Query Hard Memory Addresses

So, previously it was discussed that /dev/mem could be used to mmap a specific hardware memory into a process. Now I need to unit test some userland code that does exactly that, but I need to make sure that the unit test selects a small page that is always free in kernel land. How can I query mem to just grab the location of a free page? (my development setup is different from my other deployment; in deployment these values are hard coded, in development they occur on a qemu emulator)