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

[Konstantin Khlebnikov]

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(whichÂis pickedÂin 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