Hi all,

On Fri, 17 Nov 2017 09:44:39 +1100 Stephen Rothwell [off-list ref] wrote:

On Mon, 13 Nov 2017 12:43:08 +0100 Arnd Bergmann [off-list ref] wrote:

quoted

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

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  

This warning now exists in Linus' tree :-(

Looking closer, it seems that the above workaround doesn't work for my
compiler (gcc v5.2.0):

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:108:11: warning: the comparison will always evaluate as 'true' for the address of 'nodes' will never be NULL [-Waddress]
  ((maskp) != NULL) ? MAX_NUMNODES : 0,  \
           ^
mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
           nodemask_pr_args(&nodes));
           ^
include/linux/nodemask.h:109:11: warning: the comparison will always evaluate as 'true' for the address of 'nodes' will never be NULL [-Waddress]
  ((maskp) != NULL) ? (maskp)->bits : NULL
           ^
mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
           nodemask_pr_args(&nodes));
           ^

-- 
Cheers,
Stephen Rothwell