On Mon 13-11-17 12:43:08, Arnd Bergmann wrote:

On Mon, Nov 13, 2017 at 9:09 AM, Michal Hocko [off-list ref] wrote:

quoted

On Mon 13-11-17 16:42:06, Stephen Rothwell wrote:

quoted

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

OK, that is a reasonable workaround. I was talking to our gcc guy and
he suggested to report a bug for this. Andrew, could you fold the
explicit != NULL check into the patch please?

-- 
Michal Hocko
SUSE Labs