vm_stack
struct vm_stack *vm_stack = container_of(rh, struct vm_stack, rcu);
struct vm_struct *vm_area = vm_stack->stack_vm_area;
if (try_release_thread_stack_to_cache(vm_stack->stack_vm_area))
struct vm_stack *vm_stack = tsk->stack;
vm_stack->stack_vm_area = tsk->stack_vm_area;
call_rcu(&vm_stack->rcu, thread_stack_free_rcu);