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

[Hyeonggon Yoo]

On Fri, Feb 17, 2023 at 08:13:01AM -0800, Suren Baghdasaryan wrote:
> On Fri, Feb 17, 2023 at 2:21 AM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
> >
> > On Fri, Feb 17, 2023 at 11:15 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >
> > > 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.
> >
> > 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?

Ok. I agree it does not represent common cases.

Hmm then I wonder how it went from the initial approach of "allocate anon_vma
objects only via fork()" [1] to "populate anon_vma at page faults". [2] [3]

Maybe Hugh, Andrea or Andrew have opinions?

[1] anon_vma RFC2, lore.kernel.org
https://lore.kernel.org/lkml/20040311065254.GT30940@dualathlon.random

[2] The status of object-based reverse mapping, LWN.net
https://lwn.net/Articles/85908

[3] rmap 39 add anon_vma rmap
https://gitlab.com/hyeyoo/linux-historical/-/commit/8aa3448cabdfca146aa3fd36e852d0209fb2276a

> 
> >
> > > > 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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