writeback: move nr_pages == 0 logic to one location

Now that we have no external callers of wb_start_writeback(), we
can shuffle the passing in of 'nr_pages'. Everybody passes in 0
at this point, so just kill the argument and move the dirty
count retrieval to that function.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Chris Mason <clm@fb.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/fs-writeback.c

@@ -933,8 +933,18 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 
 #endif	/* CONFIG_CGROUP_WRITEBACK */
 
-static void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
-				enum wb_reason reason)
+/*
+ * Add in the number of potentially dirty inodes, because each inode
+ * write can dirty pagecache in the underlying blockdev.
+ */
+static unsigned long get_nr_dirty_pages(void)
+{
+	return global_node_page_state(NR_FILE_DIRTY) +
+		global_node_page_state(NR_UNSTABLE_NFS) +
+		get_nr_dirty_inodes();
+}
+
+static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)
 {
 	struct wb_writeback_work *work;
 
@@ -954,7 +964,7 @@ static void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
 	}
 
 	work->sync_mode	= WB_SYNC_NONE;
-	work->nr_pages	= nr_pages;
+	work->nr_pages	= wb_split_bdi_pages(wb, get_nr_dirty_pages());
 	work->range_cyclic = 1;
 	work->reason	= reason;
 	work->auto_free	= 1;
@@ -1814,17 +1824,6 @@ static struct wb_writeback_work *get_next_work_item(struct bdi_writeback *wb)
 	return work;
 }
 
-/*
- * Add in the number of potentially dirty inodes, because each inode
- * write can dirty pagecache in the underlying blockdev.
- */
-static unsigned long get_nr_dirty_pages(void)
-{
-	return global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS) +
-		get_nr_dirty_inodes();
-}
-
 static long wb_check_background_flush(struct bdi_writeback *wb)
 {
 	if (wb_over_bg_thresh(wb)) {
@@ -1951,7 +1950,7 @@ void wb_workfn(struct work_struct *work)
  * write back the whole world.
  */
 static void __wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
-					 long nr_pages, enum wb_reason reason)
+					 enum wb_reason reason)
 {
 	struct bdi_writeback *wb;
 
@@ -1959,17 +1958,14 @@ static void __wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
 		return;
 
 	list_for_each_entry_rcu(wb, &bdi->wb_list, bdi_node)
-		wb_start_writeback(wb, wb_split_bdi_pages(wb, nr_pages),
-					reason);
+		wb_start_writeback(wb, reason);
 }
 
 void wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
 				enum wb_reason reason)
 {
-	long nr_pages = get_nr_dirty_pages();
-
 	rcu_read_lock();
-	__wakeup_flusher_threads_bdi(bdi, nr_pages, reason);
+	__wakeup_flusher_threads_bdi(bdi, reason);
 	rcu_read_unlock();
 }
 
@@ -1979,7 +1975,6 @@ void wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
 void wakeup_flusher_threads(enum wb_reason reason)
 {
 	struct backing_dev_info *bdi;
-	long nr_pages;
 
 	/*
 	 * If we are expecting writeback progress we must submit plugged IO.
@@ -1987,11 +1982,9 @@ void wakeup_flusher_threads(enum wb_reason reason)
 	if (blk_needs_flush_plug(current))
 		blk_schedule_flush_plug(current);
 
-	nr_pages = get_nr_dirty_pages();
-
 	rcu_read_lock();
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
-		__wakeup_flusher_threads_bdi(bdi, nr_pages, reason);
+		__wakeup_flusher_threads_bdi(bdi, reason);
 	rcu_read_unlock();
 }