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

Oliver Upton
2023-05-27T18:15:24Z

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);

return 0;
}

--
Thanks,
Oliver