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

[James Houghton]

On Mon, Jun 10, 2024 at 10:34 PM Yu Zhao <yuzhao at google.com> wrote:
>
> 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.)

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/

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