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

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Message-ID: <Zn1LFyO_cww9W758@tiehlicka>
Date: Thu, 27 Jun 2024 13:20:55 +0200
From: Michal Hocko <mhocko@...e.com>
To: xiujianfeng <xiujianfeng@...wei.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 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.

-- 
Michal Hocko
SUSE Labs