mempolicy: fix reference counting bugs

Address 3 known bugs in the current memory policy reference counting method.
I have a series of patches to rework the reference counting to reduce overhead
in the allocation path.  However, that series will require testing in -mm once
I repost it.

1) alloc_page_vma() does not release the extra reference taken for
   vma/shared mempolicy when the mode == MPOL_INTERLEAVE.  This can result in
   leaking mempolicy structures.  This is probably occurring, but not being
   noticed.

   Fix:  add the conditional release of the reference.

2) hugezonelist unconditionally releases a reference on the mempolicy when
   mode == MPOL_INTERLEAVE.  This can result in decrementing the reference
   count for system default policy [should have no ill effect] or premature
   freeing of task policy.  If this occurred, the next allocation using task
   mempolicy would use the freed structure and probably BUG out.

   Fix:  add the necessary check to the release.

3) The current reference counting method assumes that vma 'get_policy()'
   methods automatically add an extra reference a non-NULL returned mempolicy.
    This is true for shmem_get_policy() used by tmpfs mappings, including
   regular page shm segments.  However, SHM_HUGETLB shm's, backed by
   hugetlbfs, just use the vma policy without the extra reference.  This
   results in freeing of the vma policy on the first allocation, with reuse of
   the freed mempolicy structure on subsequent allocations.

   Fix: Rather than add another condition to the conditional reference
   release, which occur in the allocation path, just add a reference when
   returning the vma policy in shm_get_policy() to match the assumptions.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Greg KH <greg@kroah.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: David Rientjes <rientjes@google.com>
Cc: <eric.whitney@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -271,9 +271,10 @@ static struct mempolicy *shm_get_policy(struct vm_area_struct *vma,
 
 	if (sfd->vm_ops->get_policy)
 		pol = sfd->vm_ops->get_policy(vma, addr);
-	else if (vma->vm_policy)
+	else if (vma->vm_policy) {
 		pol = vma->vm_policy;
-	else
+		mpol_get(pol);	/* get_vma_policy() expects this */
+	} else
 		pol = current->mempolicy;
 	return pol;
 }

mm/mempolicy.c

@@ -1296,7 +1296,9 @@ struct zonelist *huge_zonelist(struct vm_area_struct *vma, unsigned long addr,
 		unsigned nid;
 
 		nid = interleave_nid(pol, vma, addr, HPAGE_SHIFT);
-		__mpol_free(pol);		/* finished with pol */
+		if (unlikely(pol != &default_policy &&
+				pol != current->mempolicy))
+			__mpol_free(pol);	/* finished with pol */
 		return NODE_DATA(nid)->node_zonelists + gfp_zone(gfp_flags);
 	}
 
@@ -1360,6 +1362,9 @@ alloc_page_vma(gfp_t gfp, struct vm_area_struct *vma, unsigned long addr)
 		unsigned nid;
 
 		nid = interleave_nid(pol, vma, addr, PAGE_SHIFT);
+		if (unlikely(pol != &default_policy &&
+				pol != current->mempolicy))
+			__mpol_free(pol);	/* finished with pol */
 		return alloc_page_interleave(gfp, 0, nid);
 	}
 	zl = zonelist_policy(gfp, pol);