Date:   Tue, 29 Mar 2022 17:53:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Muchun Song <songmuchun@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize
 memcg_reparent_list_lru_node()

On 3/28/22 21:15, Muchun Song wrote:
> On Tue, Mar 29, 2022 at 3:12 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>> On Sun, Mar 27, 2022 at 08:57:15PM -0400, Waiman Long wrote:
>>> On 3/22/22 22:12, Muchun Song wrote:
>>>> On Wed, Mar 23, 2022 at 9:55 AM Waiman Long <longman@...hat.com> wrote:
>>>>> On 3/22/22 21:06, Muchun Song wrote:
>>>>>> On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@...hat.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.
>> Yes, I think it's not enough:
> I think it may be enough if we insert a smp_rmb() between those two loads.
>
>> CPU0                                       CPU1
>> READ_ONCE(&nlru->nr_items) -> 0
>>                                             spin_lock(&nlru->lock);
>>                                             nlru->nr_items++;
>                                               ^^^
>                                               |||
>                                               The nlr here is not the
> same as the one in CPU0,
>                                               since CPU0 have done the
> memcg reparting. Then
>                                               CPU0 will not miss nlru
> reparting.  If I am wrong, please
>                                               correct me.  Thanks.
>>                                             spin_unlock(&nlru->lock);
>> && !spin_is_locked(&nlru->lock) -> 0

I just realize that there is another lock/unlock pair in 
memcg_reparent_objcgs():

memcg_reparent_objcgs()
     spin_lock_irq()
     memcg reparenting
     spin_unlock_irq()
     percpu_ref_kill()
         spin_lock_irqsave()
         ...
         spin_unlock_irqrestore()

This lock/unlock pair in percpu_ref_kill() will stop the reordering of 
read/write before the memcg reparenting. Now I think just adding a 
spin_is_locked() check with smp_rmb() should be enough. However, I would 
like to change the ordering like that:

if (!spin_is_locked(&nlru->lock)) {
         smp_rmb();
         if (!READ_ONCE(nlru->nr_items))
                 return;
}

Otherwise, we will have the problem

list_lru_add()
       spin_lock(&nlru->lock)
       l = list_lru_from_kmem(memcg)
READ_ONCE(nlru->nr_items);
       // Assume 0->1
       l->nr_items++
       // Assume 0->1
       nlru->nr_items++
       spin_unlock(&nlru->lock)
                                       spin_is_locked()

If spin_is_locked() is before spin_lock() in list_lru_add(), 
list_lru_from_kmem() is guarantee to get the reparented memcg and so 
won't added to the reparented lru.

Thanks,
Longman

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