On Wed, May 31, 2023 at 5:23 PM Oliver Upton <oliver.upton@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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.
