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

[Oliver Upton]

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:
> >
> > Hi Yu,
> >
> > On Sat, May 27, 2023 at 02:13:07PM -0600, Yu Zhao wrote:
> > > On Sat, May 27, 2023 at 12:08 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > > index 3d61bd3e591d..bfbebdcb4ef0 100644
> > > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > > @@ -1019,7 +1019,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
> > > >                                                kvm_granule_size(ctx->level));
> > > >
> > > >         if (childp)
> > > > -               mm_ops->put_page(childp);
> > > > +               mm_ops->free_removed_table(childp, ctx->level);
> > >
> > > Thanks, Oliver.
> > >
> > > A couple of things I haven't had the chance to verify -- I'm hoping
> > > you could help clarify:
> > > 1. For unmapping, with free_removed_table(), wouldn't we have to look
> > > into the table we know it's empty unnecessarily?
> >
> > 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?

-- 
Thanks,
Oliver