# The `clone()` syscall
This section aims to explain how the `clone()` syscall is executed when the systme is under the NUMA architechture.

### System Call Handler for `clone()`
+ If the call is made by the glibc wrapper `clone()`, the `clone` syscall handler is used
+ If `syscall(SYS_clone3, ...)` is used (no wrapper provided by glibc), the `clone3()` syscall handler is used.

Both path ends in a `kernel_clone()` call.

#ifdef __ARCH_WANT_SYS_CLONE
#ifdef CONFIG_CLONE_BACKWARDS
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
		 int __user *, parent_tidptr,
		 unsigned long, tls,
		 int __user *, child_tidptr)
#elif defined(CONFIG_CLONE_BACKWARDS2)
SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
		 int __user *, parent_tidptr,
		 int __user *, child_tidptr,
		 unsigned long, tls)
#elif defined(CONFIG_CLONE_BACKWARDS3)
SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
		int, stack_size,
		int __user *, parent_tidptr,
		int __user *, child_tidptr,
		unsigned long, tls)
#else
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
		 int __user *, parent_tidptr,
		 int __user *, child_tidptr,
		 unsigned long, tls)
#endif
{
	struct kernel_clone_args args = {
		.flags		= (lower_32_bits(clone_flags) & ~CSIGNAL),
		.pidfd		= parent_tidptr,
		.child_tid	= child_tidptr,
		.parent_tid	= parent_tidptr,
		.exit_signal	= (lower_32_bits(clone_flags) & CSIGNAL),
		.stack		= newsp,
		.tls		= tls,
	};

	return kernel_clone(&args);
}
#endif

#ifdef __ARCH_WANT_SYS_CLONE3


/**
 * sys_clone3 - create a new process with specific properties
 * @uargs: argument structure
 * @size:  size of @uargs
 *
 * clone3() is the extensible successor to clone()/clone2().
 * It takes a struct as argument that is versioned by its size.
 *
 * Return: On success, a positive PID for the child process.
 *         On error, a negative errno number.
 */
SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size)
{
	int err;

	struct kernel_clone_args kargs;
	pid_t set_tid[MAX_PID_NS_LEVEL];

	kargs.set_tid = set_tid;

	err = copy_clone_args_from_user(&kargs, uargs, size);
	if (err)
		return err;

	if (!clone3_args_valid(&kargs))
		return -EINVAL;

	return kernel_clone(&kargs);
}
#endif

### Create a brand new task
After some initial setup, the function `copy_process(NULL, trace, NUMA_NO_NODE, args);` is called.

/*
 *  Ok, this is the main fork-routine.
 *
 * It copies the process, and if successful kick-starts
 * it and waits for it to finish using the VM if required.
 *
 * args->exit_signal is expected to be checked for sanity by the caller.
 */
pid_t kernel_clone(struct kernel_clone_args *args)
{
	u64 clone_flags = args->flags;
	struct completion vfork;
	struct pid *pid;
	struct task_struct *p;
	int trace = 0;
	pid_t nr;

	/*
	 * For legacy clone() calls, CLONE_PIDFD uses the parent_tid argument
	 * to return the pidfd. Hence, CLONE_PIDFD and CLONE_PARENT_SETTID are
	 * mutually exclusive. With clone3() CLONE_PIDFD has grown a separate
	 * field in struct clone_args and it still doesn't make sense to have
	 * them both point at the same memory location. Performing this check
	 * here has the advantage that we don't need to have a separate helper
	 * to check for legacy clone().
	 */
	if ((clone_flags & CLONE_PIDFD) &&
	    (clone_flags & CLONE_PARENT_SETTID) &&
	    (args->pidfd == args->parent_tid))
		return -EINVAL;

	/*
	 * Determine whether and which event to report to ptracer.  When
	 * called from kernel_thread or CLONE_UNTRACED is explicitly
	 * requested, no event is reported; otherwise, report if the event
	 * for the type of forking is enabled.
	 */
	if (!(clone_flags & CLONE_UNTRACED)) {
		if (clone_flags & CLONE_VFORK)
			trace = PTRACE_EVENT_VFORK;
		else if (args->exit_signal != SIGCHLD)
			trace = PTRACE_EVENT_CLONE;
		else
			trace = PTRACE_EVENT_FORK;

		if (likely(!ptrace_event_enabled(current, trace)))
			trace = 0;
	}

	p = copy_process(NULL, trace, NUMA_NO_NODE, args);
	add_latent_entropy();

	if (IS_ERR(p))
		return PTR_ERR(p);

	/*
	 * Do this prior waking up the new thread - the thread pointer
	 * might get invalid after that point, if the thread exits quickly.
	 */
	trace_sched_process_fork(current, p);

	pid = get_task_pid(p, PIDTYPE_PID);
	nr = pid_vnr(pid);

	if (clone_flags & CLONE_PARENT_SETTID)
		put_user(nr, args->parent_tid);

	if (clone_flags & CLONE_VFORK) {
		p->vfork_done = &vfork;
		init_completion(&vfork);
		get_task_struct(p);
	}

	if (IS_ENABLED(CONFIG_LRU_GEN_WALKS_MMU) && !(clone_flags & CLONE_VM)) {
		/* lock the task to synchronize with memcg migration */
		task_lock(p);
		lru_gen_add_mm(p->mm);
		task_unlock(p);
	}

	wake_up_new_task(p);

	/* forking complete and child started to run, tell ptracer */
	if (unlikely(trace))
		ptrace_event_pid(trace, pid);

	if (clone_flags & CLONE_VFORK) {
		if (!wait_for_vfork_done(p, &vfork))
			ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
	}

	put_pid(pid);
	return nr;
}

### Copying The Process

/*
 * This creates a new process as a copy of the old one,
 * but does not actually start it yet.
 *
 * It copies the registers, and all the appropriate
 * parts of the process environment (as per the clone
 * flags). The actual kick-off is left to the caller.
 */
__latent_entropy struct task_struct *copy_process(
					struct pid *pid,
					int trace,
					int node,
					struct kernel_clone_args *args)
{
	int pidfd = -1, retval;
	struct task_struct *p;
	struct multiprocess_signals delayed;
	struct file *pidfile = NULL;
	const u64 clone_flags = args->flags;
	struct nsproxy *nsp = current->nsproxy;

    /*
     * CLONE_* flags processing
     */
    
    // ...
	

    /*
     * Force any signals received before this point to be delivered
     * before the fork happens.  Collect up signals sent to multiple
     * processes that happen during the fork and delay them so that
     * they appear to happen after the fork.
     */
    
    // ...

	retval = -ENOMEM;
	p = dup_task_struct(current, node);
	if (!p)
		goto fork_out;
	p->flags &= ~PF_KTHREAD;
	if (args->kthread)
		p->flags |= PF_KTHREAD;
	if (args->user_worker) {
		/*
		 * Mark us a user worker, and block any signal that isn't
		 * fatal or STOP
		 */
		p->flags |= PF_USER_WORKER;
		siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
	}
	if (args->io_thread)
		p->flags |= PF_IO_WORKER;

	if (args->name)
		strscpy_pad(p->comm, args->name, sizeof(p->comm));

	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? args->child_tid : NULL;
	/*
	 * Clear TID on mm_release()?
	 */
	p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? args->child_tid : NULL;

	ftrace_graph_init_task(p);

	rt_mutex_init_task(p);

	lockdep_assert_irqs_enabled();
#ifdef CONFIG_PROVE_LOCKING
	DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
#endif
	retval = copy_creds(p, clone_flags);
	if (retval < 0)
		goto bad_fork_free;

	retval = -EAGAIN;
	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
		if (p->real_cred->user != INIT_USER &&
		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
			goto bad_fork_cleanup_count;
	}
	current->flags &= ~PF_NPROC_EXCEEDED;

	/*
	 * If multiple threads are within copy_process(), then this check
	 * triggers too late. This doesn't hurt, the check is only there
	 * to stop root fork bombs.
	 */
	retval = -EAGAIN;
	if (data_race(nr_threads >= max_threads))
		goto bad_fork_cleanup_count;

	delayacct_tsk_init(p);	/* Must remain after dup_task_struct() */
	p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE | PF_NO_SETAFFINITY);
	p->flags |= PF_FORKNOEXEC;
	INIT_LIST_HEAD(&p->children);
	INIT_LIST_HEAD(&p->sibling);
	rcu_copy_process(p);
	p->vfork_done = NULL;
	spin_lock_init(&p->alloc_lock);

	init_sigpending(&p->pending);

	p->utime = p->stime = p->gtime = 0;
#ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
	p->utimescaled = p->stimescaled = 0;
#endif
	prev_cputime_init(&p->prev_cputime);

#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
	seqcount_init(&p->vtime.seqcount);
	p->vtime.starttime = 0;
	p->vtime.state = VTIME_INACTIVE;
#endif

#ifdef CONFIG_IO_URING
	p->io_uring = NULL;
#endif

	p->default_timer_slack_ns = current->timer_slack_ns;

#ifdef CONFIG_PSI
	p->psi_flags = 0;
#endif

	task_io_accounting_init(&p->ioac);
	acct_clear_integrals(p);

	posix_cputimers_init(&p->posix_cputimers);

	p->io_context = NULL;
	audit_set_context(p, NULL);
	cgroup_fork(p);
	if (args->kthread) {
		if (!set_kthread_struct(p))
			goto bad_fork_cleanup_delayacct;
	}
#ifdef CONFIG_NUMA
	p->mempolicy = mpol_dup(p->mempolicy);
	if (IS_ERR(p->mempolicy)) {
		retval = PTR_ERR(p->mempolicy);
		p->mempolicy = NULL;
		goto bad_fork_cleanup_delayacct;
	}
#endif
#ifdef CONFIG_CPUSETS
	p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
	p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
	seqcount_spinlock_init(&p->mems_allowed_seq, &p->alloc_lock);
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
	memset(&p->irqtrace, 0, sizeof(p->irqtrace));
	p->irqtrace.hardirq_disable_ip	= _THIS_IP_;
	p->irqtrace.softirq_enable_ip	= _THIS_IP_;
	p->softirqs_enabled		= 1;
	p->softirq_context		= 0;
#endif

	p->pagefault_disabled = 0;

#ifdef CONFIG_LOCKDEP
	lockdep_init_task(p);
#endif

#ifdef CONFIG_DEBUG_MUTEXES
	p->blocked_on = NULL; /* not blocked yet */
#endif
#ifdef CONFIG_BCACHE
	p->sequential_io	= 0;
	p->sequential_io_avg	= 0;
#endif
#ifdef CONFIG_BPF_SYSCALL
	RCU_INIT_POINTER(p->bpf_storage, NULL);
	p->bpf_ctx = NULL;
#endif

	/* Perform scheduler related setup. Assign this task to a CPU. */
	retval = sched_fork(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_policy;

	retval = perf_event_init_task(p, clone_flags);
	if (retval)
		goto bad_fork_cleanup_policy;
	retval = audit_alloc(p);
	if (retval)
		goto bad_fork_cleanup_perf;
	/* copy all the process information */
	shm_init_task(p);
	retval = security_task_alloc(p, clone_flags);
	if (retval)
		goto bad_fork_cleanup_audit;
	retval = copy_semundo(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_security;
	retval = copy_files(clone_flags, p, args->no_files);
	if (retval)
		goto bad_fork_cleanup_semundo;
	retval = copy_fs(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_files;
	retval = copy_sighand(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_fs;
	retval = copy_signal(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_sighand;
	retval = copy_mm(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_signal;
	retval = copy_namespaces(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_mm;
	retval = copy_io(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_namespaces;
	retval = copy_thread(p, args);
	if (retval)
		goto bad_fork_cleanup_io;

	stackleak_task_init(p);

	if (pid != &init_struct_pid) {
		pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid,
				args->set_tid_size);
		if (IS_ERR(pid)) {
			retval = PTR_ERR(pid);
			goto bad_fork_cleanup_thread;
		}
	}

	/*
	 * This has to happen after we've potentially unshared the file
	 * descriptor table (so that the pidfd doesn't leak into the child
	 * if the fd table isn't shared).
	 */
	if (clone_flags & CLONE_PIDFD) {
		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;

		/* Note that no task has been attached to @pid yet. */
		retval = __pidfd_prepare(pid, flags, &pidfile);
		if (retval < 0)
			goto bad_fork_free_pid;
		pidfd = retval;

		retval = put_user(pidfd, args->pidfd);
		if (retval)
			goto bad_fork_put_pidfd;
	}

#ifdef CONFIG_BLOCK
	p->plug = NULL;
#endif
	futex_init_task(p);

	/*
	 * sigaltstack should be cleared when sharing the same VM
	 */
	if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)
		sas_ss_reset(p);

	/*
	 * Syscall tracing and stepping should be turned off in the
	 * child regardless of CLONE_PTRACE.
	 */
	user_disable_single_step(p);
	clear_task_syscall_work(p, SYSCALL_TRACE);
#if defined(CONFIG_GENERIC_ENTRY) || defined(TIF_SYSCALL_EMU)
	clear_task_syscall_work(p, SYSCALL_EMU);
#endif
	clear_tsk_latency_tracing(p);

	/* ok, now we should be set up.. */
	p->pid = pid_nr(pid);
	if (clone_flags & CLONE_THREAD) {
		p->group_leader = current->group_leader;
		p->tgid = current->tgid;
	} else {
		p->group_leader = p;
		p->tgid = p->pid;
	}

	p->nr_dirtied = 0;
	p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10);
	p->dirty_paused_when = 0;

	p->pdeath_signal = 0;
	p->task_works = NULL;
	clear_posix_cputimers_work(p);

#ifdef CONFIG_KRETPROBES
	p->kretprobe_instances.first = NULL;
#endif
#ifdef CONFIG_RETHOOK
	p->rethooks.first = NULL;
#endif

	/*
	 * Ensure that the cgroup subsystem policies allow the new process to be
	 * forked. It should be noted that the new process's css_set can be changed
	 * between here and cgroup_post_fork() if an organisation operation is in
	 * progress.
	 */
	retval = cgroup_can_fork(p, args);
	if (retval)
		goto bad_fork_put_pidfd;

	/*
	 * Now that the cgroups are pinned, re-clone the parent cgroup and put
	 * the new task on the correct runqueue. All this *before* the task
	 * becomes visible.
	 *
	 * This isn't part of ->can_fork() because while the re-cloning is
	 * cgroup specific, it unconditionally needs to place the task on a
	 * runqueue.
	 */
	sched_cgroup_fork(p, args);

	/*
	 * From this point on we must avoid any synchronous user-space
	 * communication until we take the tasklist-lock. In particular, we do
	 * not want user-space to be able to predict the process start-time by
	 * stalling fork(2) after we recorded the start_time but before it is
	 * visible to the system.
	 */

	p->start_time = ktime_get_ns();
	p->start_boottime = ktime_get_boottime_ns();

	/*
	 * Make it visible to the rest of the system, but dont wake it up yet.
	 * Need tasklist lock for parent etc handling!
	 */
	write_lock_irq(&tasklist_lock);

	/* CLONE_PARENT re-uses the old parent */
	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
		p->real_parent = current->real_parent;
		p->parent_exec_id = current->parent_exec_id;
		if (clone_flags & CLONE_THREAD)
			p->exit_signal = -1;
		else
			p->exit_signal = current->group_leader->exit_signal;
	} else {
		p->real_parent = current;
		p->parent_exec_id = current->self_exec_id;
		p->exit_signal = args->exit_signal;
	}

	klp_copy_process(p);

	sched_core_fork(p);

	spin_lock(&current->sighand->siglock);

	rv_task_fork(p);

	rseq_fork(p, clone_flags);

	/* Don't start children in a dying pid namespace */
	if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
		retval = -ENOMEM;
		goto bad_fork_cancel_cgroup;
	}

	/* Let kill terminate clone/fork in the middle */
	if (fatal_signal_pending(current)) {
		retval = -EINTR;
		goto bad_fork_cancel_cgroup;
	}

	/* No more failure paths after this point. */

	/*
	 * Copy seccomp details explicitly here, in case they were changed
	 * before holding sighand lock.
	 */
	copy_seccomp(p);

	init_task_pid_links(p);
	if (likely(p->pid)) {
		ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);

		init_task_pid(p, PIDTYPE_PID, pid);
		if (thread_group_leader(p)) {
			init_task_pid(p, PIDTYPE_TGID, pid);
			init_task_pid(p, PIDTYPE_PGID, task_pgrp(current));
			init_task_pid(p, PIDTYPE_SID, task_session(current));

			if (is_child_reaper(pid)) {
				ns_of_pid(pid)->child_reaper = p;
				p->signal->flags |= SIGNAL_UNKILLABLE;
			}
			p->signal->shared_pending.signal = delayed.signal;
			p->signal->tty = tty_kref_get(current->signal->tty);
			/*
			 * Inherit has_child_subreaper flag under the same
			 * tasklist_lock with adding child to the process tree
			 * for propagate_has_child_subreaper optimization.
			 */
			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
							 p->real_parent->signal->is_child_subreaper;
			list_add_tail(&p->sibling, &p->real_parent->children);
			list_add_tail_rcu(&p->tasks, &init_task.tasks);
			attach_pid(p, PIDTYPE_TGID);
			attach_pid(p, PIDTYPE_PGID);
			attach_pid(p, PIDTYPE_SID);
			__this_cpu_inc(process_counts);
		} else {
			current->signal->nr_threads++;
			current->signal->quick_threads++;
			atomic_inc(&current->signal->live);
			refcount_inc(&current->signal->sigcnt);
			task_join_group_stop(p);
			list_add_tail_rcu(&p->thread_node,
					  &p->signal->thread_head);
		}
		attach_pid(p, PIDTYPE_PID);
		nr_threads++;
	}
	total_forks++;
	hlist_del_init(&delayed.node);
	spin_unlock(&current->sighand->siglock);
	syscall_tracepoint_update(p);
	write_unlock_irq(&tasklist_lock);

	if (pidfile)
		fd_install(pidfd, pidfile);

	proc_fork_connector(p);
	sched_post_fork(p);
	cgroup_post_fork(p, args);
	perf_event_fork(p);

	trace_task_newtask(p, clone_flags);
	uprobe_copy_process(p, clone_flags);
	user_events_fork(p, clone_flags);

	copy_oom_score_adj(clone_flags, p);

	return p;

    /*
     * Tons of bad fork exiting failure paths, reached via goto's.
     */
	
    // ...
    return ERR_PTR(retval);
}

The main part. At line 139 of this snippet, the function also sets up the memory policy for the newly forked task, which happens at line 39 of this snippet (`dup_task_struct(current, node)`, with `node = NUMA_NO_NODE`, and `orig` pointed to the task issued the system call, `current`).

:::spoiler `dup_task_struct()`:

static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
{
	struct task_struct *tsk;
	int err;

	if (node == NUMA_NO_NODE)
		node = tsk_fork_get_node(orig);
	tsk = alloc_task_struct_node(node);
	if (!tsk)
		return NULL;

	err = arch_dup_task_struct(tsk, orig);
	if (err)
		goto free_tsk;

	err = alloc_thread_stack_node(tsk, node);
	if (err)
		goto free_tsk;

#ifdef CONFIG_THREAD_INFO_IN_TASK
	refcount_set(&tsk->stack_refcount, 1);
#endif
	account_kernel_stack(tsk, 1);

	err = scs_prepare(tsk, node);
	if (err)
		goto free_stack;

#ifdef CONFIG_SECCOMP
	/*
	 * We must handle setting up seccomp filters once we're under
	 * the sighand lock in case orig has changed between now and
	 * then. Until then, filter must be NULL to avoid messing up
	 * the usage counts on the error path calling free_task.
	 */
	tsk->seccomp.filter = NULL;
#endif

	setup_thread_stack(tsk, orig);
	clear_user_return_notifier(tsk);
	clear_tsk_need_resched(tsk);
	set_task_stack_end_magic(tsk);
	clear_syscall_work_syscall_user_dispatch(tsk);

#ifdef CONFIG_STACKPROTECTOR
	tsk->stack_canary = get_random_canary();
#endif
	if (orig->cpus_ptr == &orig->cpus_mask)
		tsk->cpus_ptr = &tsk->cpus_mask;
	dup_user_cpus_ptr(tsk, orig, node);

	/*
	 * One for the user space visible state that goes away when reaped.
	 * One for the scheduler.
	 */
	refcount_set(&tsk->rcu_users, 2);
	/* One for the rcu users */
	refcount_set(&tsk->usage, 1);
#ifdef CONFIG_BLK_DEV_IO_TRACE
	tsk->btrace_seq = 0;
#endif
	tsk->splice_pipe = NULL;
	tsk->task_frag.page = NULL;
	tsk->wake_q.next = NULL;
	tsk->worker_private = NULL;

	kcov_task_init(tsk);
	kmsan_task_create(tsk);
	kmap_local_fork(tsk);

#ifdef CONFIG_FAULT_INJECTION
	tsk->fail_nth = 0;
#endif

#ifdef CONFIG_BLK_CGROUP
	tsk->throttle_disk = NULL;
	tsk->use_memdelay = 0;
#endif

#ifdef CONFIG_ARCH_HAS_CPU_PASID
	tsk->pasid_activated = 0;
#endif

#ifdef CONFIG_MEMCG
	tsk->active_memcg = NULL;
#endif

#ifdef CONFIG_CPU_SUP_INTEL
	tsk->reported_split_lock = 0;
#endif

#ifdef CONFIG_SCHED_MM_CID
	tsk->mm_cid = -1;
	tsk->last_mm_cid = -1;
	tsk->mm_cid_active = 0;
	tsk->migrate_from_cpu = -1;
#endif
	return tsk;

free_stack:
	exit_task_stack_account(tsk);
	free_thread_stack(tsk);
free_tsk:
	free_task_struct(tsk);
	return NULL;
}

:::

Since called with `node == NUMA_NO_NODE`, `node = tak_for_get_node(orig)` would definetely get called thus, decides the node where the `task_struct` of the newly created task would be allocated.

int tsk_fork_get_node(struct task_struct *tsk)
{
#ifdef CONFIG_NUMA
	if (tsk == kthreadd_task)
		return tsk->pref_node_fork;
#endif
	return NUMA_NO_NODE;
}

The `kthreadd_task` is the task of the kthread daemon, which is the ultimate parent of all kernel threads. However, since `tsk` here refers to the user space process issued the clone call, `node` remains set to `NUMA_NO_NODE`.

static inline struct task_struct *alloc_task_struct_node(int node)
{
	return kmem_cache_alloc_node(task_struct_cachep, GFP_KERNEL, node);
}

#define kmem_cache_alloc_node(...) \
    alloc_hooks(kmem_cache_alloc_node_noprof(__VA_ARGS__))

void *kmem_cache_alloc_node_noprof(struct kmem_cache *s, gfp_t gfpflags, int node)
{
	void *ret = slab_alloc_node(s, NULL, gfpflags, node, _RET_IP_, s->object_size);

	trace_kmem_cache_alloc(_RET_IP_, ret, s, gfpflags, node);

	return ret;
}
EXPORT_SYMBOL(kmem_cache_alloc_node_noprof);

Since `task_struct` is sort of a core data structure, there exists a slap cache managing the alloation of it, whose pointer is stored within `task_struct_cachep`.

With the existing slab cache pointer, the `slab_alloc_node()` function allocates the neccessary memory for the `task_struct` on the specified node, `node` (A.k.a., the same node as the CPU the task is running on, since `node` is set to `NUMA_NO_NODES`).

The call path goes as:
+ `slab_alloc_node()` **➡** `__slab_alloc_node()`
At here, the slab allocator attempts to allocate the memory via the previous `task_struct_cachep`. If the obtained slab do not exist on the specified `node`, the check `node_match(slab, node)` will not be passed, thus the slow path is entered
::: info
Many other checks are performed to determine if the slow path is taken, but NUMA related info are focused here. The whole path is contained in :link:[linux/mm/slub.c](https://github.com/torvalds/linux/blob/master/mm/slub.c).
:::

+ `...` **➡** `__slab_alloc()` **➡** `___slab_alloc()`

----

Back to the `copy_process()` function with `CONFIG_NUMA=y`, memory policy are also setup for the newly created task:

#ifdef CONFIG_NUMA
	p->mempolicy = mpol_dup(p->mempolicy);
	if (IS_ERR(p->mempolicy)) {
		retval = PTR_ERR(p->mempolicy);
		p->mempolicy = NULL;
		goto bad_fork_cleanup_delayacct;
	}
#endif

# Memory Policy