linux-kernel - Re: [PATCH -next] mm/vmscan: fix some -Wenum-conversion warnings

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Date:   Fri, 15 Nov 2019 11:17:23 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, surenb@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/vmscan: fix some -Wenum-conversion warnings

On Fri, Nov 15, 2019 at 09:04:57AM -0500, Qian Cai wrote:
> The -next commit "mm: vmscan: enforce inactive:active ratio at the
> reclaim root" [1] introduced some GCC -Wenum-conversion warnings,
> 
> mm/vmscan.c:2216:39: warning: implicit conversion from enumeration type
> 'enum lru_list' to different enumeration type 'enum node_stat_item'
> [-Wenum-conversion]
>         inactive = lruvec_page_state(lruvec, inactive_lru);
>                    ~~~~~~~~~~~~~~~~~         ^~~~~~~~~~~~
> mm/vmscan.c:2217:37: warning: implicit conversion from enumeration type
> 'enum lru_list' to different enumeration type 'enum node_stat_item'
> [-Wenum-conversion]
>         active = lruvec_page_state(lruvec, active_lru);
>                  ~~~~~~~~~~~~~~~~~         ^~~~~~~~~~
> mm/vmscan.c:2746:42: warning: implicit conversion from enumeration type
> 'enum lru_list' to different enumeration type 'enum node_stat_item'
> [-Wenum-conversion]
>         file = lruvec_page_state(target_lruvec, LRU_INACTIVE_FILE);

Interesting, it doesn't show these for me with gcc-9.2.0.

We should definitely fix these!

> @@ -2213,8 +2213,9 @@ static bool inactive_is_low(struct lruvec *lruvec, enum lru_list inactive_lru)
>  	unsigned long inactive_ratio;
>  	unsigned long gb;
>  
> -	inactive = lruvec_page_state(lruvec, inactive_lru);
> -	active = lruvec_page_state(lruvec, active_lru);
> +	inactive = lruvec_page_state(lruvec,
> +				     (enum node_stat_item)inactive_lru);
> +	active = lruvec_page_state(lruvec, (enum node_stat_item)active_lru);

This is fragile, as it relies on the absolute values being identical,
which we don't guarantee. However, we do guarantee the relative order
between the LRU items, and we use NR_LRU_BASE for the translation.

Please use NR_LRU_BASE + (in)active_lru here.

> @@ -2743,7 +2744,8 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  	 * thrashing, try to reclaim those first before touching
>  	 * anonymous pages.
>  	 */
> -	file = lruvec_page_state(target_lruvec, LRU_INACTIVE_FILE);
> +	file = lruvec_page_state(target_lruvec,
> +				 (enum node_stat_item)LRU_INACTIVE_FILE);

This should just directly use NR_INACTIVE_FILE instead.

Thanks