mm: vma_merge: correct false positive from __vma_unlink->validate_mm_rb

The old code was always doing:

   vma->vm_end = next->vm_end
   vma_rb_erase(next) // in __vma_unlink
   vma->vm_next = next->vm_next // in __vma_unlink
   next = vma->vm_next
   vma_gap_update(next)

The new code still does the above for remove_next == 1 and 2, but for
remove_next == 3 it has been changed and it does:

   next->vm_start = vma->vm_start
   vma_rb_erase(vma) // in __vma_unlink
   vma_gap_update(next)

In the latter case, while unlinking "vma", validate_mm_rb() is told to
ignore "vma" that is being removed, but next->vm_start was reduced
instead. So for the new case, to avoid the false positive from
validate_mm_rb, it should be "next" that is ignored when "vma" is
being unlinked.

"vma" and "next" in the above comment, considered pre-swap().

Link: http://lkml.kernel.org/r/1474492522-2261-4-git-send-email-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Tested-by: Shaun Tancheff <shaun.tancheff@seagate.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Jan Vorlicek <janvorli@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 41 insertions, 18 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 4dc65be4766f..1af87c14183d 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -402,15 +402,9 @@ static inline void vma_rb_insert(struct vm_area_struct *vma,
         rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
 }
 
-static void vma_rb_erase(struct vm_area_struct *vma, struct rb_root *root)
+static void __vma_rb_erase(struct vm_area_struct *vma, struct rb_root *root)
 {
         /*
-         * All rb_subtree_gap values must be consistent prior to erase,
-         * with the possible exception of the vma being erased.
-         */
-        validate_mm_rb(root, vma);
-
-        /*
          * Note rb_erase_augmented is a fairly large inline function,
          * so make sure we instantiate it only once with our desired
          * augmented rbtree callbacks.
@@ -418,6 +412,32 @@ static void vma_rb_erase(struct vm_area_struct *vma, struct rb_root *root)
         rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
 }
 
+static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
+                                                struct rb_root *root,
+                                                struct vm_area_struct *ignore)
+{
+        /*
+         * All rb_subtree_gap values must be consistent prior to erase,
+         * with the possible exception of the "next" vma being erased if
+         * next->vm_start was reduced.
+         */
+        validate_mm_rb(root, ignore);
+
+        __vma_rb_erase(vma, root);
+}
+
+static __always_inline void vma_rb_erase(struct vm_area_struct *vma,
+                                         struct rb_root *root)
+{
+        /*
+         * All rb_subtree_gap values must be consistent prior to erase,
+         * with the possible exception of the vma being erased.
+         */
+        validate_mm_rb(root, vma);
+
+        __vma_rb_erase(vma, root);
+}
+
 /*
  * vma has some anon_vma assigned, and is already inserted on that
  * anon_vma's interval trees.
@@ -604,11 +624,12 @@ static void __insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
 static __always_inline void __vma_unlink_common(struct mm_struct *mm,
                                                 struct vm_area_struct *vma,
                                                 struct vm_area_struct *prev,
-                                                bool has_prev)
+                                                bool has_prev,
+                                                struct vm_area_struct *ignore)
 {
         struct vm_area_struct *next;
 
-        vma_rb_erase(vma, &mm->mm_rb);
+        vma_rb_erase_ignore(vma, &mm->mm_rb, ignore);
         next = vma->vm_next;
         if (has_prev)
                 prev->vm_next = next;
@@ -630,13 +651,7 @@ static inline void __vma_unlink_prev(struct mm_struct *mm,
                                      struct vm_area_struct *vma,
                                      struct vm_area_struct *prev)
 {
-        __vma_unlink_common(mm, vma, prev, true);
-}
-
-static inline void __vma_unlink(struct mm_struct *mm,
-                                struct vm_area_struct *vma)
-{
-        __vma_unlink_common(mm, vma, NULL, false);
+        __vma_unlink_common(mm, vma, prev, true, vma);
 }
 
 /*
@@ -815,8 +830,16 @@ again:
                 if (remove_next != 3)
                         __vma_unlink_prev(mm, next, vma);
                 else
-                        /* vma is not before next if they've been swapped */
-                        __vma_unlink(mm, next);
+                        /*
+                         * vma is not before next if they've been
+                         * swapped.
+                         *
+                         * pre-swap() next->vm_start was reduced so
+                         * tell validate_mm_rb to ignore pre-swap()
+                         * "next" (which is stored in post-swap()
+                         * "vma").
+                         */
+                        __vma_unlink_common(mm, next, NULL, false, vma);
                 if (file)
                         __remove_shared_vm_struct(next, file, mapping);
         } else if (insert) {