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 12:56:25 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	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 Mon, Apr 29, 2024 at 09:06:23AM -0700, Roman Gushchin wrote:
> On Fri, Apr 26, 2024 at 05:37:31PM -0700, Shakeel Butt 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);
> >  	}
> 
> I think it's generally a CONFIG_DEBUG_VM material. Do we have some extra
> concerns here?
> 
> Having pr_warn_on_once() would be nice here.

No extra concern, just want this indirection table to be up to date in
future.