The simplest case

Let's say that a kernel-space device driver (or some core code) requests 128 KB and receives a memory chunk from the order 5 list of one of the freelist data structures. At some later point in time, it will necessarily free the memory chunk by employing one of the page allocator free APIs. Now, this API's algorithm calculates – via its order – that the just-freed chunk belongs on the order 5 list; thus, it enqueues it there.