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

[Wei Yang]

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?

>> > > 
>> > > 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
>> 

-- 
Wei Yang
Help you, Help me