WB_HISTORY_SIZE
aa_state_t history[WB_HISTORY_SIZE];
BUILD_BUG_ON_NOT_POWER_OF_2(WB_HISTORY_SIZE); \
wb->pos = (wb->pos + 1) & (WB_HISTORY_SIZE - 1); \
wb->len = (wb->len + 1) > WB_HISTORY_SIZE ? WB_HISTORY_SIZE : \
pos = (pos - 1) & (WB_HISTORY_SIZE - 1);