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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH] mm/memcg: Fix incorrect flushing of lruvec data in
 obj_stock
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Vlastimil Babka <vbabka@suse.cz>, Roman Gushchin <guro@fb.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Chris Down <chris@chrisdown.name>,
        Yafang Shao <laoar.shao@gmail.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        Masayoshi Mizuma <msys.mizuma@gmail.com>,
        Xing Zhengjun <zhengjun.xing@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>
References: <20210802022827.10192-1-longman@redhat.com>
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Message-ID: <771539d4-765c-05c7-ab19-d4cae3d29efd@redhat.com>
Date:   Mon, 2 Aug 2021 09:32:16 -0400
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On 8/2/21 2:28 AM, Michal Hocko wrote:
> On Sun 01-08-21 22:28:27, Waiman Long wrote:
>> When mod_objcg_state() is called with a pgdat that is different from
>> that in the obj_stock, the old lruvec data cached in obj_stock are
>> flushed out. Unfortunately, they were flushed to the new pgdat and
>> hence the wrong node, not the one cached in obj_stock.
> It would be great to explicitly mention user observable problems here. I
> do assume this will make slab stats skewed but the effect wouldn't be
> very big, right?
It is the /sys/devices/system/node/node*/meminfo that will get skewed. 
Not /proc/meminfo. So it is a relatively minor issue. Will update the 
patch to mention that.
>> Fix that by flushing the data to the cached pgdat instead.
>>
>> Fixes: 68ac5b3c8db2 ("mm/memcg: cache vmstat data in percpu memcg_stock_pcp")
>> Signed-off-by: Waiman Long <longman@redhat.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
>> ---
>>   mm/memcontrol.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index ae1f5d0cb581..881ec4ddddcd 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3106,17 +3106,19 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
>>   		stock->cached_pgdat = pgdat;
>>   	} else if (stock->cached_pgdat != pgdat) {
>>   		/* Flush the existing cached vmstat data */
>> +		struct pglist_data *oldpg = stock->cached_pgdat;
>> +
>> +		stock->cached_pgdat = pgdat;
>>   		if (stock->nr_slab_reclaimable_b) {
>> -			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_RECLAIMABLE_B,
>> +			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_RECLAIMABLE_B,
>>   					  stock->nr_slab_reclaimable_b);
>>   			stock->nr_slab_reclaimable_b = 0;
>>   		}
>>   		if (stock->nr_slab_unreclaimable_b) {
>> -			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_UNRECLAIMABLE_B,
>> +			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_UNRECLAIMABLE_B,
>>   					  stock->nr_slab_unreclaimable_b);
>>   			stock->nr_slab_unreclaimable_b = 0;
>>   		}
>> -		stock->cached_pgdat = pgdat;
> Minor nit. Is there any reason to move the cached_pgdat? TBH I found the
> original way better from the readability POV.

Right. Will move it back to its original place.

Cheers,
Longman