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

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Date: Thu, 27 Jun 2024 19:43:06 +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 19:20, Michal Hocko wrote:
> On Thu 27-06-24 16:33:00, xiujianfeng wrote:
>>
>>
>> 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.
> 
> The default buffer size is PAGE_SIZE.

Hi Michal,

I'm sorry, I didn't understand what you meant by this sentence. What I
mean is that we can't remove both, otherwise, neither the kernel nor
user space would be aware of a buffer overflow. From my test, there was
no OOM or other exceptions when the overflow occurred; it just resulted
in the displayed information being truncated. Therefore, we need to keep
one.

>