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Date:   Thu, 16 Feb 2023 18:14:59 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net, dave@...olabs.net,
        liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, paulmck@...nel.org, mingo@...hat.com,
        will@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        peterjung1337@...il.com, rientjes@...gle.com, chriscli@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        rppt@...nel.org, jannh@...gle.com, shakeelb@...gle.com,
        tatashin@...gle.com, edumazet@...gle.com, gthelen@...gle.com,
        gurua@...gle.com, arjunroy@...gle.com, soheil@...gle.com,
        leewalsh@...gle.com, posk@...gle.com,
        michalechner92@...glemail.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is
 not yet set

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

>
> 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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