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

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Message-ID: <CAJD7tkZfkE6EyDAXetjSAKb7Zx2Mw-2naUNHRK=ihegZyZ2mHA@mail.gmail.com>
Date: Thu, 27 Jun 2024 04:33:50 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Xiu Jianfeng <xiujianfeng@...wei.com>, 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, Jun 27, 2024 at 12:13 AM Michal Hocko <mhocko@...e.com> 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.

Is it possible for userspace readers to break if the stats are
incomplete? If yes, I think WARN_ON_ONCE() may be prompted to make it
easier to catch and fix before deployment.