mm: do not drain pagevecs for mlockall(MCL_FUTURE)

MCL_FUTURE does not move pages between lru list and draining the LRU per
cpu pagevecs is a nasty activity.  Avoid doing it unecessarily.

Signed-off-by: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Acked-by: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mlock.c

@@ -549,7 +549,8 @@ SYSCALL_DEFINE1(mlockall, int, flags)
 	if (!can_do_mlock())
 		goto out;
 
-	lru_add_drain_all();	/* flush pagevec */
+	if (flags & MCL_CURRENT)
+		lru_add_drain_all();	/* flush pagevec */
 
 	down_write(&current->mm->mmap_sem);