ring-buffer: only warn on wrap if buffer is bigger than two pages

On boot up, to save memory, ftrace allocates the minimum buffer
which is two pages. Ftrace also goes through a series of tests
(when configured) on boot up. These tests can fill up a page within
a single interrupt.

The ring buffer also has a WARN_ON when it detects that the buffer was
completely filled within a single commit (other commits are allowed to
be nested).

Combine the small buffer on start up, with the tests that can fill more
than a single page within an interrupt, this can trigger the WARN_ON.

This patch makes the WARN_ON only happen when the ring buffer consists
of more than two pages.

[ Impact: prevent false WARN_ON in ftrace startup tests ]
Reported-by: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <20090421094616.GA14561@elte.hu>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/ring_buffer.c

@@ -1241,7 +1241,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 		 * about it.
 		 */
 		if (unlikely(next_page == commit_page)) {
-			WARN_ON_ONCE(1);
+			/* This can easily happen on small ring buffers */
+			WARN_ON_ONCE(buffer->pages > 2);
 			goto out_reset;
 		}