linux-kernel - Re: [PATCH] mm/vmstat: Fix -Wenum-enum-conversion warning in vmstat.h

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Date: Fri, 21 Jun 2024 19:13:57 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Shivamurthy Shastri <shivamurthy.shastri@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, vbabka@...e.cz, hannes@...xchg.org,
	anna-maria@...utronix.de, tglx@...utronix.de
Subject: Re: [PATCH] mm/vmstat: Fix -Wenum-enum-conversion warning in vmstat.h

On Fri, Jun 21, 2024 at 01:16:04PM +0200, Shivamurthy Shastri wrote:
> A W=1 build with -Wenum-enum-conversion enabled, results in the
> following build warning due to an arithmetic operation between different
> enumeration types 'enum node_stat_item' and 'enum lru_list':

OK, but why do we want -Wenum-enum-conversion enabled?  The code looks
perfectly fine before, and now it looks ugly.  What bugs does this
warning catch?

>  static inline const char *lru_list_name(enum lru_list lru)
>  {
> -	return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
> +	return node_stat_name(NR_LRU_BASE + (enum node_stat_item)lru) + 3; // skip "nr_"
>  }

and honestly, I'd convert it to an int instead of enum node_stat_item.
Because it is not a node_stat_item, and it wouldn't make sense to
add two node_stat_items together.  Just like it doesn't make sense to
add two pointers together (but it does make sense to add an integer to a
pointer).