Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node()

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@redhat.com> wrote:
>
> Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node()
> to be race free"), we are tracking the total number of lru
> entries in a list_lru_node in its nr_items field.  In the case of
> memcg_reparent_list_lru_node(), there is nothing to be done if nr_items
> is 0.  We don't even need to take the nlru->lock as no new lru entry
> could be added by a racing list_lru_add() to the draining src_idx memcg
> at this point.

Hi Waiman,

Sorry for the late reply.  Quick question: what if there is an inflight
list_lru_add()?  How about the following race?

CPU0:                               CPU1:
list_lru_add()
    spin_lock(&nlru->lock)
    l = list_lru_from_kmem(memcg)
                                    memcg_reparent_objcgs(memcg)
                                    memcg_reparent_list_lrus(memcg)
                                        memcg_reparent_list_lru()
                                            memcg_reparent_list_lru_node()
                                                if (!READ_ONCE(nlru->nr_items))
                                                    // Miss reparenting
                                                    return
    // Assume 0->1
    l->nr_items++
    // Assume 0->1
    nlru->nr_items++

IIUC, we use nlru->lock to serialise this scenario.

Thanks.