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

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Date: Thu, 27 Jun 2024 05:00:18 -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 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 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.

Right, that's exactly what I meant.

> 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

If pr_warn()'s usually get noticed in a timely manner (by testers or
bots), then I think pr_warn() would be sufficient. If they can go
unnoticed for a while, I think WARN_ON_ONCE() may be warranted to
avoid the possibility of breaking a userspace parser.