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

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Date: Thu, 27 Jun 2024 14:28:13 +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 05:00:18, Yosry Ahmed wrote:
> On Thu, Jun 27, 2024 at 4:56 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > 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 some stats go completely missing and a parser expects them to
> always be there, I think they may break.

If they break, we will eventually learn about that with or without
warning. It is true that WARN* is so vocal that people/tooling might
just report that even without breakage but that to me sounds like
abusing WARNING. There were times when this was not a big deal but now
when WARN* are getting CVEs because panic_on_warn this useful debugging
tool has become a new BUG on.

-- 
Michal Hocko
SUSE Labs