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

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Message-Id: <20210203164630.ada46d0c84e0e9f0a474b283@linux-foundation.org>
Date:   Wed, 3 Feb 2021 16:46:30 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Bibo Mao <maobibo@...ngson.cn>, Jia He <justin.he@....com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, linux-mips@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH] mm/memory.c: Remove pte_sw_mkyoung()

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:

--- 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);
_