몰랐거나, 알고싶지 않았던 것들

• task는 죽으면, 모든 resource를 release하지않고 왜 죽었는지 정도를 남겨둔다.
• 그 정보는 부모 task에서 자식의 죽음을 이용할때, 회수한다 예를 들면 join()
• do_noitfy_parent 는 task가 죽을때, 죽는 task의 부모에게 자식의 죽음을 알려주기 위한 함수이다
• do_notify_pidfd() 는 죽는 task의 FD에 대해서 죽는 event를 기다리는 task들을 깨워준다
• sig ID가 SIGCHLD가 아니더라도, parent == real_parent인 경우에는 sig ID를 SIGCHLD 로 바꿔준다.
• task->exit_code를 통해서 parent에게 왜 task가 죽었는지를 전달해준다, (info.si_code)
• 부모가 SIGCHLD 시그널을 받는다면, 부모에게 알려준다, __send_signal_locked().
• 만약 부모가 SIGCHLD 에 대해서 시그널을 안 받겠다고 해놨다면 (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN), 부모에게 알려주지않고 do_exit()에서 자체적으로 resource를 release한다
• 부모가 SIGCHLD를 안 받겠다고했지만, 부모가 자식의 죽음을 조사하는 과정에서 wait/blocked되어있는 race condition상황을 대비해서 부모를 깨워준다 (__wake_up_parent).
  
  • 이 부분에 대해서 한번 확인해볼 필요가 있음. 20년 전 코드가 아직도 유효하고 필요한지.

/*
 * Let a parent know about the death of a child.
 * For a stopped/continued status change, use do_notify_parent_cldstop instead.
 *
 * Returns true if our parent ignored us and so we have switched to
 * self-reaping.
 */
bool do_notify_parent(struct task_struct *tsk, int sig)
{
	struct kernel_siginfo info;
	unsigned long flags;
	struct sighand_struct *psig;
	bool autoreap = false;
	u64 utime, stime;

	WARN_ON_ONCE(sig == -1);

	/* do_notify_parent_cldstop should have been called instead.  */
	WARN_ON_ONCE(task_is_stopped_or_traced(tsk));

	WARN_ON_ONCE(!tsk->ptrace &&
	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
	/*
	 * tsk is a group leader and has no threads, wake up the
	 * non-PIDFD_THREAD waiters.
	 */
	if (thread_group_empty(tsk))
		do_notify_pidfd(tsk);

	if (sig != SIGCHLD) {
		/*
		 * This is only possible if parent == real_parent.
		 * Check if it has changed security domain.
		 */
		if (tsk->parent_exec_id != READ_ONCE(tsk->parent->self_exec_id))
			sig = SIGCHLD;
	}

	clear_siginfo(&info);
	info.si_signo = sig;
	info.si_errno = 0;
	/*
	 * We are under tasklist_lock here so our parent is tied to
	 * us and cannot change.
	 *
	 * task_active_pid_ns will always return the same pid namespace
	 * until a task passes through release_task.
	 *
	 * write_lock() currently calls preempt_disable() which is the
	 * same as rcu_read_lock(), but according to Oleg, this is not
	 * correct to rely on this
	 */
	rcu_read_lock();
	info.si_pid = task_pid_nr_ns(tsk, task_active_pid_ns(tsk->parent));
	info.si_uid = from_kuid_munged(task_cred_xxx(tsk->parent, user_ns),
				       task_uid(tsk));
	rcu_read_unlock();

	task_cputime(tsk, &utime, &stime);
	info.si_utime = nsec_to_clock_t(utime + tsk->signal->utime);
	info.si_stime = nsec_to_clock_t(stime + tsk->signal->stime);

	info.si_status = tsk->exit_code & 0x7f;
	if (tsk->exit_code & 0x80)
		info.si_code = CLD_DUMPED;
	else if (tsk->exit_code & 0x7f)
		info.si_code = CLD_KILLED;
	else {
		info.si_code = CLD_EXITED;
		info.si_status = tsk->exit_code >> 8;
	}

	psig = tsk->parent->sighand;
	spin_lock_irqsave(&psig->siglock, flags);
	if (!tsk->ptrace && sig == SIGCHLD &&
	    (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
	     (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {
		/*
		 * We are exiting and our parent doesn't care.  POSIX.1
		 * defines special semantics for setting SIGCHLD to SIG_IGN
		 * or setting the SA_NOCLDWAIT flag: we should be reaped
		 * automatically and not left for our parent's wait4 call.
		 * Rather than having the parent do it as a magic kind of
		 * signal handler, we just set this to tell do_exit that we
		 * can be cleaned up without becoming a zombie.  Note that
		 * we still call __wake_up_parent in this case, because a
		 * blocked sys_wait4 might now return -ECHILD.
		 *
		 * Whether we send SIGCHLD or not for SA_NOCLDWAIT
		 * is implementation-defined: we do (if you don't want
		 * it, just use SIG_IGN instead).
		 */
		autoreap = true;
		if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
			sig = 0;
	}
	/*
	 * Send with __send_signal as si_pid and si_uid are in the
	 * parent's namespaces.
	 */
	if (valid_signal(sig) && sig)
		__send_signal_locked(sig, &info, tsk->parent, PIDTYPE_TGID, false);
	__wake_up_parent(tsk, tsk->parent);
	spin_unlock_irqrestore(&psig->siglock, flags);

	return autoreap;
}

void __init arm64_memblock_init(void)
{
	const s64 linear_region_size = BIT(vabits_actual - 1);

	/* Handle linux,usable-memory-range property */
	fdt_enforce_memory_region();

	/* Remove memory above our supported physical address size */
	memblock_remove(1ULL << PHYS_MASK_SHIFT, ULLONG_MAX);

	/*
	 * Select a suitable value for the base of physical memory.
	 */
	memstart_addr = round_down(memblock_start_of_DRAM(),
				   ARM64_MEMSTART_ALIGN);

	physvirt_offset = PHYS_OFFSET - PAGE_OFFSET;

	vmemmap = ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT));

	/*
	 * If we are running with a 52-bit kernel VA config on a system that
	 * does not support it, we have to offset our vmemmap and physvirt_offset
	 * s.t. we avoid the 52-bit portion of the direct linear map
	 */
	if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52) && (vabits_actual != 52)) {
		vmemmap += (_PAGE_OFFSET(48) - _PAGE_OFFSET(52)) >> PAGE_SHIFT;
		physvirt_offset = PHYS_OFFSET - _PAGE_OFFSET(48);
	}

	/*
	 * Remove the memory that we will not be able to cover with the
	 * linear mapping. Take care not to clip the kernel which may be
	 * high in memory.
	 */
	memblock_remove(max_t(u64, memstart_addr + linear_region_size,
			__pa_symbol(_end)), ULLONG_MAX);
	if (memstart_addr + linear_region_size < memblock_end_of_DRAM()) {
		/* ensure that memstart_addr remains sufficiently aligned */
		memstart_addr = round_up(memblock_end_of_DRAM() - linear_region_size,
					 ARM64_MEMSTART_ALIGN);
		memblock_remove(0, memstart_addr);
	}

	/*
	 * Apply the memory limit if it was set. Since the kernel may be loaded
	 * high up in memory, add back the kernel region that must be accessible
	 * via the linear mapping.
	 */
	if (memory_limit != PHYS_ADDR_MAX) {
		memblock_mem_limit_remove_map(memory_limit);
		memblock_add(__pa_symbol(_text), (u64)(_end - _text));
	}

	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) {
		/*
		 * Add back the memory we just removed if it results in the
		 * initrd to become inaccessible via the linear mapping.
		 * Otherwise, this is a no-op
		 */
		u64 base = phys_initrd_start & PAGE_MASK;
		u64 size = PAGE_ALIGN(phys_initrd_start + phys_initrd_size) - base;

		/*
		 * We can only add back the initrd memory if we don't end up
		 * with more memory than we can address via the linear mapping.
		 * It is up to the bootloader to position the kernel and the
		 * initrd reasonably close to each other (i.e., within 32 GB of
		 * each other) so that all granule/#levels combinations can
		 * always access both.
		 */
		if (WARN(base < memblock_start_of_DRAM() ||
			 base + size > memblock_start_of_DRAM() +
				       linear_region_size,
			"initrd not fully accessible via the linear mapping -- please check your bootloader ...\n")) {
			phys_initrd_size = 0;
		} else {
			memblock_remove(base, size); /* clear MEMBLOCK_ flags */
			memblock_add(base, size);
			memblock_reserve(base, size);
		}
	}

	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
		extern u16 memstart_offset_seed;
		u64 range = linear_region_size -
			    (memblock_end_of_DRAM() - memblock_start_of_DRAM());

		/*
		 * If the size of the linear region exceeds, by a sufficient
		 * margin, the size of the region that the available physical
		 * memory spans, randomize the linear region as well.
		 */
		if (memstart_offset_seed > 0 && range >= ARM64_MEMSTART_ALIGN) {
			range /= ARM64_MEMSTART_ALIGN;
			memstart_addr -= ARM64_MEMSTART_ALIGN *
					 ((range * memstart_offset_seed) >> 16);
		}
	}

	/*
	 * Register the kernel text, kernel data, initrd, and initial
	 * pagetables with memblock.
	 */
	memblock_reserve(__pa_symbol(_text), _end - _text);
	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) {
		/* the generic initrd code expects virtual addresses */
		initrd_start = __phys_to_virt(phys_initrd_start);
		initrd_end = initrd_start + phys_initrd_size;
	}

	early_init_fdt_scan_reserved_mem();

	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
		zone_dma_bits = ARM64_ZONE_DMA_BITS;
		arm64_dma_phys_limit = max_zone_phys(ARM64_ZONE_DMA_BITS);
	}

	if (IS_ENABLED(CONFIG_ZONE_DMA32))
		arm64_dma32_phys_limit = max_zone_phys(32);
	else
		arm64_dma32_phys_limit = PHYS_MASK + 1;

	reserve_crashkernel();

	reserve_elfcorehdr();

	high_memory = __va(memblock_end_of_DRAM() - 1) + 1;

	dma_contiguous_reserve(arm64_dma32_phys_limit);
}