linux-kernel - Re: [PATCH -next] mm: memcg: remove redundant seq_buf_has

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Date: Thu, 27 Jun 2024 16:33:00 +0800
From: xiujianfeng <xiujianfeng@...wei.com>
To: Michal Hocko <mhocko@...e.com>
CC: <hannes@...xchg.org>, <roman.gushchin@...ux.dev>,
	<shakeel.butt@...ux.dev>, <muchun.song@...ux.dev>,
	<akpm@...ux-foundation.org>, <cgroups@...r.kernel.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] mm: memcg: remove redundant
 seq_buf_has_overflowed()



On 2024/6/27 15:13, Michal Hocko wrote:
> On Wed 26-06-24 09:42:32, Xiu Jianfeng wrote:
>> Both the end of memory_stat_format() and memcg_stat_format() will call
>> WARN_ON_ONCE(seq_buf_has_overflowed()). However, memory_stat_format()
>> is the only caller of memcg_stat_format(), when memcg is on the default
>> hierarchy, seq_buf_has_overflowed() will be executed twice, so remove
>> the reduntant one.
> 
> Shouldn't we rather remove both? Are they giving us anything useful
> actually? Would a simpl pr_warn be sufficient? Afterall all we care
> about is to learn that we need to grow the buffer size because our stats
> do not fit anymore. It is not really important whether that is an OOM or
> cgroupfs interface path.

I did a test, when I removed both of them and added a lot of prints in
memcg_stat_format() to make the seq_buf overflow, and then cat
memory.stat in user mode, no OOM occurred, and there were no warning
logs in the kernel.

> 
>> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
>> ---
>>  mm/memcontrol.c | 3 ---
>>  1 file changed, 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 974bd160838c..776d22bc66a2 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1846,9 +1846,6 @@ static void memcg_stat_format(struct mem_cgroup *memcg, struct seq_buf *s)
>>  			       vm_event_name(memcg_vm_event_stat[i]),
>>  			       memcg_events(memcg, memcg_vm_event_stat[i]));
>>  	}
>> -
>> -	/* The above should easily fit into one page */
>> -	WARN_ON_ONCE(seq_buf_has_overflowed(s));
>>  }
>>  
>>  static void memcg1_stat_format(struct mem_cgroup *memcg, struct seq_buf *s);
>> -- 
>> 2.34.1
>