`mmap stack_area()` returns area that includes a guard page

[nstilt1]

Hey, I was experimenting with your code when I found that the stack_area() function includes an extra guard page in the usable size... unless I'm using it wrong? I believe that the stack area includes 2 guard pages and only 1 of them is subtracted. I have a test that fails, and switching it to subtract 2 guard pages fixes my test.

Test (in mmap stack restore guard file):

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_stack_area() {
        for stack_size_kb in 1..64 {
            let size = stack_size_kb * 1024;
            let stack = StackRestoreGuard::new(size);
            assert_eq!(
                stack.stack_area().1,
                ((size + page_size() - 1) / page_size()) * page_size()
            )
        }
    }
}

Fix:

    pub fn stack_area(&self) -> (*mut u8, usize) {
        unsafe {
            (
                self.mapping.add(self.page_size),
                self.size_with_guard - 2 * self.page_size,
            )
        }
    }

Fixing this could allow you to pass in the allocated_stack_size instead of requested_stack_size in lib.rs:161