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

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Message-ID: <CAHVum0dpHrW3cDX9FCcd5wTsetFxzWP0B6WL3uXnqmwrVJnGcw@mail.gmail.com>
Date: Mon, 30 Sep 2024 09:42:32 -0700
From: Vipin Sharma <vipinsh@...gle.com>
To: David Matlack <dmatlack@...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 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.