Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe

Yu Zhao
2023-05-31T23:42:24Z

On Wed, May 31, 2023 at 5:23 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> On Wed, May 31, 2023 at 05:10:52PM -0600, Yu Zhao wrote:
> > On Wed, May 31, 2023 at 1:28 PM Oliver Upton <oliver.upton@linux.dev> wrote:
> > > On Tue, May 30, 2023 at 02:06:55PM -0600, Yu Zhao wrote:
> > > > On Tue, May 30, 2023 at 1:37 PM Oliver Upton <oliver.upton@linux.dev> wrote:
> > > > > As it is currently implemented, yes. But, there's potential to fast-path
> > > > > the implementation by checking page_count() before starting the walk.
> > > >
> > > > Do you mind posting another patch? I'd be happy to ack it, as well as
> > > > the one you suggested above.
> > >
> > > I'd rather not take such a patch independent of the test_clear_young
> > > series if you're OK with that. Do you mind implementing something
> > > similar to the above patch w/ the proposed optimization if you need it?
> >
> > No worries. I can take the above together with the following, which
> > would form a new series with its own merits, since apparently you
> > think the !AF case is important.
>
> Sorry if my suggestion was unclear.
>
> I thought we were talking about ->free_removed_table() being called from
> the stage-2 unmap path

Yes, we were, or in general, about how to make KVM PTs RCU safe for ARM.

So I'm thinking about taking 1) your patch above, 2) what I just
suggested and 3) what you suggested below to form a mini series, which
could land indepently and would make my job here easier.

> in which case we wind up unnecessarily visiting
> PTEs on a table known to be empty. You could fast-path that by only
> initiating a walk if page_count() > 1:

Yes, this is what I meant.

> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 95dae02ccc2e..766563dc465c 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1331,7 +1331,8 @@ void kvm_pgtable_stage2_free_removed(struct kvm_pgtable_mm_ops *mm_ops, void *pg
> .end = kvm_granule_size(level),
> };
>
> - WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
> + if (mm_ops->page_count(pgtable) > 1)
> + WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
>
> WARN_ON(mm_ops->page_count(pgtable) != 1);
> mm_ops->put_page(pgtable);
>
>
> A lock-free access fault walker is interesting, but in my testing it hasn't
> led to any significant improvements over acquiring the MMU lock for
> read. Because of that I hadn't bothered with posting the series upstream.

It's hard to measure but we have perf benchmarks on ChromeOS which should help.