writeback: make bdi_start_background_writeback() take bdi_writeback instead of backing_dev_info

bdi_start_background_writeback() currently takes @bdi and kicks the
root wb (bdi_writeback).  In preparation for cgroup writeback support,
make it take wb instead.

This patch doesn't make any functional difference.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/fs-writeback.c

@@ -228,23 +228,23 @@ void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
 }
 
 /**
- * bdi_start_background_writeback - start background writeback
- * @bdi: the backing device to write from
+ * wb_start_background_writeback - start background writeback
+ * @wb: bdi_writback to write from
  *
  * Description:
  *   This makes sure WB_SYNC_NONE background writeback happens. When
- *   this function returns, it is only guaranteed that for given BDI
+ *   this function returns, it is only guaranteed that for given wb
  *   some IO is happening if we are over background dirty threshold.
  *   Caller need not hold sb s_umount semaphore.
  */
-void bdi_start_background_writeback(struct backing_dev_info *bdi)
+void wb_start_background_writeback(struct bdi_writeback *wb)
 {
 	/*
 	 * We just wake up the flusher thread. It will perform background
 	 * writeback as soon as there is no other work to do.
 	 */
-	trace_writeback_wake_background(bdi);
-	wb_wakeup(&bdi->wb);
+	trace_writeback_wake_background(wb->bdi);
+	wb_wakeup(wb);
 }
 
 /*

include/linux/backing-dev.h

@@ -27,7 +27,7 @@ void bdi_unregister(struct backing_dev_info *bdi);
 int __must_check bdi_setup_and_register(struct backing_dev_info *, char *);
 void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
 			bool range_cyclic, enum wb_reason reason);
-void bdi_start_background_writeback(struct backing_dev_info *bdi);
+void wb_start_background_writeback(struct bdi_writeback *wb);
 void wb_workfn(struct work_struct *work);
 void wb_wakeup_delayed(struct bdi_writeback *wb);
 

mm/page-writeback.c

@@ -1456,7 +1456,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 		}
 
 		if (unlikely(!writeback_in_progress(wb)))
-			bdi_start_background_writeback(bdi);
+			wb_start_background_writeback(wb);
 
 		if (!strictlimit)
 			wb_dirty_limits(wb, dirty_thresh, background_thresh,
@@ -1588,7 +1588,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 		return;
 
 	if (nr_reclaimable > background_thresh)
-		bdi_start_background_writeback(bdi);
+		wb_start_background_writeback(wb);
 }
 
 static DEFINE_PER_CPU(int, bdp_ratelimits);