shrinker_id_to_offset
set_bit(shrinker_id_to_offset(shrinker_id), unit->map);
nr_deferred = atomic_long_xchg(&unit->nr_deferred[shrinker_id_to_offset(shrinker->id)], 0);
atomic_long_add_return(nr, &unit->nr_deferred[shrinker_id_to_offset(shrinker->id)]);