Readiness notification, like epoll, is not really suitable for file backed IO. A file on disk is not a FIFO that you can wait for data to be available; what would even mean for notification to be ready to read for a file fd? Consider that reads can be at any offset.

You need completion notifications instead (like IOCP or io_uring or POSIX aio). You could in principle start an async splice from an FD into a pipe and then poll for the pipe to be ready to read, but IIRC there is no async splice.

This is highly dependent on the filesystem - as far as I know, basically only XFS works, and only for a small subset of operations, and everything else just blocks.

Tokio's filesystem handling in Rust is the same way by default - a pool of IO threads.

Edit: I just remembered that Avi Kivity (of KVM and ScyllaDB fame) wrote a tool to detect this: https://github.com/avikivity/fsqual

They are always "ready" and they block when you use them.