linux-kernel - Re: [PATCH] mm: fix nodemask printing

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Date:   Mon, 20 Nov 2017 12:33:20 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Zhangshaokun <zhangshaokun@...ilicon.com>
Subject: Re: [PATCH] mm: fix nodemask printing

On Mon, Nov 20, 2017 at 9:22 AM, Michal Hocko <mhocko@...nel.org> wrote:
> On Fri 17-11-17 11:15:45, Arnd Bergmann wrote:
>> The cleanup caused build warnings for constant mask pointers:
>>
>> mm/mempolicy.c: In function ‘mpol_to_str’:
>> ./include/linux/nodemask.h:108:11: warning: the comparison will always evaluate as ‘true’ for the address of ‘nodes’ will never be NULL [-Waddress]
>>
>> An earlier workaround I suggested was incorporated in the version that
>> got merged, but that only solved the problem for gcc-7 and higher, while
>> gcc-4.6 through gcc-6.x still warn.
>>
>> This changes the printing again to use inline functions that make it
>> clear to the compiler that the line that does the NULL check has no
>> idea whether the argument is a constant NULL.
>>
>> Fixes: 0205f75571e3 ("mm: simplify nodemask printing")
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> Thanks for the fixup. It is sad how a questionable warning makes the
> code worse... Does it make sense to have the warning enabled?
> Gcc bug  [1] suggests there is no great interest into fixing it.
>
> Anyway to the patch
> Acked-by: Michal Hocko <mhocko@...e.com>
>
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82963

I think I found some real bugs when I started seeing that warning elsewhere,
so I'd rather keep it enabled in general. I'd still agree that the gcc behavior
in macros is questionable though, in particular since they decided to
stop warning for the "!= NULL" case. I've found the commit that changed
gcc-7 behavior and commented again in the bugzilla, but did not reopen.

The inline function was mentioned in bugzilla there[2] as the preferred
workaround.

       Arnd

[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48778