linux-kernel - Re: [PATCH v2 5/7] memcg: pr_warn_once for unexpected events and stats

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Date: Mon, 29 Apr 2024 12:54:08 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Yosry Ahmed <yosryahmed@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/7] memcg: pr_warn_once for unexpected events and
 stats

On Sat, Apr 27, 2024 at 10:22:34AM -0400, Johannes Weiner wrote:
> On Fri, Apr 26, 2024 at 06:18:13PM -0700, Shakeel Butt wrote:
> > On Fri, Apr 26, 2024 at 05:58:16PM -0700, Yosry Ahmed wrote:
> > > On Fri, Apr 26, 2024 at 5:38 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
[...]
> > > 
> > > Can we make these more compact by using WARN_ON_ONCE() instead:
> > > 
> > > if (WARN_ON_ONCE(i < 0))
> > >          return 0;
> > > 
> > > I guess the advantage of using pr_warn_once() is that we get to print
> > > the exact stat index, but the stack trace from WARN_ON_ONCE() should
> > > make it obvious in most cases AFAICT.
> 
> if (WARN_ONCE(i < 0, "stat item %d not in memcg_node_stat_items\n", i))
> 	return 0;
> 
> should work?
> 
> > > No strong opinions either way.
> > 
> > One reason I used pr_warn_once() over WARN_ON_ONCE() is the syzbot
> > trigger. No need to trip the bot over this error condition.
> 
> The warn splat is definitely quite verbose. But I think that would
> only be annoying initially, in case a site was missed. Down the line,
> it seems helpful to have this stand out to somebody who is trying to
> add a new cgroup stat and forgets to update the right enums.

Sounds good to me. I will change it to WARN_ONCE().