mm: count writeback pages per BDI

Count per BDI writeback pages.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/backing-dev.h

@@ -28,6 +28,7 @@ typedef int (congested_fn)(void *, int);
 
 enum bdi_stat_item {
 	BDI_RECLAIMABLE,
+	BDI_WRITEBACK,
 	NR_BDI_STAT_ITEMS
 };
 

mm/page-writeback.c

@@ -979,14 +979,18 @@ int test_clear_page_writeback(struct page *page)
 	int ret;
 
 	if (mapping) {
+		struct backing_dev_info *bdi = mapping->backing_dev_info;
 		unsigned long flags;
 
 		write_lock_irqsave(&mapping->tree_lock, flags);
 		ret = TestClearPageWriteback(page);
-		if (ret)
+		if (ret) {
 			radix_tree_tag_clear(&mapping->page_tree,
 						page_index(page),
 						PAGECACHE_TAG_WRITEBACK);
+			if (bdi_cap_writeback_dirty(bdi))
+				__dec_bdi_stat(bdi, BDI_WRITEBACK);
+		}
 		write_unlock_irqrestore(&mapping->tree_lock, flags);
 	} else {
 		ret = TestClearPageWriteback(page);
@@ -1002,14 +1006,18 @@ int test_set_page_writeback(struct page *page)
 	int ret;
 
 	if (mapping) {
+		struct backing_dev_info *bdi = mapping->backing_dev_info;
 		unsigned long flags;
 
 		write_lock_irqsave(&mapping->tree_lock, flags);
 		ret = TestSetPageWriteback(page);
-		if (!ret)
+		if (!ret) {
 			radix_tree_tag_set(&mapping->page_tree,
 						page_index(page),
 						PAGECACHE_TAG_WRITEBACK);
+			if (bdi_cap_writeback_dirty(bdi))
+				__inc_bdi_stat(bdi, BDI_WRITEBACK);
+		}
 		if (!PageDirty(page))
 			radix_tree_tag_clear(&mapping->page_tree,
 						page_index(page),