Can all this be a nop if a CONFIG option is not selected?

Code obfuscation, please don't do that

+typedef struct {} net_t;

No typedef for this please.

+
+#define __data_pernet 
+
+/* Look up a per network namespace variable */
+static inline unsigned long __per_net_offset(net_t net) { return 0; }
+
+/* Like per_net but returns a pseudo variable address that must be moved
+ * __per_net_offset() bytes before it will point to a real variable.
+ * Useful for static initializers.
+ */
+#define __per_net_base(name)   __pernetname(name)
+
+/* Get the network namespace reference from a per_net variable address */
+#define net_of(ptr, name) ({ net_t net; ptr; net; })
+
+/* Look up a per network namespace variable */
+#define per_net(name, net) \
+	(*(__per_net_offset(net), &__per_net_base(name)))
+ 
+/* Are the two network namespaces the same */
+static inline int net_eq(net_t a, net_t b) { return 1; }
+/* Get an unsigned value appropriate for hashing the network namespace */
+static inline unsigned int net_hval(net_t net) { return 0; }
+
+/* Convert to and from to and from void pointers */
+static inline void *net_to_voidp(net_t net) { return NULL; }
+static inline net_t net_from_voidp(void *ptr) { net_t net; return net; }
+
+static inline int null_net(net_t net) { return 0; }
+
+#define DEFINE_PER_NET(type, name) 			\
+	__data_pernet __typeof__(type) __pernetname(name)
+
+#define DECLARE_PER_NET(type, name) \
+	extern __typeof__(type) __pernetname(name)
+
+#define EXPORT_PER_NET_SYMBOL(var)	\
+	EXPORT_SYMBOL(__pernetname(var))
+#define EXPORT_PER_NET_SYMBOL_GPL(var)	\
+	EXPORT_SYMBOL_GPL(__pernetname(var))
+
+#endif /* __LINUX_NET_NAMESPACE_TYPE_H */

-- 
Stephen Hemminger [off-list ref]