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

[Waiman Long <longman@redhat.com>]

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.

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))
                 return;

Cheers,
Longman