ID_PER_SC
BUILD_BUG_ON(sizeof(unsigned short) * ID_PER_SC !=
map = vzalloc(DIV_ROUND_UP(max_pages, ID_PER_SC) *
unsigned int shift = (offset % ID_PER_SC) * ID_SHIFT;
unsigned int old_ids = atomic_read(&map[offset / ID_PER_SC].ids);
BUILD_BUG_ON(!is_power_of_2(ID_PER_SC));
struct swap_cgroup *sc = &map[offset / ID_PER_SC];
unsigned int shift = (offset % ID_PER_SC) * ID_SHIFT;