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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

01.10.2019 17:13, Max Reitz wrote:
> 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?
> 

Why not? Otherwise migration of bitmaps will not work: we may have different set
of filters on source and destination, and we still should map nodes with bitmaps
automatically.

I like John's idea of explicitly defined node mapping, but now we need simpler fix,
not involving libvirt changes if possible.

Hmm, or you mean that by this patch I touch not only migration but all users of
bdrv_get_device_or_node_name? Than I can't predict all the consequences...

Is it better to add .get_name_for_bitmaps_migration handler (with simpler name of course)?

-- 
Best regards,
Vladimir