Well, then you have to implement some kind of plan for efficient caching - some kind of LRU scheme, for example, to prevent the cache from ballooning to unusabl

Well, then you have to implement some kind of plan for efficient caching - some kind of LRU scheme, for example, to prevent the cache from ballooning to unusable sizes - at which point you're reinventing the kernel page cache (poorly). mmap does have a big advantage here if you really need a lot of random accesses.