[PATCH mm-unstable v15 01/14] mm: x86, arm64: add arch_has_hw_pte_young()

[Yu Zhao]

Some architectures automatically set the accessed bit in PTEs, e.g.,
x86 and arm64 v8.2. On architectures that do not have this capability,
clearing the accessed bit in a PTE usually triggers a page fault
following the TLB miss of this PTE (to emulate the accessed bit).

Being aware of this capability can help make better decisions, e.g.,
whether to spread the work out over a period of time to reduce bursty
page faults when trying to clear the accessed bit in many PTEs.

Note that theoretically this capability can be unreliable, e.g.,
hotplugged CPUs might be different from builtin ones. Therefore it
should not be used in architecture-independent code that involves
correctness, e.g., to determine whether TLB flushes are required (in
combination with the accessed bit).

Signed-off-by: Yu Zhao <yuzhao at google.com>
Reviewed-by: Barry Song <baohua at kernel.org>
Acked-by: Brian Geffon <bgeffon at google.com>
Acked-by: Jan Alexander Steffens (heftig) <heftig at archlinux.org>
Acked-by: Oleksandr Natalenko <oleksandr at natalenko.name>
Acked-by: Steven Barrett <steven at liquorix.net>
Acked-by: Suleiman Souhlal <suleiman at google.com>
Acked-by: Will Deacon <will at kernel.org>
Tested-by: Daniel Byrne <djbyrne at mtu.edu>
Tested-by: Donald Carr <d at chaos-reins.com>
Tested-by: Holger Hoffstätte <holger at applied-asynchrony.com>
Tested-by: Konstantin Kharlamov <Hi-Angel at yandex.ru>
Tested-by: Shuang Zhai <szhai2 at cs.rochester.edu>
Tested-by: Sofia Trinh <sofia.trinh at edi.works>
Tested-by: Vaibhav Jain <vaibhav at linux.ibm.com>
---
 arch/arm64/include/asm/pgtable.h | 15 ++-------------
 arch/x86/include/asm/pgtable.h   |  6 +++---
 include/linux/pgtable.h          | 13 +++++++++++++
 mm/memory.c                      | 14 +-------------
 4 files changed, 19 insertions(+), 29 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b5df82aa99e6..71a1af42f0e8 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1082,24 +1082,13 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
  * page after fork() + CoW for pfn mappings. We don't always have a
  * hardware-managed access flag on arm64.
  */
-static inline bool arch_faults_on_old_pte(void)
-{
-	/* The register read below requires a stable CPU to make any sense */
-	cant_migrate();
-
-	return !cpu_has_hw_af();
-}
-#define arch_faults_on_old_pte		arch_faults_on_old_pte
+#define arch_has_hw_pte_young		cpu_has_hw_af
 
 /*
  * Experimentally, it's cheap to set the access flag in hardware and we
  * benefit from prefaulting mappings as 'old' to start with.
  */
-static inline bool arch_wants_old_prefaulted_pte(void)
-{
-	return !arch_faults_on_old_pte();
-}
-#define arch_wants_old_prefaulted_pte	arch_wants_old_prefaulted_pte
+#define arch_wants_old_prefaulted_pte	cpu_has_hw_af
 
 static inline bool pud_sect_supported(void)
 {
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 44e2d6f1dbaa..dc5f7d8ef68a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1431,10 +1431,10 @@ static inline bool arch_has_pfn_modify_check(void)
 	return boot_cpu_has_bug(X86_BUG_L1TF);
 }
 
-#define arch_faults_on_old_pte arch_faults_on_old_pte
-static inline bool arch_faults_on_old_pte(void)
+#define arch_has_hw_pte_young arch_has_hw_pte_young
+static inline bool arch_has_hw_pte_young(void)
 {
-	return false;
+	return true;
 }
 
 #ifdef CONFIG_PAGE_TABLE_CHECK
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index d13b4f7cc5be..375e8e7e64f4 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -260,6 +260,19 @@ static inline int pmdp_clear_flush_young(struct vm_area_struct *vma,
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 #endif
 
+#ifndef arch_has_hw_pte_young
+/*
+ * Return whether the accessed bit is supported on the local CPU.
+ *
+ * This stub assumes accessing through an old PTE triggers a page fault.
+ * Architectures that automatically set the access bit should overwrite it.
+ */
+static inline bool arch_has_hw_pte_young(void)
+{
+	return false;
+}
+#endif
+
 #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
 static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 				       unsigned long address,
diff --git a/mm/memory.c b/mm/memory.c
index e38f9245470c..3a9b00c765c2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -126,18 +126,6 @@ int randomize_va_space __read_mostly =
 					2;
 #endif
 
-#ifndef arch_faults_on_old_pte
-static inline bool arch_faults_on_old_pte(void)
-{
-	/*
-	 * Those arches which don't have hw access flag feature need to
-	 * implement their own helper. By default, "true" means pagefault
-	 * will be hit on old pte.
-	 */
-	return true;
-}
-#endif
-
 #ifndef arch_wants_old_prefaulted_pte
 static inline bool arch_wants_old_prefaulted_pte(void)
 {
@@ -2871,7 +2859,7 @@ static inline bool __wp_page_copy_user(struct page *dst, struct page *src,
 	 * On architectures with software "accessed" bits, we would
 	 * take a double page fault, so mark it accessed here.
 	 */
-	if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
+	if (!arch_has_hw_pte_young() && !pte_young(vmf->orig_pte)) {
 		pte_t entry;
 
 		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
-- 
2.37.3.968.ga6b4b080e4-goog