linux-kernel - Re: [PATCH] mm: Always sanity check anon

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Date: Fri, 26 Apr 2024 16:50:19 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Peter Xu <peterx@...hat.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lokesh Gidra <lokeshgidra@...gle.com>,
	Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

On Fri, Apr 26, 2024 at 08:32:06AM -0700, Suren Baghdasaryan wrote:
> On Fri, Apr 26, 2024 at 8:28 AM Matthew Wilcox <willy@...radead.org> wrote:
> > > I think the only path in either do_anonymous_page() or
> > > do_huge_pmd_anonymous_page() that skips calling anon_vma_prepare() is
> > > the "Use the zero-page for reads" here:
> > > https://elixir.bootlin.com/linux/latest/source/mm/memory.c#L4265. I
> > > didn't look into this particular benchmark yet but will try it out
> > > once I have some time to benchmark your change.
> >
> > Yes, Liam and I had just brainstormed that as being a plausible
> > explanation too.  I don't know how frequent it is to use anon memory
> > read-only.  Presumably it must happen often enough that we've bothered
> > to implement the zero-page optimisation.  But probably not nearly as
> > often as this benchmark makes it happen ;-)
> 
> I also wonder if some of this improvement can be attributed to the
> last patch in your series
> (https://lore.kernel.org/all/20240426144506.1290619-5-willy@infradead.org/).
> I assume it was included in the 0day testing?

Patch 4 was where I expected to see the improvement too.  But I think
what's going on is that this benchmark evaded all our hard work on
page fault scalability.  Because it's read-only, it never assigned an
anon_vma and so all its page faults fell back to taking the mmap_sem.
So patch 4 will have no effect on this benchmark.

The report from 0day is pretty clear they bisected the performance
improvement to patch 2.