[PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set

[Suren Baghdasaryan]

On Fri, Feb 17, 2023 at 2:21 AM Hyeonggon Yoo <42.hyeyoo at gmail.com> wrote:
>
> On Fri, Feb 17, 2023 at 11:15 AM Suren Baghdasaryan <surenb at google.com> wrote:
> >
> > On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan <surenb at google.com> wrote:
> > >
> > > On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox <willy at infradead.org> wrote:
> > > >
> > > > On Wed, Feb 15, 2023 at 09:17:41PM -0800, Suren Baghdasaryan wrote:
> > > > > When vma->anon_vma is not set, page fault handler will set it by either
> > > > > reusing anon_vma of an adjacent VMA if VMAs are compatible or by
> > > > > allocating a new one. find_mergeable_anon_vma() walks VMA tree to find
> > > > > a compatible adjacent VMA and that requires not only the faulting VMA
> > > > > to be stable but also the tree structure and other VMAs inside that tree.
> > > > > Therefore locking just the faulting VMA is not enough for this search.
> > > > > Fall back to taking mmap_lock when vma->anon_vma is not set. This
> > > > > situation happens only on the first page fault and should not affect
> > > > > overall performance.
> > > >
> > > > I think I asked this before, but don't remember getting an aswer.
> > > > Why do we defer setting anon_vma to the first fault?  Why don't we
> > > > set it up at mmap time?
> > >
> > > Yeah, I remember that conversation Matthew and I could not find the
> > > definitive answer at the time. I'll look into that again or maybe
> > > someone can answer it here.
> >
> > After looking into it again I'm still under the impression that
> > vma->anon_vma is populated lazily (during the first page fault rather
> > than at mmap time) to avoid doing extra work for areas which are never
> > faulted. Though I might be missing some important detail here.
>
> I think this is because the kernel cannot merge VMAs that have
> different anon_vmas?
>
> Enabling lazy population of anon_vma could potentially increase the
> chances of merging VMAs.

Hmm. Do you have a clear explanation why merging chances increase this
way? A couple of possibilities I can think of would be:
1. If after mmap'ing a VMA and before faulting the first page into it
we often change something that affects anon_vma_compatible() decision,
like vm_policy;
2. When mmap'ing VMAs we do not map them consecutively but the final
arrangement is actually contiguous.

Don't think either of those cases would be very representative of a
usual case but maybe I'm wrong or there is another reason?

>
> > > In the end rather than changing that logic I decided to skip
> > > vma->anon_vma==NULL cases because I measured them being less than
> > > 0.01% of all page faults, so ROI from changing that would be quite
> > > low. But I agree that the logic is weird and maybe we can improve
> > > that. I will have to review that again when I'm working on eliminating
> > > all these special cases we skip, like swap/userfaults/etc.
>
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