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Date:   Wed, 16 Oct 2019 16:21:59 -0700 (PDT)
Subject:     Re: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared
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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:
>> > -----Original Message-----
>> > From: Will Deacon <will@kernel.org>
>> > Sent: 2019年10月1日 20:54
>> > To: Justin He (Arm Technology China) <Justin.He@arm.com>
>> > Cc: Catalin Marinas <Catalin.Marinas@arm.com>; Mark Rutland
>> > <Mark.Rutland@arm.com>; James Morse <James.Morse@arm.com>; Marc
>> > Zyngier <maz@kernel.org>; Matthew Wilcox <willy@infradead.org>; Kirill A.
>> > Shutemov <kirill.shutemov@linux.intel.com>; linux-arm-
>> > kernel@lists.infradead.org; linux-kernel@vger.kernel.org; linux-
>> > mm@kvack.org; Punit Agrawal <punitagrawal@gmail.com>; Thomas
>> > Gleixner <tglx@linutronix.de>; Andrew Morton <akpm@linux-
>> > foundation.org>; hejianet@gmail.com; Kaly Xin (Arm Technology China)
>> > <Kaly.Xin@arm.com>
>> > Subject: Re: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF
>> > is cleared
>> >
>> > 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().

>> Btw, currently I only observed this double pagefault on arm64's guest
>> (host is ThunderX2).  On X86 guest (host is Intel(R) Core(TM) i7-4790 CPU
>> @ 3.60GHz ), there is no such double pagefault. It has the similar setting
>> access flag way by hardware.
>
> Right, and that's why I'm not concerned about x86 for this problem.
>
> Will