writeback: pass in '0' for nr_pages writeback in laptop mode

Laptop mode really wants to writeback the number of dirty
pages and inodes. Instead of calculating this in the caller,
just pass in 0 and let wakeup_flusher_threads() handle it.

Use the new wakeup_flusher_threads_bdi() instead of rolling
our own.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Chris Mason <clm@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/page-writeback.c

@@ -1980,23 +1980,8 @@ int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
 void laptop_mode_timer_fn(unsigned long data)
 {
 	struct request_queue *q = (struct request_queue *)data;
-	int nr_pages = global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS);
-	struct bdi_writeback *wb;
 
-	/*
-	 * We want to write everything out, not just down to the dirty
-	 * threshold
-	 */
-	if (!bdi_has_dirty_io(q->backing_dev_info))
-		return;
-
-	rcu_read_lock();
-	list_for_each_entry_rcu(wb, &q->backing_dev_info->wb_list, bdi_node)
-		if (wb_has_dirty_io(wb))
-			wb_start_writeback(wb, nr_pages,
-						WB_REASON_LAPTOP_TIMER);
-	rcu_read_unlock();
+	wakeup_flusher_threads_bdi(q->backing_dev_info, WB_REASON_LAPTOP_TIMER);
 }
 
 /*