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From: "Nicholas Piggin" <npiggin@gmail.com>
To: "Yu Zhao" <yuzhao@google.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Alistair Popple" <apopple@nvidia.com>,
	"Anup Patel" <anup@brainfault.org>,
	"Ben Gardon" <bgardon@google.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Chao Peng" <chao.p.peng@linux.intel.com>,
	"Christophe Leroy" <christophe.leroy@csgroup.eu>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Fabiano Rosas" <farosas@linux.ibm.com>,
	"Gaosheng Cui" <cuigaosheng1@huawei.com>,
	"Gavin Shan" <gshan@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"James Morse" <james.morse@arm.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Marc Zyngier" <maz@kernel.org>,
	"Masami Hiramatsu" <mhiramat@kernel.org>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Michael Larabel" <michael@michaellarabel.com>,
	"Mike Rapoport" <rppt@kernel.org>,
	"Oliver Upton" <oliver.upton@linux.dev>,
	"Paul Mackerras" <paulus@ozlabs.org>,
	"Peter Xu" <peterx@redhat.com>,
	"Sean Christopherson" <seanjc@google.com>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Suzuki K Poulose" <suzuki.poulose@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Thomas Huth" <thuth@redhat.com>, "Will Deacon" <will@kernel.org>,
	"Zenghui Yu" <yuzenghui@huawei.com>, <kvmarm@lists.linux.dev>,
	<kvm@vger.kernel.org>, <linux-arm-kernel@lists.infradead.org>,
	<linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linux-mm@kvack.org>, <linuxppc-dev@lists.ozlabs.org>,
	<linux-trace-kernel@vger.kernel.org>, <x86@kernel.org>,
	<linux-mm@google.com>
Subject: Re: [PATCH mm-unstable v2 06/10] kvm/powerpc: make radix page tables RCU safe
Date: Tue, 20 Jun 2023 20:49:56 +1000	[thread overview]
Message-ID: <CTHF42QOSYR0.1Y16SFUF11F3X@wheely> (raw)
In-Reply-To: <CAOUHufY8egkNrxQwd6ms4j6ziyUW5uDjD=yhkxHLqAAOGB4Ccw@mail.gmail.com>

On Tue Jun 20, 2023 at 6:00 PM AEST, Yu Zhao wrote:
> On Tue, Jun 20, 2023 at 12:33 AM Nicholas Piggin <npiggin@gmail.com> 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 <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?
>
> 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

  reply	other threads:[~2023-06-20 10:50 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-26 23:44 [PATCH mm-unstable v2 00/10] mm/kvm: locklessly clear the accessed bit Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 01/10] mm/kvm: add mmu_notifier_ops->test_clear_young() Yu Zhao
2023-06-06  8:34   ` Tzung-Bi Shih
2023-06-09  1:00     ` Yu Zhao
     [not found]   ` <ZHedMX470b7EMwbe@ziepe.ca>
2023-06-09  9:04     ` Paolo Bonzini
2023-06-15 17:42   ` Sean Christopherson
2023-06-20  7:30   ` Nicholas Piggin
2023-05-26 23:44 ` [PATCH mm-unstable v2 02/10] mm/kvm: use mmu_notifier_ops->test_clear_young() Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 03/10] kvm/arm64: export stage2_try_set_pte() and macros Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe Yu Zhao
2023-05-27 18:08   ` Oliver Upton
2023-05-27 20:13     ` Yu Zhao
2023-05-30 19:37       ` Oliver Upton
2023-05-30 20:06         ` Yu Zhao
     [not found]           ` <ZHef0VsZvZ1Vnz0u@linux.dev>
2023-05-31 23:10             ` Yu Zhao
2023-05-31 23:22               ` Oliver Upton
2023-05-31 23:41                 ` Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 05/10] kvm/arm64: add kvm_arch_test_clear_young() Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 06/10] kvm/powerpc: make radix page tables RCU safe Yu Zhao
2023-06-20  6:32   ` Nicholas Piggin
2023-06-20  8:00     ` Yu Zhao
2023-06-20 10:49       ` Nicholas Piggin [this message]
2023-05-26 23:44 ` [PATCH mm-unstable v2 07/10] kvm/powerpc: add kvm_arch_test_clear_young() Yu Zhao
2023-06-20  7:47   ` Nicholas Piggin
2023-06-21  0:38     ` Yu Zhao
2023-06-21  2:51       ` Nicholas Piggin
2023-05-26 23:44 ` [PATCH mm-unstable v2 08/10] kvm/x86: move tdp_mmu_enabled and shadow_accessed_mask Yu Zhao
2023-06-15 16:59   ` Sean Christopherson
2023-05-26 23:44 ` [PATCH mm-unstable v2 09/10] kvm/x86: add kvm_arch_test_clear_young() Yu Zhao
2023-06-09  9:06   ` Paolo Bonzini
2023-06-15 18:26   ` Sean Christopherson
2023-05-26 23:44 ` [PATCH mm-unstable v2 10/10] mm: multi-gen LRU: use mmu_notifier_test_clear_young() Yu Zhao
2023-06-09  0:59 ` kvm/arm64: Spark benchmark Yu Zhao
2023-06-09 13:04   ` Marc Zyngier
2023-06-18 20:11     ` Yu Zhao
2023-06-09  0:59 ` kvm/powerpc: memcached benchmark Yu Zhao
2023-06-09  0:59 ` kvm/x86: multichase benchmark Yu Zhao
2023-06-18 19:19   ` Yu Zhao
2023-06-09  9:07 ` [PATCH mm-unstable v2 00/10] mm/kvm: locklessly clear the accessed bit Paolo Bonzini
2023-06-20  2:19   ` Yu Zhao

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