linux-kernel - Re: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE

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Date:   Fri, 18 Oct 2019 13:38:08 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     will@...nel.org
CC:     Justin.He@....com, Catalin.Marinas@....com, Mark.Rutland@....com,
        James.Morse@....com, maz@...nel.org, willy@...radead.org,
        kirill.shutemov@...ux.intel.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, punitagrawal@...il.com, tglx@...utronix.de,
        akpm@...ux-foundation.org, hejianet@...il.com, Kaly.Xin@....com,
        nd@....com
Subject:     Re: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

On Wed, 16 Oct 2019 16:46:08 PDT (-0700), will@...nel.org wrote:
> 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@...nel.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

OK, that fence is because we allow invalid translations to be cached, which is a 
completely different issue.

RISC-V implementations are allowed to have software managed accessed/dirty 
bits.  For some reason I thought we were relying on the firmware to handle 
this, but I can't actually find the code so I might be crazy.  Wherever it's 
done, there's no spec enforcing it so we should leave this true on RISC-V.

Thanks!

> Will