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Subject: Re: [PATCH v2 1/3] mm: Fix dropped memcg from mem cgroup soft limit
 tree
To:     Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Ying Huang <ying.huang@intel.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
References: <cover.1613584277.git.tim.c.chen@linux.intel.com>
 <8d35206601ccf0e1fe021d24405b2a0c2f4e052f.1613584277.git.tim.c.chen@linux.intel.com>
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 <72cb8618-73af-ce08-d5d5-30cab30755a3@linux.intel.com>
 <YEH1xF8xm9MCsQ+q@dhcp22.suse.cz>
From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <087bed0e-5b5f-0e25-c247-7fcb34de1513@linux.intel.com>
Date:   Fri, 5 Mar 2021 11:07:59 -0800
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On 3/5/21 1:11 AM, Michal Hocko wrote:
> On Thu 04-03-21 09:35:08, Tim Chen wrote:
>>
>>
>> On 2/18/21 11:13 AM, Michal Hocko wrote:
>>
>>>
>>> Fixes: 4e41695356fb ("memory controller: soft limit reclaim on contention")
>>> Acked-by: Michal Hocko <mhocko@suse.com>
>>>
>>> Thanks!
>>>> ---
>>>>  mm/memcontrol.c | 6 +++++-
>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index ed5cc78a8dbf..a51bf90732cb 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -3505,8 +3505,12 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
>>>>  			loop > MEM_CGROUP_MAX_SOFT_LIMIT_RECLAIM_LOOPS))
>>>>  			break;
>>>>  	} while (!nr_reclaimed);
>>>> -	if (next_mz)
>>>> +	if (next_mz) {
>>>> +		spin_lock_irq(&mctz->lock);
>>>> +		__mem_cgroup_insert_exceeded(next_mz, mctz, excess);
>>>> +		spin_unlock_irq(&mctz->lock);
>>>>  		css_put(&next_mz->memcg->css);
>>>> +	}
>>>>  	return nr_reclaimed;
>>>>  }
>>>>  
>>>> -- 
>>>> 2.20.1
>>>
>>
>> Mel,
>>
>> Reviewing this patch a bit more, I realize that there is a chance that the removed
>> next_mz could be inserted back to the tree from a memcg_check_events
>> that happen in between.  So we need to make sure that the next_mz
>> is indeed off the tree and update the excess value before adding it
>> back.  Update the patch to the patch below.
> 
> This scenario is certainly possible but it shouldn't really matter much
> as __mem_cgroup_insert_exceeded bails out when the node is on the tree
> already.
> 

Makes sense. We should still update the excess value with

+		excess = soft_limit_excess(next_mz->memcg);
+		__mem_cgroup_insert_exceeded(next_mz, mctz, excess);

before doing insertion.  The excess value was recorded from previous
mz in the loop and needs to be updated to that of next_mz.

Tim