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

[Konstantin Khlebnikov]

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(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