Re: [PATCH v4 3/4] mm/memory.c: Add memory read privilege on page fault handling

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 19 May 2020 18:03:29 +0800 Bibo Mao <maobibo@loongson.cn> wrote:

> Here add pte_sw_mkyoung function to make page readable on MIPS
> platform during page fault handling. This patch improves page
> fault latency about 10% on my MIPS machine with lmbench
> lat_pagefault case.
> 
> It is noop function on other arches, there is no negative
> influence on those architectures.
> 
> --- a/arch/mips/include/asm/pgtable.h
> +++ b/arch/mips/include/asm/pgtable.h
> @@ -414,6 +414,8 @@ static inline pte_t pte_mkyoung(pte_t pte)
>  	return pte;
>  }
>  
> +#define pte_sw_mkyoung	pte_mkyoung
> +
>  #ifdef CONFIG_MIPS_HUGE_TLB_SUPPORT
>  static inline int pte_huge(pte_t pte)	{ return pte_val(pte) & _PAGE_HUGE; }
>  
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -227,6 +227,21 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
>  }
>  #endif
>  
> +/*
> + * On some architectures hardware does not set page access bit when accessing
> + * memory page, it is responsibilty of software setting this bit. It brings
> + * out extra page fault penalty to track page access bit. For optimization page
> + * access bit can be set during all page fault flow on these arches.
> + * To be differentiate with macro pte_mkyoung, this macro is used on platforms
> + * where software maintains page access bit.
> + */
> +#ifndef pte_sw_mkyoung
> +static inline pte_t pte_sw_mkyoung(pte_t pte)
> +{
> +	return pte;
> +}
> +#endif

Yup, that's neat enough.  Thanks for making this change.  It looks like
all architectures include asm-generic/pgtable.h so that's fine.

It's conventional to add a

#define pte_sw_mkyoung pte_sw_mkyoung

immediately above the #endif there, so we can't try to implement
pte_sw_mkyoung() twice if this header gets included twice.  But the
header has #ifndef _ASM_GENERIC_PGTABLE_H around the whole thing so
that should be OK.