writeback: do uninterruptible sleep in balance_dirty_pages()

Using TASK_INTERRUPTIBLE in balance_dirty_pages() seems wrong. If it's
going to do that then it must break out if signal_pending(), otherwise
it's pretty much guaranteed to degenerate into a busywait loop. Plus we
*do* want these processes to appear in D state and to contribute to load
average.

So it should be TASK_UNINTERRUPTIBLE. -- Andrew Morton

Signed-off-by: Wu Fengguang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page-writeback.c

···
 				break;		/* We've done our duty */
 		}
 		trace_wbc_balance_dirty_wait(&wbc, bdi);
-		__set_current_state(TASK_INTERRUPTIBLE);
+		__set_current_state(TASK_UNINTERRUPTIBLE);
 		io_schedule_timeout(pause);
 
 		/*