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

[Waiman Long <longman@redhat.com>]

On 3/22/22 22:12, Muchun Song wrote:
> 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.

Thinking about this some more, I believe that adding spin_is_locked() 
check will be enough for x86. However, that will likely not be enough 
for arches with a more relaxed memory semantics. So the safest way to 
avoid this possible race is to move the check to within the lock 
critical section, though that comes with a slightly higher overhead for 
the 0 nr_items case. I will send out a patch to correct that. Thanks for 
bring this possible race to my attention.

Cheers,
Longman