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Subject: Re: [PATCH] mm: Make mem_cgroup_id_get_many dependent on MMU and
 MEMCG_SWAP
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Michal Hocko <mhocko@kernel.org>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20200304142348.48167-1-vincenzo.frascino@arm.com>
 <20200304165336.GO16139@dhcp22.suse.cz>
 <8c489836-b824-184e-7cfe-25e55ab73000@arm.com>
 <20200305160929.GA1166@cmpxchg.org>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
Message-ID: <a3e7f3a7-dee4-c05e-2d75-65a0d30e6401@arm.com>
Date:   Thu, 5 Mar 2020 16:21:28 +0000
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Hi Johannes,

On 3/5/20 4:09 PM, Johannes Weiner wrote:
> On Thu, Mar 05, 2020 at 09:49:23AM +0000, Vincenzo Frascino wrote:
>> Hi Michal,
>>
>> On 3/4/20 4:53 PM, Michal Hocko wrote:
>>> On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote:
>>>> mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP
>>>> configuration options are enabled. Having them disabled triggers the
>>>> following warning at compile time:
>>>>
>>>> linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined
>>>> but not used [-Wunused-function]
>>>>  static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned
>>>>  int n)
>>>>
>>>> Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address
>>>> the issue.
>>>
>>> A similar patch has been proposed recently
>>> http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com.
>>> The conclusion was that the warning is not really worth adding code.
>>>
>>
>> Thank you for pointing this out, I was not aware of it. I understand that you
>> are against "#ifdeffery" in this case, but isn't it the case of adding at least
>> __maybe_unused? This would prevent people from reporting it over and over again
>> and you to have to push them back :) Let me know what do you think, in case I am
>> happy to change my patch accordingly.
> 
> I would ack a patch that adds __maybe_unused.
> 
> This is a tiny function. If we keep it around a few releases after
> removing the last user, it costs us absolutely nothing. Eventually
> somebody will notice and send a patch to remove it. No big deal.
> 
> There is, however, real cost in keeping bogus warnings around and
> telling people to ignore them. It's actively lowering the
> signal-to-noise ratio and normalizing warnings to developers. That's
> the kind of thing that will actually hide problems in the kernel.
> 
> We know that the function can be unused in certain scenarios. It's
> silly to let the compiler continue to warn about it. That's exactly
> what __maybe_unused is for, so let's use it here.
> 

I agree with what you are saying. I am going to change my patch accordingly.

Thank you.

-- 
Regards,
Vincenzo