linux-kernel - Re: [PATCH] mm: memcontrol: Cast mismatched enum types passed to memcg state and event functions

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Message-Id: <20170726142309.ac40faf5eb99568e6edb064c@linux-foundation.org>
Date:   Wed, 26 Jul 2017 14:23:09 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH] mm: memcontrol: Cast mismatched enum types passed to
 memcg state and event functions

On Wed, 26 Jul 2017 12:23:56 -0700 Matthias Kaehlcke <mka@...omium.org> wrote:

> In multiple instances enum values of an incorrect type are passed to
> mod_memcg_state() and other memcg functions. Apparently this is
> intentional, however clang rightfully generates tons of warnings about
> the mismatched types. Cast the offending values to the type expected
> by the called function. The casts add noise, but this seems preferable
> over losing the typesafe interface or/and disabling the warning.
> 
> ...
>
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -576,7 +576,7 @@ static inline void __mod_lruvec_state(struct lruvec *lruvec,
>  	if (mem_cgroup_disabled())
>  		return;
>  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> -	__mod_memcg_state(pn->memcg, idx, val);
> +	__mod_memcg_state(pn->memcg, (enum memcg_stat_item)idx, val);
>  	__this_cpu_add(pn->lruvec_stat->count[idx], val);
>  }

__mod_memcg_state()'s `idx' arg can be either enum memcg_stat_item or
enum memcg_stat_item.  I think it would be better to just admit to
ourselves that __mod_memcg_state() is more general than it appears, and
change it to take `int idx'.  I assume that this implicit cast of an
enum to an int will not trigger a clang warning?