mm/list_lru.c: revert "mm/list_lru: optimize memcg_reparent_list_lru_node()"

Commit 405cc51f ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
has subtle races which are proving ugly to fix.  Revert the original
optimization.  If quantitative testing indicates that we have a
significant problem here then other implementations can be looked at.

Fixes: 405cc51f ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/list_lru.c

@@ -394,12 +394,6 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid,
 	int dst_idx = dst_memcg->kmemcg_id;
 	struct list_lru_one *src, *dst;
 
-	/*
-	 * If there is no lru entry in this nlru, we can skip it immediately.
-	 */
-	if (!READ_ONCE(nlru->nr_items))
-		return;
-
 	/*
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.