Re: [PATCH v8 2/3] arm64: mm: implement arch_faults_on_old_pte() on arm64

[Catalin Marinas <catalin.marinas@arm.com>]

On Sat, Sep 21, 2019 at 09:50:53PM +0800, Jia He wrote:
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e09760ece844..4a9939615e41 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
                                                     ^^^^^^^^
						     from user

> + * 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
                                                ^^
						We

> + * hardware-managed access flag on arm64.
> + */
> +static inline bool arch_faults_on_old_pte(void)
> +{
> +	return !cpu_has_hw_af();

I saw an early incarnation of your patch having a
WARN_ON(preemptible()). I think we need this back just in case this
function will be used elsewhere in the future.

> +}
> +#define arch_faults_on_old_pte arch_faults_on_old_pte

Otherwise,

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>