mm: softdirty: respect VM_SOFTDIRTY in PTE holes

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>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 6 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index cfa63ee92c96..dfc791c42d64 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;
 }