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

[Max Reitz]

On 14.06.19 16:31, Vladimir Sementsov-Ogievskiy wrote:
> 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 <address@hidden>
>>>> ---
>>>>    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.

Perhaps.  The op blockers are in this weird state anyway.  I don’t think
we even need them any more, because the permissions were intended to
replace them (they were originally called “fine-grained op blockers”, I
seem to remember).

I dare not touch them.

>>>>    /* 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)

Because it’s needed.  (Just not in the current form, but that’s what 37
is for.)

> 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?

I don’t quite see what you mean.  There is no “overlay” in this function.

> 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..

In theory yes, but that isn’t an option for qemu-img commit, for example.

> And I'd prefer to deprecate filename based interfaces at all.

Me too.

https://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg04878.html

:-/

Max

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