[PATCH V2] arm64: properly define SOFT_DIRTY functionality

[Nico Pache]

ARM64 has a soft-dirty bit (software dirty) but never properly defines
CONFIG_ARCH_HAS_SOFT_DIRTY or its necessary functions. This patch
introduces the ability to set/clear the soft dirty bit in a similar
manner as the other arches that utilize it.

However, we must be careful... there are cases where the DBM bit is not
available and the software dirty bit plays a essential role in determining
whether or not a page is dirty. In these cases we must not allow the
user to clear the software dirty bit. We can check for these cases by
utilizing the arch_has_hw_pte_young() function which tests the availability
of DBM.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
Cc: Liu Shixin <liushixin2@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
---
 arch/arm64/Kconfig               |   1 +
 arch/arm64/include/asm/pgtable.h | 104 ++++++++++++++++++++++++++-----
 2 files changed, 90 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7856c3a3e35a..6ea73b8148c5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -173,6 +173,7 @@ config ARM64
 	select HAVE_ARCH_PREL32_RELOCATIONS
 	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_SECCOMP_FILTER
+	select HAVE_ARCH_SOFT_DIRTY
 	select HAVE_ARCH_STACKLEAK
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_TRACEHOOK
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0bd18de9fd97..c4970c9ed114 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -51,6 +51,20 @@ static inline bool arch_thp_swp_supported(void)
 }
 #define arch_thp_swp_supported arch_thp_swp_supported
 
+/*
+ * On arm64 without hardware Access Flag, copying from user will fail because
+ * the pte is old and cannot be marked young. So we always end up with zeroed
+ * page after fork() + CoW for pfn mappings. We don't always have a
+ * hardware-managed access flag on arm64.
+ */
+#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.
+ */
+#define arch_wants_old_prefaulted_pte	cpu_has_hw_af
+
 /*
  * Outside of a few very special situations (e.g. hibernation), we always
  * use broadcast TLB invalidation instructions, therefore a spurious page
@@ -121,8 +135,9 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 })
 
 #define pte_hw_dirty(pte)	(pte_write(pte) && !(pte_val(pte) & PTE_RDONLY))
-#define pte_sw_dirty(pte)	(!!(pte_val(pte) & PTE_DIRTY))
-#define pte_dirty(pte)		(pte_sw_dirty(pte) || pte_hw_dirty(pte))
+#define pte_soft_dirty(pte)	(!!(pte_val(pte) & PTE_DIRTY))
+#define pte_dirty(pte)		(pte_soft_dirty(pte) || pte_hw_dirty(pte))
+#define pte_swp_soft_dirty(pte)	pte_soft_dirty(pte)
 
 #define pte_valid(pte)		(!!(pte_val(pte) & PTE_VALID))
 /*
@@ -189,7 +204,8 @@ static inline pte_t pte_mkwrite(pte_t pte)
 
 static inline pte_t pte_mkclean(pte_t pte)
 {
-	pte = clear_pte_bit(pte, __pgprot(PTE_DIRTY));
+	if (!arch_has_hw_pte_young())
+		pte = clear_pte_bit(pte, __pgprot(PTE_DIRTY));
 	pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
 
 	return pte;
@@ -1077,25 +1093,83 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
 #define phys_to_ttbr(addr)	(addr)
 #endif
 
-/*
- * On arm64 without hardware Access Flag, copying from user will fail because
- * the pte is old and cannot be marked young. So we always end up with zeroed
- * page after fork() + CoW for pfn mappings. We don't always have a
- * hardware-managed access flag on arm64.
- */
-#define arch_has_hw_pte_young		cpu_has_hw_af
+static inline bool pud_sect_supported(void)
+{
+	return PAGE_SIZE == SZ_4K;
+}
 
+#ifdef CONFIG_ARM64_HW_AFDBM
 /*
- * Experimentally, it's cheap to set the access flag in hardware and we
- * benefit from prefaulting mappings as 'old' to start with.
+ * if we have the DBM bit we can utilize the software dirty bit as
+ * a mechanism to introduce the soft_dirty functionality; however, without
+ * it this bit is crucial to determining if a entry is dirty and we cannot
+ * clear it via software. DBM can also be disabled or broken on some early
+ * armv8 devices, so check its availability before modifying it.
  */
-#define arch_wants_old_prefaulted_pte	cpu_has_hw_af
+static inline pte_t pte_clear_soft_dirty(pte_t pte)
+{
+	if (!arch_has_hw_pte_young())
+		return pte;
 
-static inline bool pud_sect_supported(void)
+	return clear_pte_bit(pte, __pgprot(PTE_DIRTY));
+}
+
+static inline pte_t pte_mksoft_dirty(pte_t pte)
 {
-	return PAGE_SIZE == SZ_4K;
+	if (!arch_has_hw_pte_young())
+		return pte;
+
+	return set_pte_bit(pte, __pgprot(PTE_DIRTY));
+}
+
+static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
+{
+	if (!arch_has_hw_pte_young())
+		return pte;
+
+	return clear_pte_bit(pte, __pgprot(PTE_DIRTY));
+}
+
+static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
+{
+	if (!arch_has_hw_pte_young())
+		return pte;
+
+	return set_pte_bit(pte, __pgprot(PTE_DIRTY));
+}
+
+static inline int pmd_soft_dirty(pmd_t pmd)
+{
+	return pte_soft_dirty(pmd_pte(pmd));
+}
+
+static inline pmd_t pmd_clear_soft_dirty(pmd_t pmd)
+{
+	return pte_pmd(pte_clear_soft_dirty(pmd_pte(pmd)));
+}
+
+static inline pmd_t pmd_mksoft_dirty(pmd_t pmd)
+{
+	return pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)));
 }
 
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+static inline int pmd_swp_soft_dirty(pmd_t pmd)
+{
+	return pmd_soft_dirty(pmd);
+}
+
+static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t pmd)
+{
+	return pmd_clear_soft_dirty(pmd);
+}
+
+static inline pmd_t pmd_swp_mksoft_dirty(pmd_t pmd)
+{
+	return pmd_mksoft_dirty(pmd);
+}
+#endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
+#endif /* CONFIG_ARM64_HW_AFDBM */
 
 #define __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION
 #define ptep_modify_prot_start ptep_modify_prot_start
-- 
2.41.0