All the discussion was about virtual memory. The initial thread in a process, on every operating system that is still relevant today, uses guard pages to grow

All the discussion was about virtual memory. The initial thread in a process, on every operating system that is still relevant today, uses guard pages to grow its virtual memory allocation for the stack segment. Physical memory for the stack segment's pages may or may not be mapped into the process address space (it will usually be mapped). This stack can't shrink, and also once the thread touches a page, it becomes commited memory and it will always stay commited. Other threads in a process do not use guard pages and use fixed virtual memory allocation for their stacks. Their stacks are fixed in size and can't grow. Just like for the initial thread once a page is touched, it becomes commited memory and it will always stay that way. In Go, stacks start small and use a variable amount of virtual memory. Go stacks can shrink, freeing both address space and physical memory.