linux-kernel - Re: [PATCH] mm/memory.c: Remove pte_sw

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Message-Id: <1612408831.1l3aypsvyz.astroid@bobo.none>
Date:   Thu, 04 Feb 2021 13:23:18 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Jia He <justin.he@....com>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        Bibo Mao <maobibo@...ngson.cn>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Subject: Re: [PATCH] mm/memory.c: Remove pte_sw_mkyoung()

Excerpts from Andrew Morton's message of February 4, 2021 10:46 am:
> On Wed,  3 Feb 2021 10:19:44 +0000 (UTC) Christophe Leroy <christophe.leroy@...roup.eu> wrote:
> 
>> Commit 83d116c53058 ("mm: fix double page fault on arm64 if PTE_AF
>> is cleared") introduced arch_faults_on_old_pte() helper to identify
>> platforms that don't set page access bit in HW and require a page
>> fault to set it.
>> 
>> Commit 44bf431b47b4 ("mm/memory.c: Add memory read privilege on page
>> fault handling") added pte_sw_mkyoung() which is yet another way to
>> manage platforms that don't set page access bit in HW and require a
>> page fault to set it.
>> 
>> Remove that pte_sw_mkyoung() helper and use the already existing
>> arch_faults_on_old_pte() helper together with pte_mkyoung() instead.
> 
> This conflicts with mm/memory.c changes in linux-next.  In
> do_set_pte().  Please check my efforts:

I wanted to just get rid of it completely --

https://marc.info/?l=linux-mm&m=160860750115163&w=2

Waiting for MIPs to get that patch mentioned merged or nacked but
as yet seems to be no response from maintainers.

https://lore.kernel.org/linux-arch/20201019081257.32127-1-huangpei@loongson.cn/

Thanks,
Nick

> 
> --- a/arch/mips/include/asm/pgtable.h~mm-memoryc-remove-pte_sw_mkyoung
> +++ a/arch/mips/include/asm/pgtable.h
> @@ -406,8 +406,6 @@ static inline pte_t pte_mkyoung(pte_t pt
>  	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/linux/pgtable.h~mm-memoryc-remove-pte_sw_mkyoung
> +++ a/include/linux/pgtable.h
> @@ -424,22 +424,6 @@ static inline void ptep_set_wrprotect(st
>  }
>  #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;
> -}
> -#define pte_sw_mkyoung	pte_sw_mkyoung
> -#endif
> -
>  #ifndef pte_savedwrite
>  #define pte_savedwrite pte_write
>  #endif
> --- a/mm/memory.c~mm-memoryc-remove-pte_sw_mkyoung
> +++ a/mm/memory.c
> @@ -2902,7 +2902,8 @@ static vm_fault_t wp_page_copy(struct vm
>  		}
>  		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
>  		entry = mk_pte(new_page, vma->vm_page_prot);
> -		entry = pte_sw_mkyoung(entry);
> +		if (arch_faults_on_old_pte())
> +			entry = pte_mkyoung(entry);
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>  
>  		/*
> @@ -3560,7 +3561,8 @@ static vm_fault_t do_anonymous_page(stru
>  	__SetPageUptodate(page);
>  
>  	entry = mk_pte(page, vma->vm_page_prot);
> -	entry = pte_sw_mkyoung(entry);
> +	if (arch_faults_on_old_pte())
> +		entry = pte_mkyoung(entry);
>  	if (vma->vm_flags & VM_WRITE)
>  		entry = pte_mkwrite(pte_mkdirty(entry));
>  
> @@ -3745,8 +3747,8 @@ void do_set_pte(struct vm_fault *vmf, st
>  
>  	if (prefault && arch_wants_old_prefaulted_pte())
>  		entry = pte_mkold(entry);
> -	else
> -		entry = pte_sw_mkyoung(entry);
> +	else if (arch_faults_on_old_pte())
> +		entry = pte_mkyoung(entry);
>  
>  	if (write)
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> _
> 
> 
>