Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 13/01/2020 03.33, Wei Yang wrote:
> On Sun, Jan 12, 2020 at 12:55:45PM +0300, Konstantin Khlebnikov wrote:
>>
>>
>> On 12/01/2020 01.38, Wei Yang wrote:
>>> On Fri, Jan 10, 2020 at 11:11:23AM +0300, Konstantin Khlebnikov wrote:
>>> [...]
>>>>>>>>
>>>>>>>> series of vma in parent with shared AV:
>>>>>>>>
>>>>>>>> SRC1 - AV0
>>>>>>>> SRC2 - AV0
>>>>>>>> SRC3 - AV0
>>>>>>>> ...
>>>>>>>> SRCn - AV0
>>>>>>>>
>>>>>>>> in child after fork
>>>>>>>>
>>>>>>>> DST1 - AV_OLD_1 (some old vma, picked by anon_vma_clone) plus DST1 is attached to same AVs as SRC1
>>>>>>>> DST2 - AV_OLD_2 (other old vma) plus DST1 is attached to same AVs as SRC2
>>>>>>>> DST2 - AV1 prev AV parent does not match AV0, no old vma found for reusing -> allocate new one (child of AV0)
>>>>>>>> DST3 - AV1 - DST2->AV->parent == SRC3->AV (AV0) -> share AV with prev
>>>>>>>> DST4 - AV1 - same thing
>>>>>>>> ...
>>>>>>>> DSTn - AV1
>>>>>>>>
>>>
>>> To focus on the point, I rearranged the order a little. Suppose your following
>>> comments is explaining the above behavior.
>>>
>>>      I've illustrated how two heuristics (reusing-old and sharing-prev) _could_ work together.
>>>      But they both are optional.
>>>      At cloning first vma SRC1 -> DST1 there is no prev to share anon vma,
>>>      thus works common code which _could_ reuse old vma because it have to.
>>>      If there is no old anon-vma which have to be reused then DST1 will allocate
>>>      new anon-vma (AV1) and it will be used by DST2 and so on like on your picture.
>>>
>>> I agree with your 3rd paragraph, but confused with 2nd.
>>>
>>> At cloning first vma SRC1 -> DST1, there is no prev so anon_vma_clone() would
>>> pick up a reusable anon_vma. Here you named it AV_OLD_1. This looks good to
>>> me. But I am not sure why you would picked up AV_OLD_2 for DST2? In parent,
>>> SRC1 and SRC2 has the same anon_vma, AV0. So in child, DST1 and DST2 could
>>> also share the same anon_vma, AV_OLD_1.
>>>
>>> Sorry for my poor understanding, would you mind giving me more hint on this
>>> change?
>>
>> For DST2 heuristic "share-with-prev" will not work because if prev (DST1)
>> uses old AV (AV_OLD_1) and AV_OLD_1->parent isn't SRC2->AV (AV0).
>> So DST2 could only pick another old AV or allocate new.
> 
> I know this behavior after your change, my question is why you want to do so.

Because I want to keep both heuristics.
This seems most sane way of interaction between them.

Unfortunately even this patch is slightly broken.
Condition prev->anon_vma->parent == pvma->anon_vma doesn't guarantee that
prev vma has the same set of anon-vmas like current vma.
I.e. anon_vma_clone(vma, prev) might be not enough for keeping connectivity.
Building such case isn't trivial job but I see nothing that could prevent it.

> 
>>
>> My patch uses condition dst->prev->anon_vma->parent == src->anon_vma rather
>> than obvious src->prev->anon_vma == src->anon_vma because in this way it
>> eliminates all unwanted corner cases and explicitly verifies that we going to
>> share related anon-vma.
>>
> 
> This do eliminates some corner case, but as you showed child and parent don't
> share the same AV topology. To keep the same AV topology is the purpose of my
> commit.
> 
> I agree you found some bug that previous commit doesn't do it is expected. But
> since you change the design a little, I suggest you split this idea to a
> separate patch so that reviewer and audience in the future could understand
> your approach clearly. Otherwise audience would be confused and hard to track
> this change.
> 
> For example, you describe the behavior after your change. The second vma would
> probably have a different AV from first vma.
> 
>> Heuristic "reuse-old" uses fact that VMA links and AV parent chain are tracked
>> independently: when VMA reuses old AV it still links to all related AV even
>> if VMA->AV points into some old AV in the middle of inheritance chain.
>>
>>>
>>>>>>>
>>>>>>> Yes, your code works for DST3..DSTn. They will pick up AV1 since
>>>>>>> (DST2->AV->parent == SRC3->AV).
>>>>>>>
>>>>>>> My question is why DST1 and DST2 has different AV? The purpose of my patch
>>>>>>> tries to make child has the same topology and parent. So the ideal look of
>>>>>>> child is:
>>>>>>>
>>>>>>> DST1 - AV1
>>>>>>> DST2 - AV1
>>>>>>> DST2 - AV1
>>>>>>> DST3 - AV1
>>>>>>> DST4 - AV1
>>>>>>>
>>>>>>> Would you mind putting more words on DST1 and DST2? I didn't fully understand
>>>>>>> the logic here.
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>
>>>>>> I think that the first version is doing the work as you expected, but been
>>>>>> revised in second version, to limits the number of users of reused old
>>>>>> anon(which is picked in anon_vma_clone() and keep the tree structure.
>>>>>>
>>>>>
>>>>> Any reason to reduce the reuse? Maybe I lost some point.
>>>>
>>>>>
>>>>>>> --
>>>>>>> Wei Yang
>>>>>>> Help you, Help me
>>>>>
>>>
>