ceph_pg_compare
int ceph_pg_compare(const struct ceph_pg *lhs, const struct ceph_pg *rhs);
legacy_change = ceph_pg_compare(&t->pgid, &pgid) ||
ret = ceph_pg_compare(&lhs->pgid, &rhs->pgid);
DEFINE_RB_FUNCS2(pg_mapping, struct ceph_pg_mapping, pgid, ceph_pg_compare,