hugetlb: fix get_policy for stacked shared memory files

Here's another breakage as a result of shared memory stacked files :(

The NUMA policy for a VMA is determined by checking the following (in the
order given):

1) vma->vm_ops->get_policy() (if defined)
2) vma->vm_policy (if defined)
3) task->mempolicy (if defined)
4) Fall back to default_policy

By switching to stacked files for shared memory, get_policy() is now always
set to shm_get_policy which is a wrapper function.  This causes us to stop
at step 1, which yields NULL for hugetlb instead of task->mempolicy which
was the previous (and correct) result.

This patch modifies the shm_get_policy() wrapper to maintain steps 1-3 for
the wrapped vm_ops.

(akpm: the refcounting of mempolicies is busted and this patch does nothing to
improve it)

Signed-off-by: Adam Litke <[email protected]>
Acked-by: William Irwin <[email protected]>
Cc: dean gaudet <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

ipc/shm.c

@@ -254,8 +254,10 @@ struct mempolicy *shm_get_policy(struct vm_area_struct *vma, unsigned long addr)
 
 	if (sfd->vm_ops->get_policy)
 		pol = sfd->vm_ops->get_policy(vma, addr);
-	else
+	else if (vma->vm_policy)
 		pol = vma->vm_policy;
+	else
+		pol = current->mempolicy;
 	return pol;
 }
 #endif