Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal

[Suren Baghdasaryan]

On Fri, Sep 2, 2022 at 12:43 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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 “a reader/writer semaphore could be put into the 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.” 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=1XjZ9FUUDiyhfhjZ54NA@xxxxxxxxxxxxxx/
[2] https://www.phoronix.com/scan.php?page=news_item&px=Google-Fibers-Toward-Open

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