PID will fail with the error ENOMEM; it is not possible to create a new processes)).
Starting with Linux 3.4, the reboot(2) system causes a signal to be sent to the namespace "init" process. See reboot(2) for more details. namespace..
To summarize: there is a technical requirement for each of CLONE_THREAD, CLONE_SIGHAND, and CLONE_VM to share a PID namespace. (Note furthermore that in clone(2) requires CLONE_VM to be specified if CLONE_THREAD or CLONE_SIGHAND is specified.) Thus, call sequences such as the following will fail (with the error EINVAL):
unshare(CLONE_NEWPID); clone(..., CLONE_VM, ...); /* Fails */ setns(fd, CLONE_NEWPID); clone(..., CLONE_VM, ...); /* Fails */ clone(..., CLONE_VM, ...); setns(fd, CLONE_NEWPID); /* Fails */ clone(..., CLONE_VM, ...); unshare(CLONE_NEWPID); /* Fails */.