Re: [Qemu-devel] [PATCH v5 14/42] block: Use CAFs when working with backing chains

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

14.06.2019 16:50, Max Reitz wrote:
> On 14.06.19 15:26, Vladimir Sementsov-Ogievskiy wrote:
>> 13.06.2019 1:09, Max Reitz wrote:
>>> Use child access functions when iterating through backing chains so
>>> filters do not break the chain.
>>>
>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> ---
>>>    block.c | 40 ++++++++++++++++++++++++++++------------
>>>    1 file changed, 28 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/block.c b/block.c
>>> index 11f37983d9..505b3e9a01 100644
>>> --- a/block.c
>>> +++ b/block.c
> 
> [...]
> 
>>> @@ -4273,11 +4274,18 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>>>    BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
>>>                                        BlockDriverState *bs)
>>>    {
>>> -    while (active && bs != backing_bs(active)) {
>>> -        active = backing_bs(active);
>>> +    bs = bdrv_skip_rw_filters(bs);
>>> +    active = bdrv_skip_rw_filters(active);
>>> +
>>> +    while (active) {
>>> +        BlockDriverState *next = bdrv_backing_chain_next(active);
>>> +        if (bs == next) {
>>> +            return active;
>>> +        }
>>> +        active = next;
>>>        }
>>>    
>>> -    return active;
>>> +    return NULL;
>>>    }
>>
>> Semantics changed for this function.
>> It is used in two places
>> 1. from bdrv_find_base wtih @bs=NULL, it should be unchanged, as I hope we will never have
>>      filter node as a bottom of some valid chain
>>
>> 2. from qmp_block_commit, only to check op-blocker... hmmm. I really don't understand,
>> why do we check BLOCK_OP_TYPE_COMMIT_TARGET on top_bs overlay.. top_bs overlay is out of the job,
>> what is this check for?
> 
> There is a loop before this check which checks that the same blocker is
> not set on any nodes between top and base (both inclusive).  I guess
> non-active commit checks the node above @top, too, because its backing
> file will change.

So in this case frozen chain works better.

> 
>>>    /* Given a BDS, searches for the base layer. */
> 
> [...]
> 
>>> @@ -5149,7 +5165,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>>>                char *backing_file_full_ret;
>>>    
>>>                if (strcmp(backing_file, curr_bs->backing_file) == 0) {
>>
>> hmm, interesting, what bs->backing_file now means? It's strange enough to store such field on
>> bds, when we have backing link anyway..
> 
> Patch 37 has you covered. :-)
> 

Hmm, if it has removed this field, but it doesn't)

So, we finished with some object, called "overlay", but it is not an overlay of bs, it's overlay of
first non-implicit filtered node in bs backing chain, it may be found by bdrv_find_overlay() helper (which is
almost unused and my be safely dropped), and filename of this "overlay" is stored in bs->backing_file string
variable, keeping in mind that bs->backing is pointer to backing child of bs which is completely another thing?

Oh, no, everything related to filename-based backing chain logic is not for me o_O. If something doesn't work
with filename-based logic users should use node-names.. And I'd prefer to deprecate filename based interfaces
at all.

-- 
Best regards,
Vladimir