They're... kinda free (in terms of capacity). A PTE or PDE is 64 bits per page. Even with 4kB pages, this is 8/4096 = 0.2% overhead. Churning the page tables and TLB as you walk the file is expensive, or at least historically was expensive (used to be that removing an entry required flushing the entire TLB; I don't think that's true anymore).

If you can use MAP_HUGE_2MB, this drops off to 0.00038% and the TLB impact goes way down.

> no way of handling errors.

Yeah, this aspect is really significant. Thanks for mentioning it.