> Hmm, quite slow indeed, are you sure you're not running in debug mode?
>

Well, yes, but it's no faster with a non-debug build: my laptop is
really crawling :-)

> If the performance regression is limited to read(), I don't think it's
> really an issue, but using mmap/munmap explicitly would probably benicer
> anyway (1° because it lets the glibc choose whatever heuristic is best,
> 2° because it would help release memory on more systems than just glibc
> systems). I think limiting ourselves to systems which have
> MMAP_ANONYMOUS is good enough.
>

Agreed.
Here's a patch.