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

From: "Nicholas Piggin" <[email-masked]>
Date: Tue, 20 Jun 2023 20:49:56 +1000

On Tue Jun 20, 2023 at 6:00 PM AEST, Yu Zhao wrote:
> On Tue, Jun 20, 2023 at 12:33 AM Nicholas Piggin <[email-masked]> wrote:
> >
> > 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.
>
> Thanks. All above points taken.
>
> > > 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?
>
> Correct.
>
> > > Signed-off-by: Yu Zhao <[email-masked]>
> > > ---
> > >  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?
>
> We don't. The use of the arch/powerpc allocator for PUD tables seems
> appropriate to me because, unlike PMD/PTE tables, we never free PUD
> tables during the lifetime of a VM:

Ah you're right, the pud_free only comes from the double alloc case
so it's never visible to concurrent threads.

> * We don't free PUD/PMD/PTE tables when they become empty, i.e., not
> mapping any pages but still attached. (We could in theory, as
> x86/aarch64 do.)

We may try to do that at some point, but that's not related to your
patch for now so no worries.

Thanks,
Nick