NR_SPACE_IDS
#define RECYCLE_THRESHOLD (NR_SPACE_IDS / 2)
#define SID_ARRAY_SIZE (NR_SPACE_IDS / (8 * sizeof(long)))
static unsigned long free_space_ids = NR_SPACE_IDS - 1;
index = find_next_zero_bit(space_id, NR_SPACE_IDS, space_id_index);