On 10.09.19 17:02, Kevin Wolf wrote:
> Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
>> This allows us to differentiate between filters and nodes with COW
>> backing files: Filters cannot be used as overlays at all (for this
>> function).
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
> Didn't we occasionally advertise blockdev-snapshot as the way to insert
> filters on top at runtime?

I can only remember advertising for it as the only graph manipulation
tool we had, and maybe saying “We’d want something like
blockdev-snapshot for filters, too”.

Max

> Though it seems it has always only worked for
> filters that use bs->backing, among which I think there aren't any
> user-creatable ones. So we're probably good.
> 
> Kevin
> 
>>  blockdev.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/blockdev.c b/blockdev.c
>> index 29c6c6044a..c540802127 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -1664,7 +1664,12 @@ static void external_snapshot_prepare(BlkActionState *common,
>>          goto out;
>>      }
>>  
>> -    if (state->new_bs->backing != NULL) {
>> +    if (state->new_bs->drv->is_filter) {
>> +        error_setg(errp, "Filters cannot be used as overlays");
>> +        goto out;
>> +    }
>> +
>> +    if (bdrv_filtered_cow_child(state->new_bs)) {
>>          error_setg(errp, "The overlay already has a backing image");
>>          goto out;
>>      }
>> -- 
>> 2.21.0
>>