Linux union problems

For more information about Linux union-General Linux technology-Linux programming and kernel, see the following. The task_struct and kernel stack of linux processes are stored on the same 4 K page. As follows:

Include/linux/sched. h 2.4.18

Union task_union {
Struct task_struct task;
Unsigned long stack [INIT_TASK_SIZE/sizeof (long)]
};

I am a bit puzzled here, isn't it because task_struct and the kernel stack should both exist? Why is union used? Is this not covered in the memory?
However, the picture in the book is shown on the top of the space, and on the bottom of the space. How is this implemented?