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

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Date: Thu, 27 Jun 2024 13:56:51 +0200
From: Michal Hocko <mhocko@...e.com>
To: Yosry Ahmed <yosryahmed@...gle.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 27-06-24 04:33:50, Yosry Ahmed wrote:
> 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?

They will certainly get an imprecise picture. Sufficient to break I
dunno.

> If yes, I think WARN_ON_ONCE() may be prompted to make it
> easier to catch and fix before deployment.

The only advantage of WARN_ON_ONCE is that the splat is so verbose that
it gets noticed. And also it panics the system if panic_on_warn is
enabled. I do not particularly care about the latter but to me it seems
like the warning is just an over reaction and a simple pr_warn should
just achieve the similar effect - see my other reply
-- 
Michal Hocko
SUSE Labs