Re: [PATCH mm-unstable v2 06/10] kvm/powerpc: make radix page tables RCU safe

Nicholas Piggin
2023-06-20T06:33:16Z

On Sat May 27, 2023 at 9:44 AM AEST, Yu Zhao wrote:
> KVM page tables are currently not RCU safe against remapping, i.e.,
> kvmppc_unmap_free_pmd_entry_table() et al. The previous

Minor nit but the "page table" is not RCU-safe against something. It
is RCU-freed, and therefore some algorithm that accesses it can have
the existence guarantee provided by RCU (usually there still needs
to be more to it).

> mmu_notifier_ops members rely on kvm->mmu_lock to synchronize with
> that operation.
>
> 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, orphan page tables need to
> be freed by RCU.

Short version: clear the referenced bit using RCU instead of MMU lock
to protect against page table freeing, and there is no problem with
clearing the bit in a table that has been freed.

Seems reasonable.

>
> Unmapping, specifically kvm_unmap_radix(), does not free page tables,
> hence not a concern.

Not sure if you really need to make the distinction about why the page
table is freed, we might free them via unmapping. The point is just
anything that frees them while there can be concurrent access, right?

>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
> arch/powerpc/kvm/book3s_64_mmu_radix.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> index 461307b89c3a..3b65b3b11041 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> @@ -1469,13 +1469,15 @@ int kvmppc_radix_init(void)
> {
> unsigned long size = sizeof(void *) << RADIX_PTE_INDEX_SIZE;
>
> - kvm_pte_cache = kmem_cache_create("kvm-pte", size, size, 0, pte_ctor);
> + kvm_pte_cache = kmem_cache_create("kvm-pte", size, size,
> + SLAB_TYPESAFE_BY_RCU, pte_ctor);
> if (!kvm_pte_cache)
> return -ENOMEM;
>
> size = sizeof(void *) << RADIX_PMD_INDEX_SIZE;
>
> - kvm_pmd_cache = kmem_cache_create("kvm-pmd", size, size, 0, pmd_ctor);
> + kvm_pmd_cache = kmem_cache_create("kvm-pmd", size, size,
> + SLAB_TYPESAFE_BY_RCU, pmd_ctor);
> if (!kvm_pmd_cache) {
> kmem_cache_destroy(kvm_pte_cache);
> return -ENOMEM;

KVM PPC HV radix PUD level page tables use the arch/powerpc allocators
(for some reason), which are not RCU freed. I think you need them too?
I wouldn't mind if the kvm pud table slab was moved in here instead of
shared.

Thanks,
Nick