[PATCH v5 4/9] mm: Add test_clear_young_fast_only MMU notifier

[Yu Zhao]

On Mon, Jun 10, 2024 at 6:22 PM James Houghton <jthoughton at google.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 at 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