mempolicy: sanitize the usage of get_task_policy()

Cleanup + preparation. Every user of get_task_policy() calls it
unconditionally, even if it is not going to use the result.

get_task_policy() is cheap but still this does not look clean, plus
the code looks simpler if get_task_policy() is called only when this
is really needed.

Note: I hope this is correct, but it is not clear why vma_policy_mof()
doesn't fall back to get_task_policy() if ->get_policy() returns NULL.
Signed-off-by: NOleg Nesterov <oleg@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1621,14 +1621,11 @@ COMPAT_SYSCALL_DEFINE6(mbind, compat_ulong_t, start, compat_ulong_t, len,
 struct mempolicy *get_vma_policy(struct task_struct *task,
 		struct vm_area_struct *vma, unsigned long addr)
 {
-	struct mempolicy *pol = get_task_policy(task);
+	struct mempolicy *pol = NULL;
 
 	if (vma) {
 		if (vma->vm_ops && vma->vm_ops->get_policy) {
-			struct mempolicy *vpol = vma->vm_ops->get_policy(vma,
-									addr);
-			if (vpol)
-				pol = vpol;
+			pol = vma->vm_ops->get_policy(vma, addr);
 		} else if (vma->vm_policy) {
 			pol = vma->vm_policy;
 
@@ -1643,12 +1640,15 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
 		}
 	}
 
+	if (!pol)
+		pol = get_task_policy(task);
+
 	return pol;
 }
 
 bool vma_policy_mof(struct task_struct *task, struct vm_area_struct *vma)
 {
-	struct mempolicy *pol = get_task_policy(task);
+	struct mempolicy *pol = NULL;
 
 	if (vma) {
 		if (vma->vm_ops && vma->vm_ops->get_policy) {
@@ -1660,11 +1660,14 @@ bool vma_policy_mof(struct task_struct *task, struct vm_area_struct *vma)
 			mpol_cond_put(pol);
 
 			return ret;
-		} else if (vma->vm_policy) {
-			pol = vma->vm_policy;
 		}
+
+		pol = vma->vm_policy;
 	}
 
+	if (!pol)
+		pol = get_task_policy(task);
+
 	return pol->flags & MPOL_F_MOF;
 }
 
@@ -2068,12 +2071,12 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
  */
 struct page *alloc_pages_current(gfp_t gfp, unsigned order)
 {
-	struct mempolicy *pol = get_task_policy(current);
+	struct mempolicy *pol = &default_policy;
 	struct page *page;
 	unsigned int cpuset_mems_cookie;
 
-	if (in_interrupt() || (gfp & __GFP_THISNODE))
-		pol = &default_policy;
+	if (!in_interrupt() && !(gfp & __GFP_THISNODE))
+		pol = get_task_policy(current);
 
 retry_cpuset:
 	cpuset_mems_cookie = read_mems_allowed_begin();