linux-kernel - Re: a racy access flag clearing warning when calling mmap system call

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Date:   Fri, 8 Dec 2017 11:01:40 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     chenjiankang <chenjiankang1@...wei.com>
Cc:     Will Deacon <will.deacon@....com>,
        Yisheng Xie <xieyisheng1@...wei.com>, qiuxishi@...wei.com,
        linux-kernel@...r.kernel.org, wangkefeng.wang@...wei.com
Subject: Re: a racy access flag clearing warning when calling mmap system call

On Fri, Dec 08, 2017 at 11:19:52AM +0800, chenjiankang wrote:
> 在 2017/12/7 21:23, Will Deacon 写道:
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 149d05fb9421..8fe103b1e101 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -42,6 +42,8 @@
> >  #include <asm/cmpxchg.h>
> >  #include <asm/fixmap.h>
> >  #include <linux/mmdebug.h>
> > +#include <linux/mm_types.h>
> > +#include <linux/sched.h>
> >  
> >  extern void __pte_error(const char *file, int line, unsigned long val);
> >  extern void __pmd_error(const char *file, int line, unsigned long val);
> > @@ -207,9 +209,6 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
> >  	}
> >  }
> >  
> > -struct mm_struct;
> > -struct vm_area_struct;
> > -
> >  extern void __sync_icache_dcache(pte_t pteval, unsigned long addr);
> >  
> >  /*
> > @@ -238,7 +237,8 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> >  	 * hardware updates of the pte (ptep_set_access_flags safely changes
> >  	 * valid ptes without going through an invalid entry).
> >  	 */
> > -	if (pte_valid(*ptep) && pte_valid(pte)) {
> > +	if (IS_ENABLED(CONFIG_DEBUG_VM) && pte_valid(*ptep) && pte_valid(pte) &&
> > +	   (mm == current->active_mm || atomic_read(&mm->mm_users) > 1)) {
> >  		VM_WARN_ONCE(!pte_young(pte),
> >  			     "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
> >  			     __func__, pte_val(*ptep), pte_val(pte));
[...]
>     From the print information, the only difference between pte and ptep is the PTE_SPECIAL bit.
> And the the PTE access bit is all zero.
>     diff below. Whether the access bit of the new ptep should be judged to eliminate the 
> false positive?
[...]
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 2987d5a..3c1b0c6 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -206,7 +206,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>          * valid ptes without going through an invalid entry).
>          */
>         if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM) && pte_valid(*ptep)) {
> -               VM_WARN_ONCE(!pte_young(pte),
> +               VM_WARN_ONCE(!pte_young(pte) && pte_young(*ptep),
>                              "%s: racy access flag clearing: %016llx -> %016llx",
>                              __func__, pte_val(*ptep), pte_val(pte));

It's actually the other way around: *ptep being "old" (AF = 0) could at
any point be made "young" by the hardware (AF = 1). This is racing with
the software update which keeps the AF bit 0.

-- 
Catalin