From a8e6015d9534f39abc08e6804566af059e498a60 Mon Sep 17 00:00:00 2001
From: Yu Zhao <yuzhao@google.com>
Date: Wed, 4 Aug 2021 01:31:34 -0600
Subject: [PATCH 01/10] mm: x86, arm64: add arch_has_hw_pte_young()

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 triggers a page fault following the
TLB miss of this PTE.

Being aware of this capability can help make better decisions, i.e.,
whether to limit the size of each batch of PTEs and the burst of
batches when clearing the accessed bit.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Change-Id: Ib49b44fb56df3333a2ff1fcc496fb1980b976e7a
---
 arch/arm64/include/asm/cpufeature.h |  5 +++++
 arch/arm64/include/asm/pgtable.h    | 13 ++++++++-----
 arch/arm64/kernel/cpufeature.c      | 10 ++++++++++
 arch/arm64/tools/cpucaps            |  1 +
 arch/x86/include/asm/pgtable.h      |  6 +++---
 include/linux/pgtable.h             | 13 +++++++++++++
 mm/memory.c                         | 14 +-------------
 7 files changed, 41 insertions(+), 21 deletions(-)

--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -808,6 +808,11 @@ static inline bool system_supports_tlb_r
 		cpus_have_const_cap(ARM64_HAS_TLB_RANGE);
 }
 
+static inline bool system_has_hw_af(void)
+{
+	return IS_ENABLED(CONFIG_ARM64_HW_AFDBM) && cpus_have_const_cap(ARM64_HW_AF);
+}
+
 extern int do_emulate_mrs(struct pt_regs *regs, u32 sys_reg, u32 rt);
 
 static inline u32 id_aa64mmfr0_parange_to_phys_shift(int parange)
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -999,13 +999,16 @@ static inline void update_mmu_cache(stru
  * 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)
+static inline bool arch_has_hw_pte_young(bool local)
 {
-	WARN_ON(preemptible());
+	if (local) {
+		WARN_ON(preemptible());
+		return cpu_has_hw_af();
+	}
 
-	return !cpu_has_hw_af();
+	return system_has_hw_af();
 }
-#define arch_faults_on_old_pte		arch_faults_on_old_pte
+#define arch_has_hw_pte_young		arch_has_hw_pte_young
 
 /*
  * Experimentally, it's cheap to set the access flag in hardware and we
@@ -1013,7 +1016,7 @@ static inline bool arch_faults_on_old_pt
  */
 static inline bool arch_wants_old_prefaulted_pte(void)
 {
-	return !arch_faults_on_old_pte();
+	return arch_has_hw_pte_young(true);
 }
 #define arch_wants_old_prefaulted_pte	arch_wants_old_prefaulted_pte
 
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2187,6 +2187,16 @@ static const struct arm64_cpu_capabiliti
 		.matches = has_hw_dbm,
 		.cpu_enable = cpu_enable_hw_dbm,
 	},
+	{
+		.desc = "Hardware update of the Access flag",
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.capability = ARM64_HW_AF,
+		.sys_reg = SYS_ID_AA64MMFR1_EL1,
+		.sign = FTR_UNSIGNED,
+		.field_pos = ID_AA64MMFR1_HADBS_SHIFT,
+		.min_field_value = 1,
+		.matches = has_cpuid_feature,
+	},
 #endif
 	{
 		.desc = "CRC32 instructions",
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -35,6 +35,7 @@ HAS_STAGE2_FWB
 HAS_SYSREG_GIC_CPUIF
 HAS_TLB_RANGE
 HAS_VIRT_HOST_EXTN
+HW_AF
 HW_DBM
 KVM_PROTECTED_MODE
 MISMATCHED_CACHE_TYPE
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1397,10 +1397,10 @@ static inline bool arch_has_pfn_modify_c
 	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(bool local)
 {
-	return false;
+	return true;
 }
 
 #endif	/* __ASSEMBLY__ */
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -259,6 +259,19 @@ static inline int pmdp_clear_flush_young
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 #endif
 
+#ifndef arch_has_hw_pte_young
+/*
+ * Return whether the accessed bit is supported by the local CPU or all CPUs.
+ *
+ * Those arches which have hw access flag feature need to implement their own
+ * helper. By default, "false" means pagefault will be hit on old pte.
+ */
+static inline bool arch_has_hw_pte_young(bool local)
+{
+	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,
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -121,18 +121,6 @@ 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)
 {
@@ -2782,7 +2770,7 @@ static inline bool cow_user_page(struct
 	 * 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(true) && !pte_young(vmf->orig_pte)) {
 		pte_t entry;
 
 		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
