writeback: make nr_to_write a per-file limit

@@ -330,6 +330,8 @@ static int
 writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 {
 	struct address_space *mapping = inode->i_mapping;
+	long per_file_limit = wbc->per_file_limit;
+	long nr_to_write;
 	unsigned dirty;
 	int ret;
 

@@ -365,8 +367,16 @@ writeback_single_inode(struct inode *ino
 	inode->i_state &= ~I_DIRTY_PAGES;
 	spin_unlock(&inode_lock);
 
+	if (per_file_limit) {
+		nr_to_write = wbc->nr_to_write;
+		wbc->nr_to_write = per_file_limit;
+	}
+
 	ret = do_writepages(mapping, wbc);
 
+	if (per_file_limit)
+		wbc->nr_to_write += nr_to_write - per_file_limit;
+
 	/*
 	 * Make sure to wait on the data before writing out the metadata.
 	 * This is important for filesystems that modify metadata on data

@@ -698,6 +708,7 @@ static long wb_writeback(struct bdi_writ
 
 		wbc.more_io = 0;
 		wbc.nr_to_write = write_chunk;
+		wbc.per_file_limit = write_chunk;
 		wbc.pages_skipped = 0;
 
 		trace_wbc_writeback_start(&wbc, wb->bdi);

@@ -43,6 +43,7 @@ struct writeback_control {
 					   extra jobs and livelock */
 	long nr_to_write;		/* Write this many pages, and decrement
 					   this for each page written */
+	long per_file_limit;		/* Write this many pages for one file */
 	long pages_skipped;		/* Pages which were not written */
 
 	/*