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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 2 Sep 2022 07:45:53 -0700
Message-ID: <CAJuCfpEreo5vKwKU1Qs1nXe50daGN-yaPz5v4BS7Y08no7sjiA@mail.gmail.com>
Subject: Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <michel@lespinasse.org>,
        Jerome Glisse <jglisse@google.com>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Mel Gorman <mgorman@suse.de>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Matthew Wilcox <willy@infradead.org>,
        "Liam R. Howlett" <liam.howlett@oracle.com>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Laurent Dufour <laurent.dufour@fr.ibm.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Song Liu <songliubraving@fb.com>, Peter Xu <peterx@redhat.com>,
        David Hildenbrand <david@redhat.com>, dhowells@redhat.com,
        Hugh Dickins <hughd@google.com>, bigeasy@linutronix.de,
        Kent Overstreet <kent.overstreet@linux.dev>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Minchan Kim <minchan@google.com>,
        kernel-team <kernel-team@android.com>,
        linux-mm <linux-mm@kvack.org>,
        linux-arm-kernel@lists.infradead.org,
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On Fri, Sep 2, 2022 at 12:43 AM Peter Zijlstra <peterz@infradead.org> wrote=
:
>
> On Thu, Sep 01, 2022 at 10:34:48AM -0700, Suren Baghdasaryan wrote:
> > This is a proof of concept for per-vma locks idea that was discussed
> > during SPF [1] discussion at LSF/MM this year [2], which concluded with
> > suggestion that =E2=80=9Ca reader/writer semaphore could be put into th=
e VMA
> > itself; that would have the effect of using the VMA as a sort of range
> > lock. There would still be contention at the VMA level, but it would be=
 an
> > improvement.=E2=80=9D This patchset implements this suggested approach.
>
> The whole reason I started the SPF thing waay back when was because one
> of the primary reporters at the time had very large VMAs and a per-vma
> lock wouldn't actually help anything at all.
>
> IIRC it was either scientific code initializing a huge matrix or a
> database with a giant table; I'm sure the archives have better memory
> than me.

Regardless of the initial intent, SPF happens to be very useful for
cases when we have multiple threads establishing some mappings
concurrently with page faults (see details at [1]). Android vendors
independently from each other were backporting your and Laurent's
patchset for years. I found internal reports of similar mmap_lock
contention issues in Google Fibers [2] and I suspect there are more
places this happens if people looked closer.

[1] https://lore.kernel.org/all/CAJuCfpE10y78SNPQ+LRY5EonDFhOG=3D1XjZ9FUUDi=
yhfhjZ54NA@mail.gmail.com/
[2] https://www.phoronix.com/scan.php?page=3Dnews_item&px=3DGoogle-Fibers-T=
oward-Open

>
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