[PATCH v2 06/16] mm, x86: support the access bit on non-leaf PMD entries

[Yu Zhao]

Some architectures support the accessed bit on non-leaf PMD entries
(parents) in addition to leaf PTE entries (children) where pages are
mapped, e.g., x86_64 sets the accessed bit on a parent when using it
as part of linear-address translation [1]. Page table walkers who are
interested in the accessed bit on children can take advantage of this:
they do not need to search the children when the accessed bit is not
set on a parent, given that they have previously cleared the accessed
bit on this parent.

[1]: Intel 64 and IA-32 Architectures Software Developer's Manual
     Volume 3 (October 2019), section 4.8

Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
---
 arch/Kconfig                   | 9 +++++++++
 arch/x86/Kconfig               | 1 +
 arch/x86/include/asm/pgtable.h | 2 +-
 arch/x86/mm/pgtable.c          | 5 ++++-
 include/linux/pgtable.h        | 4 ++--
 5 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index ecfd3520b676..cbd7f66734ee 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -782,6 +782,15 @@ config HAVE_ARCH_TRANSPARENT_HUGEPAGE
 config HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 	bool
 
+config HAVE_ARCH_PARENT_PMD_YOUNG
+	bool
+	depends on PGTABLE_LEVELS > 2
+	help
+	  Architectures that select this are able to set the accessed bit on
+	  non-leaf PMD entries in addition to leaf PTE entries where pages are
+	  mapped. For them, page table walkers that clear the accessed bit may
+	  stop at non-leaf PMD entries when they do not see the accessed bit.
+
 config HAVE_ARCH_HUGE_VMAP
 	bool
 
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2792879d398e..b5972eb82337 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -163,6 +163,7 @@ config X86
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
+	select HAVE_ARCH_PARENT_PMD_YOUNG	if X86_64
 	select HAVE_ARCH_USERFAULTFD_WP         if X86_64 && USERFAULTFD
 	select HAVE_ARCH_VMAP_STACK		if X86_64
 	select HAVE_ARCH_WITHIN_STACK_FRAMES
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index a02c67291cfc..a6b5cfe1fc5a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -846,7 +846,7 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
 
 static inline int pmd_bad(pmd_t pmd)
 {
-	return (pmd_flags(pmd) & ~_PAGE_USER) != _KERNPG_TABLE;
+	return ((pmd_flags(pmd) | _PAGE_ACCESSED) & ~_PAGE_USER) != _KERNPG_TABLE;
 }
 
 static inline unsigned long pages_to_mb(unsigned long npg)
diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index f6a9e2e36642..1c27e6f43f80 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -550,7 +550,7 @@ int ptep_test_and_clear_young(struct vm_area_struct *vma,
 	return ret;
 }
 
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HAVE_ARCH_PARENT_PMD_YOUNG)
 int pmdp_test_and_clear_young(struct vm_area_struct *vma,
 			      unsigned long addr, pmd_t *pmdp)
 {
@@ -562,6 +562,9 @@ int pmdp_test_and_clear_young(struct vm_area_struct *vma,
 
 	return ret;
 }
+#endif
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
 int pudp_test_and_clear_young(struct vm_area_struct *vma,
 			      unsigned long addr, pud_t *pudp)
 {
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 5e772392a379..08dd9b8c055a 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -193,7 +193,7 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HAVE_ARCH_PARENT_PMD_YOUNG)
 static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
 					    unsigned long address,
 					    pmd_t *pmdp)
@@ -214,7 +214,7 @@ static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
 	BUILD_BUG();
 	return 0;
 }
-#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HAVE_ARCH_PARENT_PMD_YOUNG */
 #endif
 
 #ifndef __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
-- 
2.31.1.295.g9ea45b61b8-goog