On Mon, Jun 10, 2024 at 10:34 PM Yu Zhao [off-list ref] wrote:

On Mon, Jun 10, 2024 at 6:22 PM James Houghton [off-list ref] wrote:

quoted

This new notifier is for multi-gen LRU specifically

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

quoted

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 <redacted>

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.)

I think consolidating the callbacks is cleanest, like you had it in
v2. I really wasn't sure about this change honestly, but it was my
attempt to incorporate feedback like this[3] from v4. I'll consolidate
the callbacks like you had in v2.

Instead of the bitmap like you had, I imagine we'll have some kind of
flags argument that has bits like MMU_NOTIFIER_YOUNG_CLEAR,
MMU_NOTIFIER_YOUNG_FAST_ONLY, and other ones as they come up. Does
that sound ok?

Do idle page tracking and DAMON need this new "fast-only" notifier? Or
do they benefit from a generic API in other ways? Sorry if I missed
this from some other mail.

I've got feedback saying that tying the definition of "fast" to MGLRU
specifically is helpful. So instead of MMU_NOTIFIER_YOUNG_FAST_ONLY,
maybe MMU_NOTIFIER_YOUNG_LRU_GEN_FAST to mean "do fast-for-MGLRU
notifier". It sounds like you'd prefer the more generic one.

Thanks for the feedback -- I don't want to keep this series lingering
on the list, so I'll try and get newer versions out sooner rather than
later.

[3]: https://lore.kernel.org/linux-mm/Zl5LqcusZ88QOGQY@google.com/ (local)

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

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