[PATCH] mm: softdirty: respect VM_SOFTDIRTY in PTE holes

[Peter Feiner <pfeiner@google.com>]

After a VMA is created with the VM_SOFTDIRTY flag set,
/proc/pid/pagemap should report that the VMA's virtual pages are
soft-dirty until VM_SOFTDIRTY is cleared (i.e., by the next write of
"4" to /proc/pid/clear_refs). However, pagemap ignores the
VM_SOFTDIRTY flag for virtual addresses that fall in PTE holes (i.e.,
virtual addresses that don't have a PMD, PUD, or PGD allocated yet).

To observe this bug, use mmap to create a VMA large enough such that
there's a good chance that the VMA will occupy an unused PMD, then
test the soft-dirty bit on its pages. In practice, I found that a VMA
that covered a PMD's worth of address space was big enough.

This patch adds the necessary VMA lookup to the PTE hole callback in
/proc/pid/pagemap's page walk and sets soft-dirty according to the
VMAs' VM_SOFTDIRTY flag.

Signed-off-by: Peter Feiner <pfeiner@google.com>
---
 fs/proc/task_mmu.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index cfa63ee..dfc791c 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -925,15 +925,30 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
 				struct mm_walk *walk)
 {
 	struct pagemapread *pm = walk->private;
-	unsigned long addr;
+	unsigned long addr = start;
 	int err = 0;
-	pagemap_entry_t pme = make_pme(PM_NOT_PRESENT(pm->v2));
 
-	for (addr = start; addr < end; addr += PAGE_SIZE) {
-		err = add_to_pagemap(addr, &pme, pm);
-		if (err)
-			break;
+	while (addr < end) {
+		struct vm_area_struct *vma = find_vma(walk->mm, addr);
+		pagemap_entry_t pme = make_pme(PM_NOT_PRESENT(pm->v2));
+		unsigned long vm_end;
+
+		if (!vma) {
+			vm_end = end;
+		} else {
+			vm_end = min(end, vma->vm_end);
+			if (vma->vm_flags & VM_SOFTDIRTY)
+				pme.pme |= PM_STATUS2(pm->v2, __PM_SOFT_DIRTY);
+		}
+
+		for (; addr < vm_end; addr += PAGE_SIZE) {
+			err = add_to_pagemap(addr, &pme, pm);
+			if (err)
+				goto out;
+		}
 	}
+
+out:
 	return err;
 }
 
-- 
2.0.0.526.g5318336

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