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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Wed, 23 Mar 2022 10:12:32 +0800
Message-ID: <CAMZfGtVG2YcmxY0fECkAYNb=sKXJQhWJqgtMTEpQwxXEXmSOLw@mail.gmail.com>
Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node()
To:     Waiman Long <longman@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>
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On Wed, Mar 23, 2022 at 9:55 AM Waiman Long <longman@redhat.com> wrote:
>
> On 3/22/22 21:06, Muchun Song wrote:
> > 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.
>
> I guess this race is theoretically possible but very unlikely since it
> means a very long pause between list_lru_from_kmem() and the increment
> of nr_items.

It is more possible in a VM.

>
> How about the following changes to make sure that this race can't happen?
>
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index c669d87001a6..c31a0a8ad4e7 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -395,9 +395,10 @@ static void memcg_reparent_list_lru_node(struct
> list_lru *lru, int nid,
>          struct list_lru_one *src, *dst;
>
>          /*
> -        * If there is no lru entry in this nlru, we can skip it
> immediately.
> +        * If there is no lru entry in this nlru and the nlru->lock is free,
> +        * we can skip it immediately.
>           */
> -       if (!READ_ONCE(nlru->nr_items))
> +       if (!READ_ONCE(nlru->nr_items) && !spin_is_locked(&nlru->lock))

I think we also should insert a smp_rmb() between those two loads.

Thanks.