[PATCH v7 01/12] mm: x86, arm64: add arch_has_hw_pte_young()

Yu Zhao
2022-02-08T08:19:42Z

Some architectures automatically set the accessed bit in PTEs, e.g.,
x86 and arm64 v8.2. On architectures that don't 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
shouldn't 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@google.com>
Acked-by: Brian Geffon <bgeffon@google.com>
Acked-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Acked-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Acked-by: Steven Barrett <steven@liquorix.net>
Acked-by: Suleiman Souhlal <suleiman@google.com>
Tested-by: Daniel Byrne <djbyrne@mtu.edu>
Tested-by: Donald Carr <d@chaos-reins.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Tested-by: Shuang Zhai <szhai2@cs.rochester.edu>
Tested-by: Sofia Trinh <sofia.trinh@edi.works>
---
arch/arm64/include/asm/pgtable.h | 14 ++------------
arch/x86/include/asm/pgtable.h | 6 +++---
include/linux/pgtable.h | 13 +++++++++++++
mm/memory.c | 14 +-------------
4 files changed, 19 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index c4ba047a82d2..990358eca359 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -999,23 +999,13 @@ 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)
-{
- WARN_ON(preemptible());
-
- 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 pgprot_t arch_filter_pgprot(pgprot_t prot)
{
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 8a9432fb3802..60b6ce45c2e3 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1423,10 +1423,10 @@ 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;
}

#endif /* __ASSEMBLY__ */
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index f4f4077b97aa..c799635f4d79 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -259,6 +259,19 @@ 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 by 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_CLEAR
static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
pte_t *ptep)
diff --git a/mm/memory.c b/mm/memory.c
index c125c4969913..a7379196a47e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -122,18 +122,6 @@ 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)
{
@@ -2778,7 +2766,7 @@ static inline bool cow_user_page(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);
--
2.35.0.263.gb82422642f-goog