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

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Date:   Tue, 19 May 2020 18:30:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Bibo Mao <maobibo@...ngson.cn>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Huacai Chen <chenhc@...ote.com>,
        Paul Burton <paulburton@...nel.org>,
        Dmitry Korotin <dkorotin@...ecomp.com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>,
        Stafford Horne <shorne@...il.com>,
        Steven Price <steven.price@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        "Maciej W. Rozycki" <macro@....com>, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4 3/4] mm/memory.c: Add memory read privilege on page
 fault handling

On Tue, 19 May 2020 18:03:29 +0800 Bibo Mao <maobibo@...ngson.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.