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 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.

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.

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
>>>
>