Quoting Pavel Emelyanov (xemul@openvz.org):
> Hi, guys!
> 
> I've noticed that compiling out all the core related to
> cloning and cleaning the new namespace saves us more than
> a Kbyte (!) from the vmlinux.
> 
> add/remove: 19/0 grow/shrink: 6/6 up/down: 1532/-336 (1196)
> function                                     old     new   delta
> copy_user_ns                                   -     181    +181
> copy_ipcs                                      -     149    +149
> copy_utsname                                   -     120    +120
> shm_exit_ns                                    -     106    +106
> sem_exit_ns                                    -     106    +106
> msg_exit_ns                                    -     106    +106
> freeary                                        -     100    +100
> release_uids                                   -      95     +95
> freeque                                        -      92     +92
> free_nsproxy                                  48      99     +51
> __sem_init_ns                                  -      45     +45
> shm_init_ns                                    -      42     +42
> sem_init_ns                                    -      42     +42
> msg_init_ns                                    -      42     +42
> __shm_init_ns                                  -      38     +38
> create_new_namespaces                        300     335     +35
> __msg_init_ns                                  -      31     +31
> sysvipc_proc_release                           5      35     +30
> free_ipc_ns                                    -      30     +30
> do_shm_rmid                                    -      29     +29
> shm_release                                   18      39     +21
> free_user_ns                                   -      16     +16
> sysvipc_proc_open                            100     111     +11
> do_shmat                                     778     787      +9
> free_uts_ns                                    -       5      +5
> sys_shmctl                                  1934    1907     -27
> msg_init                                      82      47     -35
> shm_init                                      92      47     -45
> sem_init                                      99      44     -55
> sys_msgctl                                  1394    1311     -83
> sys_semctl                                  2123    2032     -91
> 
> Since there already were some questions like "do I need it
> on my cellphone?" in reply to pid namespaces patches and 
> so on, why don't we make ALL the namespaces cloning code 
> under the config option to make those people happy?
> 
> Here's the proposed patch.

How about a single config variable for all namespaces?

-serge

> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> 
> ---
> 
> diff --git a/include/linux/ipc.h b/include/linux/ipc.h
> index 96988d1..b882610 100644
> --- a/include/linux/ipc.h
> +++ b/include/linux/ipc.h
> @@ -100,56 +100,6 @@ struct kern_ipc_perm
>  	void		*security;
>  };
> 
> -struct ipc_ids;
> -struct ipc_namespace {
> -	struct kref	kref;
> -	struct ipc_ids	*ids[3];
> -
> -	int		sem_ctls[4];
> -	int		used_sems;
> -
> -	int		msg_ctlmax;
> -	int		msg_ctlmnb;
> -	int		msg_ctlmni;
> -
> -	size_t		shm_ctlmax;
> -	size_t		shm_ctlall;
> -	int		shm_ctlmni;
> -	int		shm_tot;
> -};
> -
> -extern struct ipc_namespace init_ipc_ns;
> -
> -#ifdef CONFIG_SYSVIPC
> -#define INIT_IPC_NS(ns)		.ns		= &init_ipc_ns,
> -extern void free_ipc_ns(struct kref *kref);
> -extern struct ipc_namespace *copy_ipcs(unsigned long flags,
> -						struct ipc_namespace *ns);
> -#else
> -#define INIT_IPC_NS(ns)
> -static inline struct ipc_namespace *copy_ipcs(unsigned long flags,
> -						struct ipc_namespace *ns)
> -{
> -	return ns;
> -}
> -#endif
> -
> -static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
> -{
> -#ifdef CONFIG_SYSVIPC
> -	if (ns)
> -		kref_get(&ns->kref);
> -#endif
> -	return ns;
> -}
> -
> -static inline void put_ipc_ns(struct ipc_namespace *ns)
> -{
> -#ifdef CONFIG_SYSVIPC
> -	kref_put(&ns->kref, free_ipc_ns);
> -#endif
> -}
> -
>  #endif /* __KERNEL__ */
> 
>  #endif /* _LINUX_IPC_H */
> diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> new file mode 100644
> index 0000000..89f51f8
> --- /dev/null
> +++ b/include/linux/ipc_namespace.h
> @@ -0,0 +1,67 @@
> +#ifndef __IPC_NAMESPACE_H__
> +#define __IPC_NAMESPACE_H__
> +
> +#include <linux/err.h>
> +
> +struct ipc_ids;
> +struct ipc_namespace {
> +	struct kref	kref;
> +	struct ipc_ids	*ids[3];
> +
> +	int		sem_ctls[4];
> +	int		used_sems;
> +
> +	int		msg_ctlmax;
> +	int		msg_ctlmnb;
> +	int		msg_ctlmni;
> +
> +	size_t		shm_ctlmax;
> +	size_t		shm_ctlall;
> +	int		shm_ctlmni;
> +	int		shm_tot;
> +};
> +
> +extern struct ipc_namespace init_ipc_ns;
> +
> +#ifdef CONFIG_SYSVIPC
> +#define INIT_IPC_NS(ns)		.ns		= &init_ipc_ns,
> +#else
> +#define INIT_IPC_NS(ns)
> +#endif
> +
> +#ifdef CONFIG_NS_IPC
> +extern void free_ipc_ns(struct kref *kref);
> +extern struct ipc_namespace *copy_ipcs(unsigned long flags,
> +						struct ipc_namespace *ns);
> +
> +static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
> +{
> +	if (ns)
> +		kref_get(&ns->kref);
> +	return ns;
> +}
> +
> +static inline void put_ipc_ns(struct ipc_namespace *ns)
> +{
> +	kref_put(&ns->kref, free_ipc_ns);
> +}
> +#else
> +static inline struct ipc_namespace *copy_ipcs(unsigned long flags,
> +		struct ipc_namespace *ns)
> +{
> +	if (flags & CLONE_NEWIPC)
> +		return ERR_PTR(-EINVAL);
> +
> +	return ns;
> +}
> +
> +static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
> +{
> +	return ns;
> +}
> +
> +static inline void put_ipc_ns(struct ipc_namespace *ns)
> +{
> +}
> +#endif
> +#endif
> diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 4817c66..ac1b47f 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -122,7 +122,6 @@ extern struct pid *find_ge_pid(int nr, s
> 
>  extern struct pid *alloc_pid(struct pid_namespace *ns);
>  extern void FASTCALL(free_pid(struct pid *pid));
> -extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
> 
>  /*
>   * the helpers to get the pid's id seen from different namespaces
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index 0135c76..1f5f915 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -6,6 +6,7 @@
>  #include <linux/threads.h>
>  #include <linux/nsproxy.h>
>  #include <linux/kref.h>
> +#include <linux/err.h>
> 
>  struct pidmap {
>         atomic_t nr_free;
> @@ -29,6 +30,7 @@ struct pid_namespace {
> 
>  extern struct pid_namespace init_pid_ns;
> 
> +#ifdef CONFIG_NS_PID
>  static inline struct pid_namespace *get_pid_ns(struct pid_namespace *ns)
>  {
>  	if (ns != &init_pid_ns)
> @@ -38,12 +40,37 @@ static inline struct pid_namespace *get_
> 
>  extern struct pid_namespace *copy_pid_ns(unsigned long flags, struct pid_namespace *ns);
>  extern void free_pid_ns(struct kref *kref);
> +extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
> 
>  static inline void put_pid_ns(struct pid_namespace *ns)
>  {
>  	if (ns != &init_pid_ns)
>  		kref_put(&ns->kref, free_pid_ns);
>  }
> +#else
> +static inline struct pid_namespace *get_pid_ns(struct pid_namespace *ns)
> +{
> +	return ns;
> +}
> +
> +static inline void put_pid_ns(struct pid_namespace *ns)
> +{
> +}
> +
> +static inline struct pid_namespace *copy_pid_ns(unsigned long flags,
> +		struct pid_namespace *ns)
> +{
> +	if (flags & CLONE_NEWPID)
> +		return ERR_PTR(-EINVAL);
> +
> +	return ns;
> +}
> +
> +static inline void zap_pid_ns_processes(struct pid_namespace *ns)
> +{
> +	BUG();
> +}
> +#endif
> 
>  static inline struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
>  {
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> diff --git a/include/linux/sem.h b/include/linux/sem.h
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index b5f41d4..d73080c 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -17,7 +17,7 @@ struct user_namespace {
> 
>  extern struct user_namespace init_user_ns;
> 
> -#ifdef CONFIG_USER_NS
> +#ifdef CONFIG_NS_UID
> 
>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
>  {
> diff --git a/include/linux/utsname.h b/include/linux/utsname.h
> index 923db99..cea08a9 100644
> --- a/include/linux/utsname.h
> +++ b/include/linux/utsname.h
> @@ -35,6 +35,7 @@ struct new_utsname {
>  #include <linux/sched.h>
>  #include <linux/kref.h>
>  #include <linux/nsproxy.h>
> +#include <linux/err.h>
>  #include <asm/atomic.h>
> 
>  struct uts_namespace {
> @@ -43,6 +44,7 @@ struct uts_namespace {
>  };
>  extern struct uts_namespace init_uts_ns;
> 
> +#ifdef CONFIG_NS_UTS
>  static inline void get_uts_ns(struct uts_namespace *ns)
>  {
>  	kref_get(&ns-&