[PATCH v4 2/7] mm: multi-gen LRU: Have secondary MMUs participate in aging

[Yu Zhao]

On Wed, May 29, 2024 at 12:05 PM James Houghton <jthoughton at google.com> wrote:
>
> Secondary MMUs are currently consulted for access/age information at
> eviction time, but before then, we don't get accurate age information.
> That is, pages that are mostly accessed through a secondary MMU (like
> guest memory, used by KVM) will always just proceed down to the oldest
> generation, and then at eviction time, if KVM reports the page to be
> young, the page will be activated/promoted back to the youngest
> generation.

Correct, and as I explained offline, this is the only reasonable
behavior if we can't locklessly walk secondary MMUs.

Just for the record, the (crude) analogy I used was:
Imagine a large room with many bills ($1, $5, $10, ...) on the floor,
but you are only allowed to pick up 10 of them (and put them in your
pocket). A smart move would be to survey the room *first and then*
pick up the largest ones. But if you are carrying a 500 lbs backpack,
you would just want to pick up whichever that's in front of you rather
than walk the entire room.

MGLRU should only scan (or lookaround) secondary MMUs if it can be
done lockless. Otherwise, it should just fall back to the existing
approach, which existed in previous versions but is removed in this
version.

> Do not do look around if there is a secondary MMU we have to interact
> with.
>
> The added feature bit (0x8), if disabled, will make MGLRU behave as if
> there are no secondary MMUs subscribed to MMU notifiers except at
> eviction time.
>
> Suggested-by: Yu Zhao <yuzhao at google.com>
> Signed-off-by: James Houghton <jthoughton at google.com>

This is not what I suggested, and it would have been done in the first
place if it hadn't regressed the non-lockless case.

NAK.