On Mon, Nov 13, 2017 at 9:09 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Mon 13-11-17 16:42:06, Stephen Rothwell wrote:
>> Hi Andrew,
>>
>> After merging the akpm-current tree, today's linux-next build (powerpc
>> ppc64_defconfig) produced this warning:
>>
>> In file included from include/linux/mmzone.h:17:0,
>> from include/linux/mempolicy.h:10,
>> from mm/mempolicy.c:70:
>> mm/mempolicy.c: In function 'mpol_to_str':
>> include/linux/nodemask.h:107:41: warning: the address of 'nodes' will always evaluate as 'true' [-Waddress]
>> #define nodemask_pr_args(maskp) (maskp) ? MAX_NUMNODES : 0, (maskp) ? (maskp)->bits : NULL
>> ^
>> mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
>> nodemask_pr_args(&nodes));
>> ^
>
> Hmm, this warning is quite surprising to me. Sure in this particular
> case maskp will always be non-NULL so we always expand to
> MAX_NUMNODES, maskp->bits
> which is what we want. But we have other users which may be NULL. Does
> anybody understan why this warns at all?
As I understand it, the warning tries to address a common typo of accidentally
testing the pointer to a stack object for being non-NULL, rather than the object
pointed to for being non-zero.
Adding an extra '!= NULL' comparison gets rid of the warning for me:
#define nodemask_pr_args(maskp) \
((maskp) != NULL) ? MAX_NUMNODES : 0, \
((maskp) != NULL) ?(maskp)->bits : NULL
Arnd
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