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

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Date: Fri, 26 Apr 2024 18:18:13 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.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 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:
> >
> > To reduce memory usage by the memcg events and stats, the kernel uses
> > indirection table and only allocate stats and events which are being
> > used by the memcg code. To make this more robust, let's add warnings
> > where unexpected stats and events indexes are used.
> >
> > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > ---
> >  mm/memcontrol.c | 43 ++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 34 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 103e0e53e20a..36145089dcf5 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -671,9 +671,11 @@ unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx)
> >                 return node_page_state(lruvec_pgdat(lruvec), idx);
> >
> >         i = memcg_stats_index(idx);
> > -       if (i >= 0) {
> > +       if (likely(i >= 0)) {
> >                 pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >                 x = READ_ONCE(pn->lruvec_stats->state[i]);
> > +       } else {
> > +               pr_warn_once("%s: stat item index: %d\n", __func__, idx);
> >         }
> 
> 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.
> 
> 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.