From: Will Deacon <will@kernel.org>
To: Palmer Dabbelt <palmer@sifive.com>
Cc: Justin.He@arm.com, Catalin.Marinas@arm.com, Mark.Rutland@arm.com,
James.Morse@arm.com, maz@kernel.org, willy@infradead.org,
kirill.shutemov@linux.intel.com,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
punitagrawal@gmail.com, tglx@linutronix.de,
akpm@linux-foundation.org, hejianet@gmail.com, Kaly.Xin@arm.com,
nd@arm.com
Subject: Re: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared
Date: Thu, 17 Oct 2019 00:46:08 +0100 [thread overview]
Message-ID: <20191016234607.626nzv5kf5fgz25x@willie-the-truck> (raw)
In-Reply-To: <mhng-dd251518-8ac0-40fa-9f62-20715d9ba906@palmer-si-x1e>
Hey Palmer,
On Wed, Oct 16, 2019 at 04:21:59PM -0700, Palmer Dabbelt wrote:
> On Tue, 08 Oct 2019 05:39:44 PDT (-0700), will@kernel.org wrote:
> > On Tue, Oct 08, 2019 at 02:19:05AM +0000, Justin He (Arm Technology China) wrote:
> > > > On Mon, Sep 30, 2019 at 09:57:40AM +0800, Jia He wrote:
> > > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > > index b1ca51a079f2..1f56b0118ef5 100644
> > > > > --- a/mm/memory.c
> > > > > +++ b/mm/memory.c
> > > > > @@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
> > > > > 2;
> > > > > #endif
> > > > >
> > > > > +#ifndef arch_faults_on_old_pte
> > > > > +static inline bool arch_faults_on_old_pte(void)
> > > > > +{
> > > > > + return false;
> > > > > +}
> > > > > +#endif
> > > >
> > > > Kirill has acked this, so I'm happy to take the patch as-is, however isn't
> > > > it the case that /most/ architectures will want to return true for
> > > > arch_faults_on_old_pte()? In which case, wouldn't it make more sense for
> > > > that to be the default, and have x86 and arm64 provide an override? For
> > > > example, aren't most architectures still going to hit the double fault
> > > > scenario even with your patch applied?
> > >
> > > No, after applying my patch series, only those architectures which don't provide
> > > setting access flag by hardware AND don't implement their arch_faults_on_old_pte
> > > will hit the double page fault.
> > >
> > > The meaning of true for arch_faults_on_old_pte() is "this arch doesn't have the hardware
> > > setting access flag way, it might cause page fault on an old pte"
> > > I don't want to change other architectures' default behavior here. So by default,
> > > arch_faults_on_old_pte() is false.
> >
> > ...and my complaint is that this is the majority of supported architectures,
> > so you're fixing something for arm64 which also affects arm, powerpc,
> > alpha, mips, riscv, ...
> >
> > Chances are, they won't even realise they need to implement
> > arch_faults_on_old_pte() until somebody runs into the double fault and
> > wastes lots of time debugging it before they spot your patch.
>
> If I understand the semantics correctly, we should have this set to true. I
> don't have any context here, but we've got
>
> /*
> * The kernel assumes that TLBs don't cache invalid
> * entries, but in RISC-V, SFENCE.VMA specifies an
> * ordering constraint, not a cache flush; it is
> * necessary even after writing invalid entries.
> */
> local_flush_tlb_page(addr);
>
> in do_page_fault().
Ok, although I think this is really about whether or not your hardware can
make a pte young when accessed, or whether you take a fault and do it
by updating the pte explicitly.
v12 of the patches did change the default, so you should be "safe" with
those either way:
http://lists.infradead.org/pipermail/linux-arm-kernel/2019-October/686030.html
Will
prev parent reply other threads:[~2019-10-16 23:46 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-30 1:57 [PATCH v10 0/3] fix double page fault on arm64 Jia He
2019-09-30 1:57 ` [PATCH v10 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af() Jia He
2019-10-01 12:54 ` Will Deacon
2019-10-01 13:18 ` Marc Zyngier
2019-10-08 1:12 ` Justin He (Arm Technology China)
2019-10-08 15:32 ` Suzuki K Poulose
2019-10-09 6:29 ` Jia He
2019-09-30 1:57 ` [PATCH v10 2/3] arm64: mm: implement arch_faults_on_old_pte() on arm64 Jia He
2019-10-01 12:50 ` Will Deacon
2019-10-01 13:32 ` Marc Zyngier
2019-10-08 1:55 ` Justin He (Arm Technology China)
2019-10-08 2:30 ` Justin He (Arm Technology China)
2019-10-08 7:46 ` Marc Zyngier
2019-09-30 1:57 ` [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared Jia He
2019-10-01 12:54 ` Will Deacon
2019-10-08 2:19 ` Justin He (Arm Technology China)
2019-10-08 12:39 ` Will Deacon
2019-10-08 12:58 ` Justin He (Arm Technology China)
2019-10-08 14:32 ` Kirill A. Shutemov
2019-10-16 23:21 ` Palmer Dabbelt
2019-10-16 23:46 ` Will Deacon [this message]
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