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

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Date:   Tue, 8 Oct 2019 17:32:32 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Justin He (Arm Technology China)" <Justin.He@....com>
Cc:     Will Deacon <will@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Mark Rutland <Mark.Rutland@....com>,
        James Morse <James.Morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "hejianet@...il.com" <hejianet@...il.com>,
        "Kaly Xin (Arm Technology China)" <Kaly.Xin@....com>,
        nd <nd@....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 12:58:57PM +0000, Justin He (Arm Technology China) wrote:
> Hi Will
> 
> > -----Original Message-----
> > From: Will Deacon <will@...nel.org>
> > Sent: 2019年10月8日 20:40
> > To: Justin He (Arm Technology China) <Justin.He@....com>
> > Cc: Catalin Marinas <Catalin.Marinas@....com>; Mark Rutland
> > <Mark.Rutland@....com>; James Morse <James.Morse@....com>; Marc
> > Zyngier <maz@...nel.org>; Matthew Wilcox <willy@...radead.org>; Kirill A.
> > Shutemov <kirill.shutemov@...ux.intel.com>; linux-arm-
> > kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; linux-
> > mm@...ck.org; Punit Agrawal <punitagrawal@...il.com>; Thomas
> > Gleixner <tglx@...utronix.de>; Andrew Morton <akpm@...ux-
> > foundation.org>; hejianet@...il.com; Kaly Xin (Arm Technology China)
> > <Kaly.Xin@....com>; nd <nd@....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@...nel.org>
> > > > Sent: 2019年10月1日 20:54
> > > > To: Justin He (Arm Technology China) <Justin.He@....com>
> > > > Cc: Catalin Marinas <Catalin.Marinas@....com>; Mark Rutland
> > > > <Mark.Rutland@....com>; James Morse <James.Morse@....com>;
> > Marc
> > > > Zyngier <maz@...nel.org>; Matthew Wilcox <willy@...radead.org>;
> > Kirill A.
> > > > Shutemov <kirill.shutemov@...ux.intel.com>; linux-arm-
> > > > kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; linux-
> > > > mm@...ck.org; Punit Agrawal <punitagrawal@...il.com>; Thomas
> > > > Gleixner <tglx@...utronix.de>; Andrew Morton <akpm@...ux-
> > > > foundation.org>; hejianet@...il.com; Kaly Xin (Arm Technology China)
> > > > <Kaly.Xin@....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

Sounds sane to me.

-- 
 Kirill A. Shutemov