mempolicy: remove the "task" arg of vma_policy_mof() and simplify it

1. vma_policy_mof(task) is simply not safe unless task == current,
   it can race with do_exit()->mpol_put(). Remove this arg and update
   its single caller.

2. vma can not be NULL, remove this check and simplify the code.
Signed-off-by: Oleg 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mempolicy.h

@@ -136,7 +136,7 @@ struct mempolicy *mpol_shared_policy_lookup(struct shared_policy *sp,
 
 struct mempolicy *get_vma_policy(struct task_struct *tsk,
 		struct vm_area_struct *vma, unsigned long addr);
-bool vma_policy_mof(struct task_struct *task, struct vm_area_struct *vma);
+bool vma_policy_mof(struct vm_area_struct *vma);
 
 extern void numa_default_policy(void);
 extern void numa_policy_init(void);

kernel/sched/fair.c

@@ -1946,7 +1946,7 @@ void task_numa_work(struct callback_head *work)
 		vma = mm->mmap;
 	}
 	for (; vma; vma = vma->vm_next) {
-		if (!vma_migratable(vma) || !vma_policy_mof(p, vma))
+		if (!vma_migratable(vma) || !vma_policy_mof(vma))
 			continue;
 
 		/*

mm/mempolicy.c

@@ -1646,11 +1646,10 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
 	return pol;
 }
 
-bool vma_policy_mof(struct task_struct *task, struct vm_area_struct *vma)
+bool vma_policy_mof(struct vm_area_struct *vma)
 {
-	struct mempolicy *pol = NULL;
+	struct mempolicy *pol;
 
-	if (vma) {
 	if (vma->vm_ops && vma->vm_ops->get_policy) {
 		bool ret = false;
 
@@ -1663,10 +1662,8 @@ bool vma_policy_mof(struct task_struct *task, struct vm_area_struct *vma)
 	}
 
 	pol = vma->vm_policy;
-	}
-
 	if (!pol)
-		pol = get_task_policy(task);
+		pol = get_task_policy(current);
 
 	return pol->flags & MPOL_F_MOF;
 }