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

[Yu Zhao <yuzhao@google.com>]

On Tue, May 30, 2023 at 1:37 PM Oliver Upton <oliver.upton@linux.dev> 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@linux.dev> 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.

> > 2. For remapping and unmapping, how does free_removed_table() put the
> > final refcnt on the table passed in? (Previously we had
> > put_page(childp) in stage2_map_walk_table_post(). So I'm assuming we'd
> > have to do something equivalent with free_removed_table().)
>
> Heh, that's a bug, and an embarrassing one at that!
>
> Sent out a fix for that, since it would appear we leak memory on
> table->block transitions. PTAL if you have a chance.
>
> https://lore.kernel.org/all/20230530193213.1663411-1-oliver.upton@linux.dev/

Awesome.