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

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Message-ID: <20170727141742.GA19738@cmpxchg.org>
Date:   Thu, 27 Jul 2017 10:17:42 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthias Kaehlcke <mka@...omium.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 Thu, Jul 27, 2017 at 09:24:51AM +0200, Michal Hocko wrote:
> On Wed 26-07-17 15:03:32, Andrew Morton wrote:
> > On Wed, 26 Jul 2017 14:49:14 -0700 Matthias Kaehlcke <mka@...omium.org> wrote:
> > 
> > > El Wed, Jul 26, 2017 at 02:23:09PM -0700 Andrew Morton ha dit:
> > > 
> > > > 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?
> > > 
> > > Sure, no warnings are raised for implicit casts from enum to
> > > int.
> > > 
> > > __mod_memcg_state() is not the only function though, all these
> > > functions are called with conflicting types:
> > > 
> > > memcg_page_state()
> > > __mod_memcg_state()
> > > mod_memcg_state()
> > > count_memcg_events()
> > > count_memcg_page_event()
> > > memcg_sum_events()
> > > 
> > > Should we change all of them to reveice an int instead of an enum?
> > 
> > I suspect so - the current implementation is denying reality and your
> > original proposal is a bit of a fudge.  But I'll await input from the
> > memcg peeps.
> 
> well, those enums do not help type safety much I am afraid so turining
> the idx to int sounds like a more preferable way to me. Johannes?

I agree, turning the parameter into an int makes for much nicer code
than the casts.

Matthias, would you care to update your patch to change these over?