mm: track vma changes with VM_SOFTDIRTY bit

Pavel reported that in case if vma area get unmapped and then mapped (or
expanded) in-place, the soft dirty tracker won't be able to recognize this
situation since it works on pte level and ptes are get zapped on unmap,
loosing soft dirty bit of course.

So to resolve this situation we need to track actions on vma level, there
VM_SOFTDIRTY flag comes in.  When new vma area created (or old expanded)
we set this bit, and keep it here until application calls for clearing
soft dirty bit.

Thus when user space application track memory changes now it can detect if
vma area is renewed.
Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/soft-dirty.txt

@@ -28,6 +28,13 @@ 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.
 
+  While in most cases tracking memory changes by #PF-s is more than enough
+there is still a scenario when we can lose soft dirty bits -- a task
+unmaps a previously mapped memory region and then maps a new one at exactly
+the same place. When unmap is called, the kernel internally clears PTE values
+including soft dirty bits. To notify user space application about such
+memory region renewal the kernel always marks new memory regions (and
+expanded regions) as soft dirty.
 
   This feature is actively used by the checkpoint-restore project. You
 can find more details about it on http://criu.org

fs/exec.c

@@ -266,7 +266,7 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	BUILD_BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
 	vma->vm_end = STACK_TOP_MAX;
 	vma->vm_start = vma->vm_end - PAGE_SIZE;
-	vma->vm_flags = VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
+	vma->vm_flags = VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
 

fs/proc/task_mmu.c

@@ -740,6 +740,9 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
 		ptent = pte_file_clear_soft_dirty(ptent);
 	}
 
+	if (vma->vm_flags & VM_SOFTDIRTY)
+		vma->vm_flags &= ~VM_SOFTDIRTY;
+
 	set_pte_at(vma->vm_mm, addr, pte, ptent);
 #endif
 }
@@ -949,13 +952,15 @@ static void pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
 		if (is_migration_entry(entry))
 			page = migration_entry_to_page(entry);
 	} else {
-		*pme = make_pme(PM_NOT_PRESENT(pm->v2));
+		if (vma->vm_flags & VM_SOFTDIRTY)
+			flags2 |= __PM_SOFT_DIRTY;
+		*pme = make_pme(PM_NOT_PRESENT(pm->v2) | PM_STATUS2(pm->v2, flags2));
 		return;
 	}
 
 	if (page && !PageAnon(page))
 		flags |= PM_FILE;
-	if (pte_soft_dirty(pte))
+	if ((vma->vm_flags & VM_SOFTDIRTY) || pte_soft_dirty(pte))
 		flags2 |= __PM_SOFT_DIRTY;
 
 	*pme = make_pme(PM_PFRAME(frame) | PM_STATUS2(pm->v2, flags2) | flags);
@@ -974,7 +979,7 @@ static void thp_pmd_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *p
 		*pme = make_pme(PM_PFRAME(pmd_pfn(pmd) + offset)
 				| PM_STATUS2(pm->v2, pmd_flags2) | PM_PRESENT);
 	else
-		*pme = make_pme(PM_NOT_PRESENT(pm->v2));
+		*pme = make_pme(PM_NOT_PRESENT(pm->v2) | PM_STATUS2(pm->v2, pmd_flags2));
 }
 #else
 static inline void thp_pmd_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
@@ -997,7 +1002,11 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	if (vma && pmd_trans_huge_lock(pmd, vma) == 1) {
 		int pmd_flags2;
 
-		pmd_flags2 = (pmd_soft_dirty(*pmd) ? __PM_SOFT_DIRTY : 0);
+		if ((vma->vm_flags & VM_SOFTDIRTY) || pmd_soft_dirty(*pmd))
+			pmd_flags2 = __PM_SOFT_DIRTY;
+		else
+			pmd_flags2 = 0;
+
 		for (; addr != end; addr += PAGE_SIZE) {
 			unsigned long offset;
 
@@ -1015,12 +1024,17 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	if (pmd_trans_unstable(pmd))
 		return 0;
 	for (; addr != end; addr += PAGE_SIZE) {
+		int flags2;
 
 		/* check to see if we've left 'vma' behind
 		 * and need a new, higher one */
 		if (vma && (addr >= vma->vm_end)) {
 			vma = find_vma(walk->mm, addr);
-			pme = make_pme(PM_NOT_PRESENT(pm->v2));
+			if (vma && (vma->vm_flags & VM_SOFTDIRTY))
+				flags2 = __PM_SOFT_DIRTY;
+			else
+				flags2 = 0;
+			pme = make_pme(PM_NOT_PRESENT(pm->v2) | PM_STATUS2(pm->v2, flags2));
 		}
 
 		/* check that 'vma' actually covers this address,
@@ -1044,13 +1058,15 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 
 #ifdef CONFIG_HUGETLB_PAGE
 static void huge_pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
-					pte_t pte, int offset)
+					pte_t pte, int offset, int flags2)
 {
 	if (pte_present(pte))
-		*pme = make_pme(PM_PFRAME(pte_pfn(pte) + offset)
-				| PM_STATUS2(pm->v2, 0) | PM_PRESENT);
+		*pme = make_pme(PM_PFRAME(pte_pfn(pte) + offset)	|
+				PM_STATUS2(pm->v2, flags2)		|
+				PM_PRESENT);
 	else
-		*pme = make_pme(PM_NOT_PRESENT(pm->v2));
+		*pme = make_pme(PM_NOT_PRESENT(pm->v2)			|
+				PM_STATUS2(pm->v2, flags2));
 }
 
 /* This function walks within one hugetlb entry in the single call */
@@ -1059,12 +1075,22 @@ static int pagemap_hugetlb_range(pte_t *pte, unsigned long hmask,
 				 struct mm_walk *walk)
 {
 	struct pagemapread *pm = walk->private;
+	struct vm_area_struct *vma;
 	int err = 0;
+	int flags2;
 	pagemap_entry_t pme;
 
+	vma = find_vma(walk->mm, addr);
+	WARN_ON_ONCE(!vma);
+
+	if (vma && (vma->vm_flags & VM_SOFTDIRTY))
+		flags2 = __PM_SOFT_DIRTY;
+	else
+		flags2 = 0;
+
 	for (; addr != end; addr += PAGE_SIZE) {
 		int offset = (addr & ~hmask) >> PAGE_SHIFT;
-		huge_pte_to_pagemap_entry(&pme, pm, *pte, offset);
+		huge_pte_to_pagemap_entry(&pme, pm, *pte, offset, flags2);
 		err = add_to_pagemap(addr, &pme, pm);
 		if (err)
 			return err;

include/linux/mm.h

@@ -115,6 +115,12 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_ARCH_1	0x01000000	/* Architecture-specific flag */
 #define VM_DONTDUMP	0x04000000	/* Do not include in the core dump */
 
+#ifdef CONFIG_MEM_SOFT_DIRTY
+# define VM_SOFTDIRTY	0x08000000	/* Not soft dirty clean area */
+#else
+# define VM_SOFTDIRTY	0
+#endif
+
 #define VM_MIXEDMAP	0x10000000	/* Can contain "struct page" and pure PFN pages */
 #define VM_HUGEPAGE	0x20000000	/* MADV_HUGEPAGE marked this vma */
 #define VM_NOHUGEPAGE	0x40000000	/* MADV_NOHUGEPAGE marked this vma */

mm/mmap.c

@@ -1609,6 +1609,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	if (file)
 		uprobe_mmap(vma);
 
+	/*
+	 * New (or expanded) vma always get soft dirty status.
+	 * Otherwise user-space soft-dirty page tracker won't
+	 * be able to distinguish situation when vma area unmapped,
+	 * then new mapped in-place (which must be aimed as
+	 * a completely new data area).
+	 */
+	vma->vm_flags |= VM_SOFTDIRTY;
+
 	return addr;
 
 unmap_and_free_vma:
@@ -2652,6 +2661,7 @@ static unsigned long do_brk(unsigned long addr, unsigned long len)
 	mm->total_vm += len >> PAGE_SHIFT;
 	if (flags & VM_LOCKED)
 		mm->locked_vm += (len >> PAGE_SHIFT);
+	vma->vm_flags |= VM_SOFTDIRTY;
 	return addr;
 }
 
@@ -2916,7 +2926,7 @@ int install_special_mapping(struct mm_struct *mm,
 	vma->vm_start = addr;
 	vma->vm_end = addr + len;
 
-	vma->vm_flags = vm_flags | mm->def_flags | VM_DONTEXPAND;
+	vma->vm_flags = vm_flags | mm->def_flags | VM_DONTEXPAND | VM_SOFTDIRTY;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 
 	vma->vm_ops = &special_mapping_vmops;