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

Oliver Upton
2023-05-30T19:37:42Z

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.

> 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/

--
Thanks,
Oliver