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

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Message-ID: <Zi_Ff_iAs_PTph4l@P9FQF9L96D>
Date: Mon, 29 Apr 2024 09:06:23 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Shakeel Butt <shakeel.butt@...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 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.