Still, there is some sort of racing condition I cannot reliably
reproduce, but often enough observe. It looks like this:

    $ ./cachedel z && ./nocache cat z > /dev/null && ./cachestats -v z
    pages in cache: 2/16 (12.5%)  [filesize=64.0K, pagesize=4K]

    cache map:
         0: | | | | | | | | | | | | | | |x|x|

Although of course the file should not be cached at all.