writeback: fix occasional slow sync(1)

In case when system contains no dirty pages, wakeup_flusher_threads() will
submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
immediately without doing anything, even though there are dirty inodes in
the system. Thus sync(1) will write all the dirty inodes from a
WB_SYNC_ALL writeback pass which is slow.

Fix the problem by using get_nr_dirty_pages() in wakeup_flusher_threads()
instead of calculating number of dirty pages manually. That function also
takes number of dirty inodes into account.

Signed-off-by: Jan Kara <[e-mail]>
Reported-by: Paul Taysom <[e-mail]>
Cc: Wu Fengguang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fs-writeback.c

···
 {
 	struct backing_dev_info *bdi;
 
-	if (!nr_pages) {
-		nr_pages = global_page_state(NR_FILE_DIRTY) +
-				global_page_state(NR_UNSTABLE_NFS);
-	}
+	if (!nr_pages)
+		nr_pages = get_nr_dirty_pages();
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {