Would you like to change the comment from "undefined" to "sysconf failed"? Here is what `man` says:

     If the call to sysconf() is not successful, -1 is returned and errno is
     set appropriately.  Otherwise, if the variable is associated with func-
     tionality that is not supported, -1 is returned and errno is not modi-
     fied.  Otherwise, the current variable value is returned.

I don't see a guarantee that page_size will not be -1 at some point.