mm: pagewalk: fix misbehavior of walk_page_range for vma(VM_PFNMAP)

walk_page_range() silently skips vma having VM_PFNMAP set, which leads to
undesirable behaviour at client end (who called walk_page_range).  For
example for pagemap_read(), when no callbacks are called against VM_PFNMAP
vma, pagemap_read() may prepare pagemap data for next virtual address
range at wrong index.  That could confuse and/or break userspace
applications.

This patch avoid this misbehavior caused by vma(VM_PFNMAP) like follows:
- for pagemap_read() which has its own ->pte_hole(), call the ->pte_hole()
  over vma(VM_PFNMAP),
- for clear_refs and queue_pages which have their own ->tests_walk,
  just return 1 and skip vma(VM_PFNMAP). This is no problem because
  these are not interested in hole regions,
- for other callers, just skip the vma(VM_PFNMAP) as a default behavior.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 8 deletions

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 4c9a653ba563..75c1f2878519 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -35,7 +35,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
         do {
 again:
                 next = pmd_addr_end(addr, end);
-                if (pmd_none(*pmd)) {
+                if (pmd_none(*pmd) || !walk->vma) {
                         if (walk->pte_hole)
                                 err = walk->pte_hole(addr, next, walk);
                         if (err)
@@ -165,9 +165,6 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
  * or skip it via the returned value. Return 0 if we do walk over the
  * current vma, and return 1 if we skip the vma. Negative values means
  * error, where we abort the current walk.
- *
- * Default check (only VM_PFNMAP check for now) is used when the caller
- * doesn't define test_walk() callback.
  */
 static int walk_page_test(unsigned long start, unsigned long end,
                         struct mm_walk *walk)
@@ -178,11 +175,19 @@ static int walk_page_test(unsigned long start, unsigned long end,
                 return walk->test_walk(start, end, walk);
 
         /*
-         * Do not walk over vma(VM_PFNMAP), because we have no valid struct
-         * page backing a VM_PFNMAP range. See also commit a9ff785e4437.
+         * vma(VM_PFNMAP) doesn't have any valid struct pages behind VM_PFNMAP
+         * range, so we don't walk over it as we do for normal vmas. However,
+         * Some callers are interested in handling hole range and they don't
+         * want to just ignore any single address range. Such users certainly
+         * define their ->pte_hole() callbacks, so let's delegate them to handle
+         * vma(VM_PFNMAP).
          */
-        if (vma->vm_flags & VM_PFNMAP)
-                return 1;
+        if (vma->vm_flags & VM_PFNMAP) {
+                int err = 1;
+                if (walk->pte_hole)
+                        err = walk->pte_hole(start, end, walk);
+                return err ? err : 1;
+        }
         return 0;
 }