fincore()

Hmm, it seems there's an assumption here about the page size. It's not immediately clear if you mean 4K or 1K pages. So I think that should be made explicit somehow, like by vec getting a length argument.

But other than that I think it's a fabulous idea. Although you could achieve much the same benefits if you could do a read() and have it also return a flag indicating if the data was all in memory or not.