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

[Kirill A. Shutemov]

On Tue, Oct 08, 2019 at 12:58:57PM +0000, Justin He (Arm Technology China) wrote:
> Hi Will
> 
> > -----Original Message-----
> > From: Will Deacon <will@xxxxxxxxxx>
> > Sent: 2019å10æ8æ 20:40
> > To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>
> > Cc: Catalin Marinas <Catalin.Marinas@xxxxxxx>; Mark Rutland
> > <Mark.Rutland@xxxxxxx>; James Morse <James.Morse@xxxxxxx>; Marc
> > Zyngier <maz@xxxxxxxxxx>; Matthew Wilcox <willy@xxxxxxxxxxxxx>; Kirill A.
> > Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> > mm@xxxxxxxxx; Punit Agrawal <punitagrawal@xxxxxxxxx>; Thomas
> > Gleixner <tglx@xxxxxxxxxxxxx>; Andrew Morton <akpm@linux-
> > foundation.org>; hejianet@xxxxxxxxx; Kaly Xin (Arm Technology China)
> > <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> > 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@xxxxxxxxxx>
> > > > Sent: 2019å10æ1æ 20:54
> > > > To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>
> > > > Cc: Catalin Marinas <Catalin.Marinas@xxxxxxx>; Mark Rutland
> > > > <Mark.Rutland@xxxxxxx>; James Morse <James.Morse@xxxxxxx>;
> > Marc
> > > > Zyngier <maz@xxxxxxxxxx>; Matthew Wilcox <willy@xxxxxxxxxxxxx>;
> > Kirill A.
> > > > Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>; linux-arm-
> > > > kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> > > > mm@xxxxxxxxx; Punit Agrawal <punitagrawal@xxxxxxxxx>; Thomas
> > > > Gleixner <tglx@xxxxxxxxxxxxx>; Andrew Morton <akpm@linux-
> > > > foundation.org>; hejianet@xxxxxxxxx; Kaly Xin (Arm Technology China)
> > > > <Kaly.Xin@xxxxxxx>
> > > > 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

Sounds sane to me.

-- 
 Kirill A. Shutemov