linux-kernel - Re: [PATCH 0/2] KVM: x86/mmu: Repurpose MMU shrinker into page cache shrinker

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Message-ID: <CALzav=dmv8U8O6cFnyXNbUeTDrLs5Wj-ufYHNxDEJ14UZqrL9A@mail.gmail.com>
Date: Mon, 30 Sep 2024 09:50:40 -0700
From: David Matlack <dmatlack@...gle.com>
To: Vipin Sharma <vipinsh@...gle.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com, zhi.wang.linux@...il.com, 
	weijiang.yang@...el.com, mizhang@...gle.com, liangchen.linux@...il.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] KVM: x86/mmu: Repurpose MMU shrinker into page cache shrinker

On Mon, Sep 30, 2024 at 9:43 AM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> On Wed, Sep 25, 2024 at 4:51 PM David Matlack <dmatlack@...gle.com> wrote:
> >
> > On 2024-09-13 02:43 PM, Vipin Sharma wrote:
> > > This series is extracted out from the NUMA aware page table series[1].
> > > MMU shrinker changes were in patches 1 to 9 in the old series.
> >
> > I'm curious how you tested this series. Would it be posisble to write a
> > selftest to exercise KVM's shrinker interactions? I don't think it needs
> > to be anything fancy to be useful (e.g. just run a VM, trigger lots of
> > shrinking, and make sure nothing blows up).
>
> My testing was dropping caches (echo 2 > /proc/sys/vm/drop_caches) in
> background while running dirty_log_perf_test selftest multiple times.
> I added printk in shrink_count() and shrink_scan() to make sure pages
> are being reported and released.
>
> I can write a test which can spawn a thread to drop caches and a VM
> which touches all of its pages to generate page faults. Only downside
> is it will not detect if KVM MMU shrinker is being invoked, counting
> and freeing pages.
>
> >
> > There appears to be a debugfs interface which could be used to trigger
> > shrinking from a selftest.
> >
> > https://docs.kernel.org/admin-guide/mm/shrinker_debugfs.html
>
> This is interesting and it does what is needed to test KVM MMU
> shrinker. However, this needs CONFIG_DEBUG_FS and
> CONFIG_SHRINKER_DEBUG. I think using shrinker_debugfs will be better,
> selftest can just skip if it cannot find shrinker_debugfs files. One
> downside is that this test will not run if these configs are not
> enabled.
>
> Which one do you prefer? I am preferring shrinker_debugfs but
> concerned about its dependency on those two configs, not sure if it is
> okay to have this kind of dependency in a selftests.

Configs are there to be enabled. If shrinker_debugfs is the right way
to test this, I don't see any reason to shy away from using it.