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

[Yu Zhao <yuzhao@google.com>]

On Sat, May 27, 2023 at 12:08 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> Yu,
>
> On Fri, May 26, 2023 at 05:44:29PM -0600, Yu Zhao wrote:
> > Stage2 page tables are currently not RCU safe against unmapping or VM
> > destruction. The previous mmu_notifier_ops members rely on
> > kvm->mmu_lock to synchronize with those operations.
> >
> > However, the new mmu_notifier_ops member test_clear_young() provides
> > a fast path that does not take kvm->mmu_lock. To implement
> > kvm_arch_test_clear_young() for that path, unmapped page tables need
> > to be freed by RCU and kvm_free_stage2_pgd() needs to be after
> > mmu_notifier_unregister().
> >
> > Remapping, specifically stage2_free_removed_table(), is already RCU
> > safe.
> >
> > Signed-off-by: Yu Zhao <yuzhao@google.com>
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h |  2 ++
> >  arch/arm64/kvm/arm.c                 |  1 +
> >  arch/arm64/kvm/hyp/pgtable.c         |  8 ++++++--
> >  arch/arm64/kvm/mmu.c                 | 17 ++++++++++++++++-
> >  4 files changed, 25 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index ff520598b62c..5cab52e3a35f 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -153,6 +153,7 @@ static inline bool kvm_level_supports_block_mapping(u32 level)
> >   * @put_page:                        Decrement the refcount on a page. When the
> >   *                           refcount reaches 0 the page is automatically
> >   *                           freed.
> > + * @put_page_rcu:            RCU variant of the above.
>
> You don't need to add yet another hook to implement this. I was working
> on lock-free walks in a separate context and arrived at the following:
>
> commit f82d264a37745e07ee28e116c336f139f681fd7f
> Author: Oliver Upton <oliver.upton@linux.dev>
> Date:   Mon May 1 08:53:37 2023 +0000
>
>     KVM: arm64: Consistently use free_removed_table() for stage-2
>
>     free_removed_table() is essential to the RCU-protected parallel walking
>     scheme, as behind the scenes the cleanup is deferred until an RCU grace
>     period. Nonetheless, the stage-2 unmap path calls put_page() directly,
>     which leads to table memory being freed inline with the table walk.
>
>     This is safe for the time being, as the stage-2 unmap walker is called
>     while holding the write lock. A future change to KVM will further relax
>     the locking mechanics around the stage-2 page tables to allow lock-free
>     walkers protected only by RCU. As such, switch to the RCU-safe mechanism
>     for freeing table memory.
>
>     Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
>
> 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?
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().)