mm: multi-gen LRU: improve lru_gen_exit_memcg()

Add warnings and poison ->next.

Link: https://lkml.kernel.org/r/20230118001827.1040870-6-talumbau@google.comSigned-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -6168,12 +6168,17 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg)
 	int i;
 	int nid;
 
+	VM_WARN_ON_ONCE(!list_empty(&memcg->mm_list.fifo));
+
 	for_each_node(nid) {
 		struct lruvec *lruvec = get_lruvec(memcg, nid);
 
+		VM_WARN_ON_ONCE(lruvec->mm_state.nr_walkers);
 		VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
 					   sizeof(lruvec->lrugen.nr_pages)));
 
+		lruvec->lrugen.list.next = LIST_POISON1;
+
 		for (i = 0; i < NR_BLOOM_FILTERS; i++) {
 			bitmap_free(lruvec->mm_state.filters[i]);
 			lruvec->mm_state.filters[i] = NULL;