mm,vmacache: optimize overflow system-wide flushing

For single threaded workloads, we can avoid flushing and iterating through
the entire list of tasks, making the whole function a lot faster,
requiring only a single atomic read for the mm_users.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmacache.c

@@ -17,6 +17,16 @@ void vmacache_flush_all(struct mm_struct *mm)
 {
 	struct task_struct *g, *p;
 
+	/*
+	 * Single threaded tasks need not iterate the entire
+	 * list of process. We can avoid the flushing as well
+	 * since the mm's seqnum was increased and don't have
+	 * to worry about other threads' seqnum. Current's
+	 * flush will occur upon the next lookup.
+	 */
+	if (atomic_read(&mm->mm_users) == 1)
+		return;
+
 	rcu_read_lock();
 	for_each_process_thread(g, p) {
 		/*