writeback: consolidate variable names in balance_dirty_pages()

Introduce

	nr_dirty = NR_FILE_DIRTY + NR_WRITEBACK + NR_UNSTABLE_NFS

in order to simplify many tests in the following patches.

balance_dirty_pages() will eventually care only about the dirty sums
besides nr_writeback.
Acked-by: NJan Kara <jack@suse.cz>
Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>

mm/page-writeback.c

@@ -565,8 +565,9 @@ static void bdi_update_bandwidth(struct backing_dev_info *bdi,
 static void balance_dirty_pages(struct address_space *mapping,
 				unsigned long write_chunk)
 {
-	long nr_reclaimable, bdi_nr_reclaimable;
-	long nr_writeback, bdi_nr_writeback;
+	unsigned long nr_reclaimable, bdi_nr_reclaimable;
+	unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
+	unsigned long bdi_dirty;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
@@ -579,7 +580,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 	for (;;) {
 		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
 					global_page_state(NR_UNSTABLE_NFS);
-		nr_writeback = global_page_state(NR_WRITEBACK);
+		nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
 
 		global_dirty_limits(&background_thresh, &dirty_thresh);
 
@@ -588,8 +589,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 		 * catch-up. This avoids (excessively) small writeouts
 		 * when the bdi limits are ramping up.
 		 */
-		if (nr_reclaimable + nr_writeback <=
-				(background_thresh + dirty_thresh) / 2)
+		if (nr_dirty <= (background_thresh + dirty_thresh) / 2)
 			break;
 
 		bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
@@ -607,10 +607,12 @@ static void balance_dirty_pages(struct address_space *mapping,
 		 */
 		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
 			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
-			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
+			bdi_dirty = bdi_nr_reclaimable +
+				    bdi_stat_sum(bdi, BDI_WRITEBACK);
 		} else {
 			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
-			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+			bdi_dirty = bdi_nr_reclaimable +
+				    bdi_stat(bdi, BDI_WRITEBACK);
 		}
 
 		/*
@@ -619,9 +621,8 @@ static void balance_dirty_pages(struct address_space *mapping,
 		 * bdi or process from holding back light ones; The latter is
 		 * the last resort safeguard.
 		 */
-		dirty_exceeded =
-			(bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
-			|| (nr_reclaimable + nr_writeback > dirty_thresh);
+		dirty_exceeded = (bdi_dirty > bdi_thresh) ||
+				  (nr_dirty > dirty_thresh);
 
 		if (!dirty_exceeded)
 			break;