get_memcg_gen
new = get_memcg_gen(pgdat->memcg_lru.seq);
new = get_memcg_gen(pgdat->memcg_lru.seq + 1);
if (!pgdat->memcg_lru.nr_memcgs[old] && old == get_memcg_gen(pgdat->memcg_lru.seq))
gen = get_memcg_gen(pgdat->memcg_lru.seq);
if (!pgdat->memcg_lru.nr_memcgs[gen] && gen == get_memcg_gen(pgdat->memcg_lru.seq))
gen = get_memcg_gen(READ_ONCE(pgdat->memcg_lru.seq));