On Wed, Mar 23, 2022 at 9:55 AM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> On 3/22/22 21:06, Muchun Song wrote:
> > On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@xxxxxxxxxx> 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.
