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

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Date:   Fri, 15 Nov 2019 11:22:05 -0500
From:   Qian Cai <cai@....pw>
To:     Johannes Weiner <hannes@...xchg.org>
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, 2019-11-15 at 11:17 -0500, Johannes Weiner wrote:
> 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.

Sorry, I meant to say it is clang.

> 
> 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