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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 27 Jan 2023 16:00:16 -0800
Message-ID: <CAJuCfpEq2F4EwWAeP6nLqS9m9XLpUss8n=35ZTgYgtiAJyvsxQ@mail.gmail.com>
Subject: Re: [PATCH v2 00/33] Per-VMA locks
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
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On Fri, Jan 27, 2023 at 3:26 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Jan 27, 2023 at 02:51:38PM -0800, Andrew Morton wrote:
> > On Fri, 27 Jan 2023 11:40:37 -0800 Suren Baghdasaryan <surenb@google.co=
m> wrote:
> >
> > > Per-vma locks idea that was discussed during SPF [1] discussion at LS=
F/MM
> > > last year [2], which concluded with suggestion that =E2=80=9Ca reader=
/writer
> > > semaphore could be put into the VMA itself; that would have the effec=
t of
> > > using the VMA as a sort of range lock. There would still be contentio=
n at
> > > the VMA level, but it would be an improvement.=E2=80=9D This patchset=
 implements
> > > this suggested approach.
> >
> > I think I'll await reviewer/tester input for a while.

Sure, I don't expect the review to be very quick considering the
complexity, however I would appreciate any testing that can be done.

> >
> > > The patchset implements per-VMA locking only for anonymous pages whic=
h
> > > are not in swap and avoids userfaultfs as their implementation is mor=
e
> > > complex. Additional support for file-back page faults, swapped and us=
er
> > > pages can be added incrementally.
> >
> > This is a significant risk.  How can we be confident that these as yet
> > unimplemented parts are implementable and that the result will be good?
>
> They don't need to be implementable for this patchset to be evaluated
> on its own terms.  This patchset improves scalability for anon pages
> without making file/swap/uffd pages worse (or if it does, I haven't
> seen the benchmarks to prove it).

Making it work for all kinds of page faults would require much more
time. So, this incremental approach, when we tackle the mmap_lock
scalability problem part-by-part seems more doable. Even with
anonymous-only support, the patch shows considerable improvements.
Therefore I would argue that the patch is viable even if it does not
support the above-mentioned cases.

>
> That said, I'm confident that I have a good handle on how to make
> file-backed page faults work under RCU.

Looking forward to collaborating on that!
Thanks,
Suren.