RE: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

["Justin He (Arm Technology China)" <Justin.He@arm.com>]

Hi Will

> -----Original Message-----
> From: Will Deacon <will@kernel.org>
> Sent: 2019年10月8日 20:40
> 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>; nd <nd@arm.com>
> Subject: Re: [PATCH v10 3/3] mm: fix double page fault on arm64 if PTE_AF
> is cleared
> 
> 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, ...

So, IIUC, you suggested that:
1. by default, arch_faults_on_old_pte() return true
2. on X86, let arch_faults_on_old_pte() be overrided as returning false
3. on arm64, let it be as-is my patch set.
4. let other architectures decide the behavior. (But by default, it will set
pte_young)

I am ok with that if no objections from others.

@Kirill A. Shutemov Do you have any comments? Thanks
> 
> 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.

As to this point, I added a WARN_ON in patch 03 to speed up the debugging
process.

--
Cheers,
Justin (Jia He)



> 
> > 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