WB_DIRTIED
stats->nr_dirtied += wb_stat(wb, WB_DIRTIED);
dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
wb->dirtied_stamp = wb_stat(wb, WB_DIRTIED);
wb_stat_mod(wb, WB_DIRTIED, nr);
wb_stat_mod(wb, WB_DIRTIED, -nr);