[PATCH 5/5] mm: Soft-dirty bits for user memory changes tracking

From: Pavel Emelyanov
Date: Thu Apr 11 2013 - 07:30:22 EST

The soft-dirty is a bit on a PTE which helps to track which pages a task
writes to. In order to do this tracking one should

1. Clear soft-dirty bits from PTEs ("echo 4 > /proc/PID/clear_refs)
2. Wait some time.
3. Read soft-dirty bits (55'th in /proc/PID/pagemap2 entries)

To do this tracking, the writable bit is cleared from PTEs when the
soft-dirty bit is. Thus, after this, when the task tries to modify a page
at some virtual address the #PF occurs and the kernel sets the soft-dirty
bit on the respective PTE.

Note, that although all the task's address space is marked as r/o after the
soft-dirty bits clear, the #PF-s that occur after that are processed fast.
This is so, since the pages are still mapped to physical memory, and thus
all the kernel does is finds this fact out and puts back writable, dirty
and soft-dirty bits on the PTE.

Another thing to note, is that when mremap moves PTEs they are marked with
soft-dirty as well, since from the user perspective mremap modifies the
virtual memory at mremap's new address.

Signed-off-by: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
---
Documentation/filesystems/proc.txt | 7 +++++-
Documentation/vm/pagemap.txt | 4 ++-
Documentation/vm/soft-dirty.txt | 36 ++++++++++++++++++++++++++++++++++
arch/x86/include/asm/pgtable.h | 26 ++++++++++++++++++++++-
arch/x86/include/asm/pgtable_types.h | 6 +++++
fs/proc/task_mmu.c | 36 +++++++++++++++++++++++++++++----
include/asm-generic/pgtable.h | 22 ++++++++++++++++++++
mm/Kconfig | 12 +++++++++++
mm/huge_memory.c | 2 +-
mm/mremap.c | 2 +-
10 files changed, 142 insertions(+), 11 deletions(-)
create mode 100644 Documentation/vm/soft-dirty.txt

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 22c47ec..488c094 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -473,7 +473,8 @@ This file is only present if the CONFIG_MMU kernel configuration option is
enabled.

The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG
-bits on both physical and virtual pages associated with a process.
+bits on both physical and virtual pages associated with a process, and the
+soft-dirty bit on pte (see Documentation/vm/soft-dirty.txt for details).
To clear the bits for all the pages associated with the process
> echo 1 > /proc/PID/clear_refs

@@ -482,6 +483,10 @@ To clear the bits for the anonymous pages associated with the process

To clear the bits for the file mapped pages associated with the process
> echo 3 > /proc/PID/clear_refs
+
+To clear the soft-dirty bit
+ > echo 4 > /proc/PID/clear_refs
+
Any other value written to /proc/PID/clear_refs will have no effect.

The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags
diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
index 4350397..394cc03 100644
--- a/Documentation/vm/pagemap.txt
+++ b/Documentation/vm/pagemap.txt
@@ -31,7 +31,9 @@ There are three components to pagemap:
skip over unmapped regions.

* /proc/pid/pagemap2. This file provides the same info as the pagemap
- does, but bits 55-60 are reserved for future use and thus zero
+ does, but bits 56-60 are reserved for future use and thus zero
+
+ Bit 55 means pte is soft-dirty (see Documentation/vm/soft-dirty.txt)

* /proc/kpagecount. This file contains a 64-bit count of the number of
times each page is mapped, indexed by PFN.
diff --git a/Documentation/vm/soft-dirty.txt b/Documentation/vm/soft-dirty.txt
new file mode 100644
index 0000000..9a12a59
--- /dev/null
+++ b/Documentation/vm/soft-dirty.txt
@@ -0,0 +1,36 @@
+ SOFT-DIRTY PTEs
+
+ The soft-dirty is a bit on a PTE which helps to track which pages a task
+writes to. In order to do this tracking one should
+
+ 1. Clear soft-dirty bits from the task's PTEs.
+
+ This is done by writing "4" into the /proc/PID/clear_refs file of the
+ task in question.
+
+ 2. Wait some time.
+
+ 3. Read soft-dirty bits from the PTEs.
+
+ This is done by reading from the /proc/PID/pagemap. The bit 55 of the
+ 64-bit qword is the soft-dirty one. If set, the respective PTE was
+ written to since step 1.
+
+
+ Internally, to do this tracking, the writable bit is cleared from PTEs
+when the soft-dirty bit is cleared. So, after this, when the task tries to
+modify a page at some virtual address the #PF occurs and the kernel sets
+the soft-dirty bit on the respective PTE.
+
+ Note, that although all the task's address space is marked as r/o after the
+soft-dirty bits clear, the #PF-s that occur after that are processed fast.
+This is so, since the pages are still mapped to physical memory, and thus all
+the kernel does is finds this fact out and puts both writable and soft-dirty
+bits on the PTE.
+
+
+ This feature is actively used by the checkpoint-restore project. You
+can find more details about it on http://criu.org
+
+
+-- Pavel Emelyanov, Apr 9, 2013
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 1e67223..eb97470 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -207,7 +207,7 @@ static inline pte_t pte_mkexec(pte_t pte)

static inline pte_t pte_mkdirty(pte_t pte)
{
- return pte_set_flags(pte, _PAGE_DIRTY);
+ return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
}

static inline pte_t pte_mkyoung(pte_t pte)
@@ -271,7 +271,7 @@ static inline pmd_t pmd_wrprotect(pmd_t pmd)

static inline pmd_t pmd_mkdirty(pmd_t pmd)
{
- return pmd_set_flags(pmd, _PAGE_DIRTY);
+ return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
}

static inline pmd_t pmd_mkhuge(pmd_t pmd)
@@ -294,6 +294,28 @@ static inline pmd_t pmd_mknotpresent(pmd_t pmd)
return pmd_clear_flags(pmd, _PAGE_PRESENT);
}

+#define __HAVE_SOFT_DIRTY
+
+static inline int pte_soft_dirty(pte_t pte)
+{
+ return pte_flags(pte) & _PAGE_SOFT_DIRTY;
+}
+
+static inline int pmd_soft_dirty(pmd_t pmd)
+{
+ return pmd_flags(pmd) & _PAGE_SOFT_DIRTY;
+}
+
+static inline pte_t pte_mksoft_dirty(pte_t pte)
+{
+ return pte_set_flags(pte, _PAGE_SOFT_DIRTY);
+}
+
+static inline pmd_t pmd_mksoft_dirty(pmd_t pmd)
+{
+ return pmd_set_flags(pmd, _PAGE_SOFT_DIRTY);
+}
+
/*
* Mask out unsupported bits in a present pgprot. Non-present pgprots
* can use those bits for other purposes, so leave them be.
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 567b5d0..dcf718c 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -55,6 +55,18 @@
#define _PAGE_HIDDEN (_AT(pteval_t, 0))
#endif

+/*
+ * The same hidden bit is used by kmemcheck, but since kmemcheck
+ * works on kernel pages while soft-dirty engine on user space,
+ * they do not conflict with each other.
+ */
+
+#ifdef CONFIG_MEM_SOFT_DIRTY
+#define _PAGE_SOFT_DIRTY (_AT(pteval_t, 1) << _PAGE_BIT_HIDDEN)
+#else
+#define _PAGE_SOFT_DIRTY (_AT(pteval_t, 0))
+#endif
+
#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
#define _PAGE_NX (_AT(pteval_t, 1) << _PAGE_BIT_NX)
#else
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 3138009..aae2474 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -692,13 +692,32 @@ enum clear_refs_types {
CLEAR_REFS_ALL = 1,
CLEAR_REFS_ANON,
CLEAR_REFS_MAPPED,
+ CLEAR_REFS_SOFT_DIRTY,
CLEAR_REFS_LAST,
};

struct clear_refs_private {
struct vm_area_struct *vma;
+ enum clear_refs_types type;
};

+static inline void clear_soft_dirty(struct vm_area_struct *vma,
+ unsigned long addr, pte_t *pte)
+{
+#ifdef CONFIG_MEM_SOFT_DIRTY
+ /*
+ * The soft-dirty tracker uses #PF-s to catch writes
+ * to pages, so write-protect the pte as well. See the
+ * Documentation/vm/soft-dirty.txt for full description
+ * of how soft-dirty works.
+ */
+ pte_t ptent = *pte;
+ ptent = pte_wrprotect(ptent);
+ ptent = pte_clear_flags(ptent, _PAGE_SOFT_DIRTY);
+ set_pte_at(vma->vm_mm, addr, pte, ptent);
+#endif
+}
+
static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
unsigned long end, struct mm_walk *walk)
{
@@ -718,6 +731,11 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
if (!pte_present(ptent))
continue;

+ if (cp->type == CLEAR_REFS_SOFT_DIRTY) {
+ clear_soft_dirty(vma, addr, pte);
+ continue;
+ }
+
page = vm_normal_page(vma, addr, ptent);
if (!page)
continue;
@@ -757,6 +775,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
mm = get_task_mm(task);
if (mm) {
struct clear_refs_private cp = {
+ .type = type,
};
struct mm_walk clear_refs_walk = {
.pmd_entry = clear_refs_pte_range,
@@ -825,6 +844,7 @@ struct pagemapread {
/* in pagemap2 pshift bits are occupied with more status bits */
#define PM_STATUS2(v2, x) (__PM_PSHIFT(v2 ? x : PAGE_SHIFT))

+#define __PM_SOFT_DIRTY (1LL)
#define PM_PRESENT PM_STATUS(4LL)
#define PM_SWAP PM_STATUS(2LL)
#define PM_FILE PM_STATUS(1LL)
@@ -866,6 +886,7 @@ static void pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
{
u64 frame, flags;
struct page *page = NULL;
+ int flags2 = 0;

if (pte_present(pte)) {
frame = pte_pfn(pte);
@@ -886,13 +907,15 @@ static void pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,

if (page && !PageAnon(page))
flags |= PM_FILE;
+ if (pte_soft_dirty(pte))
+ flags2 |= __PM_SOFT_DIRTY;

- *pme = make_pme(PM_PFRAME(frame) | PM_STATUS2(pm->v2, 0) | flags);
+ *pme = make_pme(PM_PFRAME(frame) | PM_STATUS2(pm->v2, flags2) | flags);
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
static void thp_pmd_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
- pmd_t pmd, int offset)
+ pmd_t pmd, int offset, int pmd_flags2)
{
/*
* Currently pmd for thp is always present because thp can not be
@@ -901,13 +924,13 @@ static void thp_pmd_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *p
*/
if (pmd_present(pmd))
*pme = make_pme(PM_PFRAME(pmd_pfn(pmd) + offset)
- | PM_STATUS2(pm->v2, 0) | PM_PRESENT);
+ | PM_STATUS2(pm->v2, pmd_flags2) | PM_PRESENT);
else
*pme = make_pme(PM_NOT_PRESENT(pm->v2));
}
#else
static inline void thp_pmd_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
- pmd_t pmd, int offset)
+ pmd_t pmd, int offset, int pmd_flags2)
{
}
#endif
@@ -924,12 +947,15 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
/* find the first VMA at or above 'addr' */
vma = find_vma(walk->mm, addr);
if (vma && pmd_trans_huge_lock(pmd, vma) == 1) {
+ int pmd_flags2;
+
+ pmd_flags2 = (pmd_soft_dirty(*pmd) ? __PM_SOFT_DIRTY : 0);
for (; addr != end; addr += PAGE_SIZE) {
unsigned long offset;

offset = (addr & ~PAGEMAP_WALK_MASK) >>
PAGE_SHIFT;
- thp_pmd_to_pagemap_entry(&pme, pm, *pmd, offset);
+ thp_pmd_to_pagemap_entry(&pme, pm, *pmd, offset, pmd_flags2);
err = add_to_pagemap(addr, &pme, pm);
if (err)
break;
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index bfd8768..d74bdd2 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -386,6 +386,28 @@ static inline void ptep_modify_prot_commit(struct mm_struct *mm,
#define arch_start_context_switch(prev) do {} while (0)
#endif

+#ifndef __HAVE_SOFT_DIRTY
+static inline int pte_soft_dirty(pte_t pte)
+{
+ return 0;
+}
+
+static inline int pmd_soft_dirty(pmd_t pmd)
+{
+ return 0;
+}
+
+static inline pte_t pte_mksoft_dirty(pte_t pte)
+{
+ return pte;
+}
+
+static inline pmd_t pmd_mksoft_dirty(pmd_t pmd)
+{
+ return pmd;
+}
+#endif
+
#ifndef __HAVE_PFNMAP_TRACKING
/*
* Interfaces that can be used by architecture code to keep track of
diff --git a/mm/Kconfig b/mm/Kconfig
index 3bea74f..147689e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -471,3 +471,15 @@ config FRONTSWAP
and swap data is stored as normal on the matching swap device.

If unsure, say Y to enable frontswap.
+
+config MEM_SOFT_DIRTY
+ bool "Track memory changes"
+ depends on CHECKPOINT_RESTORE && X86
+ select PROC_PAGE_MONITOR
+ help
+ This option enables memory changes tracking by introducing a
+ soft-dirty bit on pte-s. This bit it set when someone writes
+ into a page just as regular dirty bit, but unlike the latter
+ it can be cleared by hands.
+
+ See Documentation/vm/soft-dirty.txt for more details.
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e2f7f5aa..eef1606 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1431,7 +1431,7 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
if (ret == 1) {
pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
VM_BUG_ON(!pmd_none(*new_pmd));
- set_pmd_at(mm, new_addr, new_pmd, pmd);
+ set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
spin_unlock(&mm->page_table_lock);
}
out:
diff --git a/mm/mremap.c b/mm/mremap.c
index 463a257..3708655 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -126,7 +126,7 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
continue;
pte = ptep_get_and_clear(mm, old_addr, old_pte);
pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
- set_pte_at(mm, new_addr, new_pte, pte);
+ set_pte_at(mm, new_addr, new_pte, pte_mksoft_dirty(pte));
}

arch_leave_lazy_mmu_mode();
--
1.7.6.5
--
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