Re: [PATCH v3 1/2] arm64: mm: implement arch_faults_on_old_pte() on arm64

[Kirill A. Shutemov]

On Sat, Sep 14, 2019 at 12:32:38AM +0800, Jia He wrote:
> On arm64 without hardware Access Flag, copying fromuser will fail because
> the pte is old and cannot be marked young. So we always end up with zeroed
> page after fork() + CoW for pfn mappings. we don't always have a
> hardware-managed access flag on arm64.
> 
> Hence implement arch_faults_on_old_pte on arm64 to indicate that it might
> cause page fault when accessing old pte.
> 
> Signed-off-by: Jia He <justin.he@xxxxxxx>
> ---
>  arch/arm64/include/asm/pgtable.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e09760ece844..b41399d758df 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -868,6 +868,18 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
>  #define phys_to_ttbr(addr)	(addr)
>  #endif
>  
> +/*
> + * On arm64 without hardware Access Flag, copying fromuser will fail because
> + * the pte is old and cannot be marked young. So we always end up with zeroed
> + * page after fork() + CoW for pfn mappings. we don't always have a
> + * hardware-managed access flag on arm64.
> + */
> +static inline bool arch_faults_on_old_pte(void)
> +{
> +	return true;

Shouldn't youc check if this particular machine supports hardware access
bit?

> +}
> +#define arch_faults_on_old_pte arch_faults_on_old_pte
> +
>  #endif /* !__ASSEMBLY__ */
>  
>  #endif /* __ASM_PGTABLE_H */
> -- 
> 2.17.1
> 
> 

-- 
 Kirill A. Shutemov