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Subject[RFC PATCH v1 3/5] arm64/mm: Add soft-dirty page tracking support
Date
Use the final remaining PTE SW bit (63) for soft-dirty tracking. The
standard handlers are implemented for set/test/clear for both pte and
pmd. Additionally we must also track the soft-dirty state as a pte swp
bit, so use a free swap entry pte bit (61).

There are a few complexities worth calling out:

- The semantic of soft-dirty calls for having it auto-set by
pte_mkdirty(). But the arch code would previously call pte_mkdirty()
for various house-keeping operations such as gathering dirty bits
into a pte across a contpte block. These operations must not cause
soft-dirty to be set. So an internal version, __pte_mkdirty(), has
been created that does not manipulate soft-dirty, and pte_mkdirty()
is now a wrapper around that, which also sets the soft-dirty bit.

- For a region with soft-dirty tracking enabled, it works by
wrprotecting the ptes, causing a write to fault, where the handler
calls pte_mkdirty(ptep_get()) (which causes soft-dirty to be set),
then the resulting pte is written back with ptep_set_access_flags().
So the arm64 version of ptep_set_access_flags() now needs to
explicitly also set the soft-dirty bit to prevent loss.

The patch is very loosely based on a similar patch posted by Shivansh
Vij <shivanshvij@outlook.com>, at the below link.

Primary motivation for adding soft-dirty support is to allow
Checkpoint-Restore in Userspace (CRIU) to be able to track a memory
page's changes if we want to enable pre-dumping, which is important for
live migration.

Link: https://lore.kernel.org/linux-arm-kernel/MW4PR12MB687563EFB56373E8D55DDEABB92B2@MW4PR12MB6875.namprd12.prod.outlook.com/
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/pgtable-prot.h | 8 +++++
arch/arm64/include/asm/pgtable.h | 47 +++++++++++++++++++++++++--
arch/arm64/mm/contpte.c | 6 ++--
arch/arm64/mm/fault.c | 3 +-
arch/arm64/mm/hugetlbpage.c | 6 ++--
6 files changed, 61 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 763e221f2169..3a5e22208e38 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -178,6 +178,7 @@ 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-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index f1e1f6306e03..7fce22ed3fda 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -28,6 +28,14 @@
#define PTE_SWP_UFFD_WP (_AT(pteval_t, 0))
#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */

+#ifdef CONFIG_MEM_SOFT_DIRTY
+#define PTE_SOFT_DIRTY (_AT(pteval_t, 1) << 63) /* soft-dirty tracking */
+#define PTE_SWP_SOFT_DIRTY (_AT(pteval_t, 1) << 61) /* only for swp ptes */
+#else
+#define PTE_SOFT_DIRTY (_AT(pteval_t, 0))
+#define PTE_SWP_SOFT_DIRTY (_AT(pteval_t, 0))
+#endif /* CONFIG_MEM_SOFT_DIRTY */
+
/*
* This bit indicates that the entry is present i.e. pmd_page()
* still points to a valid huge page in memory even if the pmd
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 3f4748741fdb..0118e6e0adde 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -114,6 +114,7 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
#define pte_user_exec(pte) (!(pte_val(pte) & PTE_UXN))
#define pte_cont(pte) (!!(pte_val(pte) & PTE_CONT))
#define pte_devmap(pte) (!!(pte_val(pte) & PTE_DEVMAP))
+#define pte_soft_dirty(pte) (!!(pte_val(pte) & PTE_SOFT_DIRTY))
#define pte_tagged(pte) ((pte_val(pte) & PTE_ATTRINDX_MASK) == \
PTE_ATTRINDX(MT_NORMAL_TAGGED))

@@ -206,7 +207,7 @@ static inline pte_t pte_mkclean(pte_t pte)
return pte;
}

-static inline pte_t pte_mkdirty(pte_t pte)
+static inline pte_t __pte_mkdirty(pte_t pte)
{
pte = set_pte_bit(pte, __pgprot(PTE_DIRTY));

@@ -216,6 +217,11 @@ static inline pte_t pte_mkdirty(pte_t pte)
return pte;
}

+static inline pte_t pte_mkdirty(pte_t pte)
+{
+ return __pte_mkdirty(set_pte_bit(pte, __pgprot(PTE_SOFT_DIRTY)));
+}
+
static inline pte_t pte_wrprotect(pte_t pte)
{
/*
@@ -299,6 +305,16 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
}
#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */

+static inline pte_t pte_mksoft_dirty(pte_t pte)
+{
+ return set_pte_bit(pte, __pgprot(PTE_SOFT_DIRTY));
+}
+
+static inline pte_t pte_clear_soft_dirty(pte_t pte)
+{
+ return clear_pte_bit(pte, __pgprot(PTE_SOFT_DIRTY));
+}
+
static inline void __set_pte(pte_t *ptep, pte_t pte)
{
WRITE_ONCE(*ptep, pte);
@@ -508,6 +524,21 @@ static inline pte_t pte_swp_clear_uffd_wp(pte_t pte)
}
#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */

+static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
+{
+ return set_pte_bit(pte, __pgprot(PTE_SWP_SOFT_DIRTY));
+}
+
+static inline bool pte_swp_soft_dirty(pte_t pte)
+{
+ return !!(pte_val(pte) & PTE_SWP_SOFT_DIRTY);
+}
+
+static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
+{
+ return clear_pte_bit(pte, __pgprot(PTE_SWP_SOFT_DIRTY));
+}
+
#ifdef CONFIG_NUMA_BALANCING
/*
* See the comment in include/linux/pgtable.h
@@ -562,6 +593,15 @@ static inline int pmd_trans_huge(pmd_t pmd)
#define pmd_swp_clear_uffd_wp(pmd) \
pte_pmd(pte_swp_clear_uffd_wp(pmd_pte(pmd)))
#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
+#define pmd_soft_dirty(pmd) pte_soft_dirty(pmd_pte(pmd))
+#define pmd_mksoft_dirty(pmd) pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)))
+#define pmd_clear_soft_dirty(pmd) \
+ pte_pmd(pte_clear_soft_dirty(pmd_pte(pmd)))
+#define pmd_swp_soft_dirty(pmd) pte_swp_soft_dirty(pmd_pte(pmd))
+#define pmd_swp_mksoft_dirty(pmd) \
+ pte_pmd(pte_swp_mksoft_dirty(pmd_pte(pmd)))
+#define pmd_swp_clear_soft_dirty(pmd) \
+ pte_pmd(pte_swp_clear_soft_dirty(pmd_pte(pmd)))

static inline pmd_t pmd_mkinvalid(pmd_t pmd)
{
@@ -1093,7 +1133,7 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
* dirtiness again.
*/
if (pte_sw_dirty(pte))
- pte = pte_mkdirty(pte);
+ pte = __pte_mkdirty(pte);
return pte;
}

@@ -1228,7 +1268,7 @@ static inline pte_t __get_and_clear_full_ptes(struct mm_struct *mm,
addr += PAGE_SIZE;
tmp_pte = __ptep_get_and_clear(mm, addr, ptep);
if (pte_dirty(tmp_pte))
- pte = pte_mkdirty(pte);
+ pte = __pte_mkdirty(pte);
if (pte_young(tmp_pte))
pte = pte_mkyoung(pte);
}
@@ -1307,6 +1347,7 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
* bit 54: PTE_PROT_NONE (overlays PTE_UXN) (must be zero)
* bits 55-59: swap type
* bit 60: PMD_PRESENT_INVALID (must be zero)
+ * bit 61: remember soft-dirty state
*/
#define __SWP_TYPE_SHIFT 55
#define __SWP_TYPE_BITS 5
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index 1b64b4c3f8bf..c6f52fcf5d9a 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -62,7 +62,7 @@ static void contpte_convert(struct mm_struct *mm, unsigned long addr,
pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);

if (pte_dirty(ptent))
- pte = pte_mkdirty(pte);
+ pte = __pte_mkdirty(pte);

if (pte_young(ptent))
pte = pte_mkyoung(pte);
@@ -170,7 +170,7 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
pte = __ptep_get(ptep);

if (pte_dirty(pte))
- orig_pte = pte_mkdirty(orig_pte);
+ orig_pte = __pte_mkdirty(orig_pte);

if (pte_young(pte))
orig_pte = pte_mkyoung(orig_pte);
@@ -227,7 +227,7 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
goto retry;

if (pte_dirty(pte))
- orig_pte = pte_mkdirty(orig_pte);
+ orig_pte = __pte_mkdirty(orig_pte);

if (pte_young(pte))
orig_pte = pte_mkyoung(orig_pte);
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 8251e2fea9c7..678171fd88bd 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -220,7 +220,8 @@ int __ptep_set_access_flags(struct vm_area_struct *vma,
return 0;

/* only preserve the access flags and write permission */
- pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY;
+ pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE |
+ PTE_DIRTY | PTE_SOFT_DIRTY;

/*
* Setting the flags must be done atomically to avoid racing with the
diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 0f0e10bb0a95..4605eb146a2f 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -155,7 +155,7 @@ pte_t huge_ptep_get(pte_t *ptep)
pte_t pte = __ptep_get(ptep);

if (pte_dirty(pte))
- orig_pte = pte_mkdirty(orig_pte);
+ orig_pte = __pte_mkdirty(orig_pte);

if (pte_young(pte))
orig_pte = pte_mkyoung(orig_pte);
@@ -189,7 +189,7 @@ static pte_t get_clear_contig(struct mm_struct *mm,
* so check them all.
*/
if (pte_dirty(pte))
- orig_pte = pte_mkdirty(orig_pte);
+ orig_pte = __pte_mkdirty(orig_pte);

if (pte_young(pte))
orig_pte = pte_mkyoung(orig_pte);
@@ -464,7 +464,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,

/* Make sure we don't lose the dirty or young state */
if (pte_dirty(orig_pte))
- pte = pte_mkdirty(pte);
+ pte = __pte_mkdirty(pte);

if (pte_young(orig_pte))
pte = pte_mkyoung(pte);
--
2.25.1

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