>> found and thought you'd like to see it:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=1652424#c20
>>
>> Very, very briefly: drive-mirror inserts a filter node that changes what
>> bdrv_get_device_or_node_name() returns, which causes a migration problem.
>>
>>
>> Ignorant question #1: Can we multi-parent the filter node and
>> source-node? It looks like at the moment both consider their only parent
>> to be the block-job and don't have a link back to their parents otherwise.
>>
>>
>> Otherwise: I have a lot of cloudy ideas on how to solve this, but
>> ultimately what we want is to be able to find the "addressable" name for
>> the node the bitmap is attached to, which would be the name of the first
>> ancestor node that isn't a filter. (OR, the name of the block-backend
>> above that node.)
> 
> Not the name of ancestor node, it will break mapping: it must be name of the
> node itself or name of parent (may be through several filters) block-backend
> 
>>
>> A simple way to do this might be a "child_unfiltered" BdrvChild role
>> that simply bypasses the filter that was inserted and serves no real
>> purpose other than to allow the child to have a parent link and find who
>> it's """real""" parent is.
>>
>> Because of flushing, reopen, sync, drain &c &c &c I'm not sure how
>> feasible this quick idea might be, though.
>>
>>
>> - Corollary fix #1: call error_setg if the bitmap node name that's about
>> to go over the wire is an autogenerated node: this is never correct!
>>
>> (Why not? because the target is incapable of matching the node-name
>> because they are randomly generated AND you cannot specify node-names
>> with # prefixes as they are especially reserved!
>>
>> (This raises a related problem: if you explicitly add bitmaps to nodes
>> with autogenerated names, you will be unable to migrate them.))
>>
>> --js
>>
> 
> What about the following:
> 
> diff --git a/block.c b/block.c
> index 5944124845..6739c19be9 100644
> --- a/block.c
> +++ b/block.c
> @@ -1009,8 +1009,20 @@ static void bdrv_inherited_options(int *child_flags, QDict *child_options,
>       *child_flags = flags;
>   }
> 
> +static const char *bdrv_child_get_name(BdrvChild *child)
> +{
> +    BlockDriverState *parent = child->opaque;
> +
> +    if (parent->drv && parent->drv->is_filter) {
> +        return bdrv_get_parent_name(parent);
> +    }
> +
> +    return NULL;
> +}
> +

Why would we skip filters explicitly added by the user?

Max


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]