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

[Suren Baghdasaryan]

On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

>
> 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.