mempolicy: kill do_set_mempolicy()->down_write(&mm->mmap_sem)

Remove down_write(&mm->mmap_sem) in do_set_mempolicy(). This logic
was never correct and it is no longer needed, see the previous patch.

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>

mm/mempolicy.c

@@ -802,7 +802,6 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags,
 			     nodemask_t *nodes)
 {
 	struct mempolicy *new, *old;
-	struct mm_struct *mm = current->mm;
 	NODEMASK_SCRATCH(scratch);
 	int ret;
 
@@ -814,20 +813,11 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags,
 		ret = PTR_ERR(new);
 		goto out;
 	}
-	/*
-	 * prevent changing our mempolicy while show_numa_maps()
-	 * is using it.
-	 * Note:  do_set_mempolicy() can be called at init time
-	 * with no 'mm'.
-	 */
-	if (mm)
-		down_write(&mm->mmap_sem);
+
 	task_lock(current);
 	ret = mpol_set_nodemask(new, nodes, scratch);
 	if (ret) {
 		task_unlock(current);
-		if (mm)
-			up_write(&mm->mmap_sem);
 		mpol_put(new);
 		goto out;
 	}
@@ -837,9 +827,6 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags,
 	    nodes_weight(new->v.nodes))
 		current->il_next = first_node(new->v.nodes);
 	task_unlock(current);
-	if (mm)
-		up_write(&mm->mmap_sem);
-
 	mpol_put(old);
 	ret = 0;
 out: