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From: Yu Zhao <yuzhao@google.com>
Date: Mon, 10 Jun 2024 23:33:34 -0600
Message-ID: <CAOUHufYGqbd45shZkGCpqeTV9wcBDUoo3iw1SKiDeFLmrP0+=w@mail.gmail.com>
Subject: Re: [PATCH v5 4/9] mm: Add test_clear_young_fast_only MMU notifier
To: James Houghton <jthoughton@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Paolo Bonzini <pbonzini@redhat.com>, 
	Ankit Agrawal <ankita@nvidia.com>, Axel Rasmussen <axelrasmussen@google.com>, 
	Catalin Marinas <catalin.marinas@arm.com>, David Matlack <dmatlack@google.com>, 
	David Rientjes <rientjes@google.com>, James Morse <james.morse@arm.com>, 
	Jonathan Corbet <corbet@lwn.net>, Marc Zyngier <maz@kernel.org>, Oliver Upton <oliver.upton@linux.dev>, 
	Raghavendra Rao Ananta <rananta@google.com>, Ryan Roberts <ryan.roberts@arm.com>, 
	Sean Christopherson <seanjc@google.com>, Shaoqin Huang <shahuang@redhat.com>, 
	Suzuki K Poulose <suzuki.poulose@arm.com>, Wei Xu <weixugc@google.com>, 
	Will Deacon <will@kernel.org>, Zenghui Yu <yuzenghui@huawei.com>, kvmarm@lists.linux.dev, 
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On Mon, Jun 10, 2024 at 6:22=E2=80=AFPM James Houghton <jthoughton@google.c=
om> wrote:
>
> This new notifier is for multi-gen LRU specifically

Let me call it out before others do: we can't be this self-serving.

> as it wants to be
> able to get and clear age information from secondary MMUs only if it can
> be done "fast".
>
> By having this notifier specifically created for MGLRU, what "fast"
> means comes down to what is "fast" enough to improve MGLRU's ability to
> reclaim most of the time.
>
> Signed-off-by: James Houghton <jthoughton@google.com>

If we'd like this to pass other MM reviewers, especially the MMU
notifier maintainers, we'd need to design a generic API that can
benefit all the *existing* users: idle page tracking [1], DAMON [2]
and MGLRU.

Also I personally prefer to extend the existing callbacks by adding
new parameters, and on top of that, I'd try to consolidate the
existing callbacks -- it'd be less of a hard sell if my changes result
in less code, not more.

(v2 did all these, btw.)

[1] https://docs.kernel.org/admin-guide/mm/idle_page_tracking.html
[2] https://www.kernel.org/doc/html/latest/mm/damon/index.html