> I'm surprised this is seen as a liability of mmap rather than a cooperative scheduler that isn't using native kernel threads

Indeed. In practice, though, it's easier to write high performance servers and storage systems with async runtimes (like tokio) than with native threads, at least with the current state of the ecosystem. That's not for some fundamental reason - it's possible to get great threaded performance - just the current reality.

So, whoever's fault this is, it's useful to have good evidence of this downside of async runtimes (and worth thinking about ways that OSs could let runtimes know when they were about to block on IO).