Re: [Qemu-devel] [PATCH v6 14/42] block: Use CAFs when working with backing chains

[Max Reitz <mreitz@redhat.com>]

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On 09.09.19 11:55, Kevin Wolf wrote:
> Am 09.09.2019 um 10:25 hat Max Reitz geschrieben:
>> On 05.09.19 16:05, Kevin Wolf wrote:
>>> Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
>>>> 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 86b84bea21..42abbaf0ba 100644
>>>> --- a/block.c
>>>> +++ b/block.c
>>>> @@ -4376,7 +4376,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>>>>  }
>>>>  
>>>>  /*
>>>> - * Finds the image layer in the chain that has 'bs' as its backing file.
>>>> + * Finds the image layer in the chain that has 'bs' (or a filter on
>>>> + * top of it) as its backing file.
>>>>   *
>>>>   * active is the current topmost image.
>>>>   *
>>>> @@ -4388,11 +4389,18 @@ 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);
>>>
>>> This does more than the commit message says. In addition to iterating
>>> through filters instead of stopping, it also changes the semantics of
>>> the function to return the next non-filter on top of bs instead of the
>>> next node.
>>
>> Which is to say the overlay.
>>
>> (I think we only ever use “overlay” in the COW sense.)
> 
> I think we do, but so far also only ever for immediate COW childs, not
> for skipping through intermediate node.

Yes, because it’s broken so far.

>>> The block jobs seem to use it only for bdrv_is_allocated_above(), which
>>> should return the same thing in both cases, so the behaviour stays the
>>> same. qmp_block_commit() will check op blockers on a different node now,
>>> which could be a fix or a bug, I can't tell offhand. Probably the
>>> blocking doesn't really work anyway.
>>
>> You mean that the op blocker could have been on a block job filter node
>> before?  I think that‘s pretty much the point of this fix; that that
>> doesn’t make sense.  (We didn’t have mirror_top_bs and the like at
>> 058223a6e3b.)
> 
> On the off chance that the op blocker actually works, it can't be a job
> filter. I was thinking more of throttling, blkdebug etc.

Well, that’s just broken altogether right now.

>>> All of this should be mentioned in the commit message at least. Maybe
>>> it's also worth splitting in two patches.
>>
>> I don’t know.  The function was written when there were no filters.
> 
> I doubt it. blkdebug is a really old filter.
> 
>> This change would have been a no-op then.  The fact that it isn’t to me
>> just means that introducing filters broke it.
>>
>> So I don’t know what I would write.  Maybe “bdrv_find_overlay() now
>> actually finds the overlay, that is, it will not return a filter node.
>> This is the behavior that all callers expect (because they work on COW
>> backing chains).”
> 
> Maybe just something like "In addition, filter nodes are not returned as
> overlays any more. Instead, the first non-filter node on top of bs is
> considered the overlay now."?

Isn’t that basically the same? :-)

Max


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