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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 11, 2024, James Houghton wrote:
> On Tue, Jun 11, 2024 at 12:42 PM Sean Christopherson <seanjc@google.com> wrote:
> > --
> > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> > index 7b77ad6cf833..07872ae00fa6 100644
> > --- a/mm/mmu_notifier.c
> > +++ b/mm/mmu_notifier.c
> > @@ -384,7 +384,8 @@ int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
> >
> >  int __mmu_notifier_clear_young(struct mm_struct *mm,
> >                                unsigned long start,
> > -                              unsigned long end)
> > +                              unsigned long end,
> > +                              bool fast_only)
> >  {
> >         struct mmu_notifier *subscription;
> >         int young = 0, id;
> > @@ -393,9 +394,12 @@ int __mmu_notifier_clear_young(struct mm_struct *mm,
> >         hlist_for_each_entry_rcu(subscription,
> >                                  &mm->notifier_subscriptions->list, hlist,
> >                                  srcu_read_lock_held(&srcu)) {
> > -               if (subscription->ops->clear_young)
> > -                       young |= subscription->ops->clear_young(subscription,
> > -                                                               mm, start, end);
> > +               if (!subscription->ops->clear_young ||
> > +                   fast_only && !subscription->ops->has_fast_aging)
> > +                       continue;
> > +
> > +               young |= subscription->ops->clear_young(subscription,
> > +                                                       mm, start, end);
> 
> KVM changing has_fast_aging dynamically would be slow, wouldn't it?

No, it could/would be done quite quickly.  But, I'm not suggesting has_fast_aging
be dynamic, i.e. it's not an "all aging is guaranteed to be fast", it's a "this
MMU _can_ do fast aging".  It's a bit fuzzy/weird mostly because KVM can essentially
have multiple secondary MMUs wired up to the same mmu_notifier.

> I feel like it's simpler to just pass in fast_only into `clear_young` itself
> (and this is how I interpreted what you wrote above anyway).

Eh, maybe?  A "has_fast_aging" flag is more robust in the sense that it requires
secondary MMUs to opt-in, i.e. all secondary MMUs will be considered "slow" by
default.
 
It's somewhat of a moot point because KVM is the only secondary MMU that implements
.clear_young() and .test_young() (which I keep forgetting), and that seems unlikely
to change.

A flag would also avoid an indirect call and thus a RETPOLINE when CONFIG_RETPOLINE=y,
i.e. would be a minor optimization when KVM doesn't suppport fast aging.  But that's
probably a pretty unlikely combination, so it's probably not a valid argument.

So, I guess I don't have a strong opinion?

> > Double ugh.  Peeking ahead at the "failure" code, NAK to adding
> > kvm_arch_young_notifier_likely_fast for all the same reasons I objected to
> > kvm_arch_has_test_clear_young() in v1.  Please stop trying to do anything like
> > that, I will NAK each every attempt to have core mm/ code call directly into KVM.
> 
> Sorry to make you repeat yourself; I'll leave it out of v6. I don't
> like it either, but I wasn't sure how important it was to avoid
> calling into unnecessary notifiers if the TDP MMU were completely
> disabled.

If it's important, e.g. for performance, then the mmu_notifier should have a flag
so that the behavior doesn't assume a KVM backend.   Hence my has_fast_aging
suggestion.

> > Anyways, back to this code, before we spin another version, we need to agree on
> > exactly what behavior we want out of secondary MMUs.  Because to me, the behavior
> > proposed in this version doesn't make any sense.
> >
> > Signalling failure because KVM _might_ have relevant aging information in SPTEs
> > that require taking kvm->mmu_lock is a terrible tradeoff.  And for the test_young
> > case, it's flat out wrong, e.g. if a page is marked Accessed in the TDP MMU, then
> > KVM should return "young", not "failed".
> 
> Sorry for this oversight. What about something like:
> 
> 1. test (and maybe clear) A bits on TDP MMU
> 2. If accessed && !should_clear: return (fast)
> 3. if (fast_only): return (fast)
> 4. If !(must check shadow MMU): return (fast)
> 5. test (and maybe clear) A bits in shadow MMU
> 6. return (slow)

I don't understand where the "must check shadow MMU" in #4 comes from.  I also
don't think it's necessary; see below.
 
> Some of this reordering (and maybe a change from
> kvm_shadow_root_allocated() to checking indirect_shadow_pages or
> something else) can be done in its own patch.
>
> > So rather than failing the fast aging, I think what we want is to know if an
> > mmu_notifier found a young SPTE during a fast lookup.  E.g. something like this
> > in KVM, where using kvm_has_shadow_mmu_sptes() instead of kvm_memslots_have_rmaps()
> > is an optional optimization to avoid taking mmu_lock for write in paths where a
> > (very rare) false negative is acceptable.
> >
> >   static bool kvm_has_shadow_mmu_sptes(struct kvm *kvm)
> >   {
> >         return !tdp_mmu_enabled || READ_ONCE(kvm->arch.indirect_shadow_pages);
> >   }
> >
> >   static int __kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range,
> >                          bool fast_only)
> >   {
> >         int young = 0;
> >
> >         if (!fast_only && kvm_has_shadow_mmu_sptes(kvm)) {
> >                 write_lock(&kvm->mmu_lock);
> >                 young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
> >                 write_unlock(&kvm->mmu_lock);
> >         }
> >
> >         if (tdp_mmu_enabled && kvm_tdp_mmu_age_gfn_range(kvm, range))
> >                 young = 1 | MMU_NOTIFY_WAS_FAST;
> 
> I don't think this line is quite right. We might set
> MMU_NOTIFY_WAS_FAST even when we took the mmu_lock. I understand what
> you mean though, thanks.

The name sucks, but I believe the logic is correct.  As posted here in v5, the
MGRLU code wants to age both fast _and_ slow MMUs.  AIUI, the intent is to always
get aging information, but only look around at other PTEs if it can be done fast.

	if (should_walk_secondary_mmu()) {
		notifier_result =
			mmu_notifier_test_clear_young_fast_only(
					vma->vm_mm, addr, addr + PAGE_SIZE,
					/*clear=*/true);
	}

	if (notifier_result & MMU_NOTIFIER_FAST_FAILED)
		secondary_young = mmu_notifier_clear_young(vma->vm_mm, addr,
							   addr + PAGE_SIZE);
	else {
		secondary_young = notifier_result & MMU_NOTIFIER_FAST_YOUNG;
		notifier_was_fast = true;
	}

The change, relative to v5, that I am proposing is that MGLRU looks around if
the page was young in _a_ "fast" secondary MMU, whereas v5 looks around if and
only if _all_ secondary MMUs are fast.

In other words, if a fast MMU had a young SPTE, look around _that_ MMU, via the
fast_only flag.