writeback: add comments to bdi_work structure

And document its retriever, get_next_work_item().
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/fs-writeback.c

@@ -49,15 +49,15 @@ struct wb_writeback_args {
  * Work items for the bdi_writeback threads
  */
 struct bdi_work {
-	struct list_head list;
-	struct rcu_head rcu_head;
+	struct list_head list;		/* pending work list */
+	struct rcu_head rcu_head;	/* for RCU free/clear of work */
 
-	unsigned long seen;
-	atomic_t pending;
+	unsigned long seen;		/* threads that have seen this work */
+	atomic_t pending;		/* number of threads still to do work */
 
-	struct wb_writeback_args args;
+	struct wb_writeback_args args;	/* writeback arguments */
 
-	unsigned long state;
+	unsigned long state;		/* flag bits, see WS_* */
 };
 
 enum {
@@ -758,7 +758,11 @@ static long wb_writeback(struct bdi_writeback *wb,
 
 /*
  * Return the next bdi_work struct that hasn't been processed by this
- * wb thread yet
+ * wb thread yet. ->seen is initially set for each thread that exists
+ * for this device, when a thread first notices a piece of work it
+ * clears its bit. Depending on writeback type, the thread will notify
+ * completion on either receiving the work (WB_SYNC_NONE) or after
+ * it is done (WB_SYNC_ALL).
  */
 static struct bdi_work *get_next_work_item(struct backing_dev_info *bdi,
 					   struct bdi_writeback *wb)