linux-kernel - Re: [PATCH v5 4/9] mm: Add test_clear_young_fast

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Date: Mon, 10 Jun 2024 23:33:34 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Ankit Agrawal <ankita@...dia.com>, Axel Rasmussen <axelrasmussen@...gle.com>, 
	Catalin Marinas <catalin.marinas@....com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, James Morse <james.morse@....com>, 
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Raghavendra Rao Ananta <rananta@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	Sean Christopherson <seanjc@...gle.com>, Shaoqin Huang <shahuang@...hat.com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Wei Xu <weixugc@...gle.com>, 
	Will Deacon <will@...nel.org>, Zenghui Yu <yuzenghui@...wei.com>, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 4/9] mm: Add test_clear_young_fast_only MMU notifier

On Mon, Jun 10, 2024 at 6:22 PM James Houghton <jthoughton@...gle.com> 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@...gle.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