mm: softdirty: addresses before VMAs in PTE holes aren't softdirty

In PTE holes that contain VM_SOFTDIRTY VMAs, unmapped addresses before
VM_SOFTDIRTY VMAs are reported as softdirty by /proc/pid/pagemap.  This
bug was introduced in commit 68b5a6524856 ("mm: softdirty: respect
VM_SOFTDIRTY in PTE holes").  That commit made /proc/pid/pagemap look at
VM_SOFTDIRTY in PTE holes but neglected to observe the start of VMAs
returned by find_vma.

Tested:
  Wrote a selftest that creates a PMD-sized VMA then unmaps the first
  page and asserts that the page is not softdirty. I'm going to send the
  pagemap selftest in a later commit.

Signed-off-by: Peter Feiner <pfeiner@google.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Jamie Liu <jamieliu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -931,23 +931,32 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
 	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;
+		/* End of address space hole, which we mark as non-present. */
+		unsigned long hole_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);
+		if (vma)
+			hole_end = min(end, vma->vm_start);
+		else
+			hole_end = end;
+
+		for (; addr < hole_end; addr += PAGE_SIZE) {
+			err = add_to_pagemap(addr, &pme, pm);
+			if (err)
+				goto out;
 		}
 
-		for (; addr < vm_end; addr += PAGE_SIZE) {
+		if (!vma)
+			break;
+
+		/* Addresses in the VMA. */
+		if (vma->vm_flags & VM_SOFTDIRTY)
+			pme.pme |= PM_STATUS2(pm->v2, __PM_SOFT_DIRTY);
+		for (; addr < min(end, vma->vm_end); addr += PAGE_SIZE) {
 			err = add_to_pagemap(addr, &pme, pm);
 			if (err)
 				goto out;
 		}
 	}
-
 out:
 	return err;
 }