writeback: introduce writeback_control.inodes_written

The flusher works on dirty inodes in batches, and may quit prematurely
if the batch of inodes happen to be metadata-only dirtied: in this case
wbc->nr_to_write won't be decreased at all, which stands for "no pages
written" but also mis-interpreted as "no progress".

So introduce writeback_control.inodes_written to count the inodes get
cleaned from VFS POV.  A non-zero value means there are some progress on
writeback, in which case more writeback can be tried.
Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

fs/fs-writeback.c

@@ -464,6 +464,7 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 			 * No need to add it back to the LRU.
 			 */
 			list_del_init(&inode->i_wb_list);
+			wbc->inodes_written++;
 		}
 	}
 	inode_sync_complete(inode);
@@ -725,6 +726,7 @@ static long wb_writeback(struct bdi_writeback *wb,
 		wbc.more_io = 0;
 		wbc.nr_to_write = write_chunk;
 		wbc.pages_skipped = 0;
+		wbc.inodes_written = 0;
 
 		trace_wbc_writeback_start(&wbc, wb->bdi);
 		if (work->sb)
@@ -741,6 +743,8 @@ static long wb_writeback(struct bdi_writeback *wb,
 		 */
 		if (wbc.nr_to_write <= 0)
 			continue;
+		if (wbc.inodes_written)
+			continue;
 		/*
 		 * Didn't write everything and we don't have more IO, bail
 		 */

include/linux/writeback.h

@@ -34,6 +34,7 @@ struct writeback_control {
 	long nr_to_write;		/* Write this many pages, and decrement
 					   this for each page written */
 	long pages_skipped;		/* Pages which were not written */
+	long inodes_written;		/* # of inodes written (at least) */
 
 	/*
 	 * For a_ops->writepages(): is start or end are non-zero then this is