revert "mm: mempolicy: Let vma_merge and vma_split handle vma->vm_policy linkages"

Commit 05f144a0d5c2 ("mm: mempolicy: Let vma_merge and vma_split handle
vma->vm_policy linkages") removed vma->vm_policy updates code but it is
the purpose of mbind_range().  Now, mbind_range() is virtually a no-op
and while it does not allow memory corruption it is not the right fix.
This patch is a revert.

[mgorman@suse.de: Edited changelog]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: Josh Boyer <jwboyer@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -607,6 +607,27 @@ check_range(struct mm_struct *mm, unsigned long start, unsigned long end,
 	return first;
 }
 
+/* Apply policy to a single VMA */
+static int policy_vma(struct vm_area_struct *vma, struct mempolicy *new)
+{
+	int err = 0;
+	struct mempolicy *old = vma->vm_policy;
+
+	pr_debug("vma %lx-%lx/%lx vm_ops %p vm_file %p set_policy %p\n",
+		 vma->vm_start, vma->vm_end, vma->vm_pgoff,
+		 vma->vm_ops, vma->vm_file,
+		 vma->vm_ops ? vma->vm_ops->set_policy : NULL);
+
+	if (vma->vm_ops && vma->vm_ops->set_policy)
+		err = vma->vm_ops->set_policy(vma, new);
+	if (!err) {
+		mpol_get(new);
+		vma->vm_policy = new;
+		mpol_put(old);
+	}
+	return err;
+}
+
 /* Step 2: apply policy to a range and do splits. */
 static int mbind_range(struct mm_struct *mm, unsigned long start,
 		       unsigned long end, struct mempolicy *new_pol)
@@ -655,23 +676,9 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
 			if (err)
 				goto out;
 		}
-
-		/*
-		 * Apply policy to a single VMA. The reference counting of
-		 * policy for vma_policy linkages has already been handled by
-		 * vma_merge and split_vma as necessary. If this is a shared
-		 * policy then ->set_policy will increment the reference count
-		 * for an sp node.
-		 */
-		pr_debug("vma %lx-%lx/%lx vm_ops %p vm_file %p set_policy %p\n",
-			vma->vm_start, vma->vm_end, vma->vm_pgoff,
-			vma->vm_ops, vma->vm_file,
-			vma->vm_ops ? vma->vm_ops->set_policy : NULL);
-		if (vma->vm_ops && vma->vm_ops->set_policy) {
-			err = vma->vm_ops->set_policy(vma, new_pol);
-			if (err)
-				goto out;
-		}
+		err = policy_vma(vma, new_pol);
+		if (err)
+			goto out;
 	}
 
  out: