To make it work properly, you need to adjust CONFIG_SWAP_PAGES to the number of free pages your system has, to make sure that the program can swap out all memory. When a page is reaccessed (mincore 0/1) is printed - 0 means it was swapped out, 1 means it was not, so you can verify whether the swapout is working; it is needed to test certain code paths.
Make sure to have a swap enabled in UML; also if you test on a real host you'll probably want to boot in text mode or single user mode and pass mem=64M to the kernel command line (on UML I test with mem=32M, on my previous PC I did the same IIRC, on my current box it doesn't boot with 64M of ram, so I set it to 96M).
Many thanks to Ingo Molnar for writing the initial code which I then completed and perfected, and to Jeff Dike for all his work on UML.