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

[Yu Zhao]

On Fri, May 31, 2024 at 1:24 AM Oliver Upton <oliver.upton at linux.dev> wrote:
>
> On Wed, May 29, 2024 at 03:03:21PM -0600, Yu Zhao wrote:
> > 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.
>
> Grabbing the MMU lock for write to scan sucks, no argument there. But
> can you please be specific about the impact of read lock v. RCU in the
> case of arm64? I had asked about this before and you never replied.
>
> My concern remains that adding support for software table walkers
> outside of the MMU lock entirely requires more work than just deferring
> the deallocation to an RCU callback. Walkers that previously assumed
> 'exclusive' access while holding the MMU lock for write must now cope
> with volatile PTEs.
>
> Yes, this problem already exists when hardware sets the AF, but the
> lock-free walker implementation needs to be generic so it can be applied
> for other PTE bits.

Direct reclaim is multi-threaded and each reclaimer can take the mmu
lock for read (testing the A-bit) or write (unmapping before paging
out) on arm64. The fundamental problem of using the readers-writer
lock in this case is priority inversion: the readers have lower
priority than the writers, so ideally, we don't want the readers to
block the writers at all.

Using my previous (crude) analogy: puting the bill right in front of
you (the writers) profits immediately whereas searching for the
largest bill (the readers) can be futile.

As I said earlier, I prefer we drop the arm64 support for now, but I
will not object to taking the mmu lock for read when clearing the
A-bit, as long as we fully understand the problem here and document it
clearly.