[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 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.

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.