> Did they change libaio to work without O_DIRECT at some point?

No, and I don't really forsee that happening at this point.

> Or are you talking about io_uring for async file io?

Yep. io_uring can do async buffered file IO.

Initially, for buffered file IO, everything not in the page cache (e.g. a cache miss read, or a write without a page cache page already existing) was done via kernel threads inside the kernel, but that's being incrementally improved. Now most buffered reads don't need a kernel thread anymore (instead they are submitted during the io_uring_enter, and completed in task context, avoiding a lot of the overhead of "synchronous" execution in a kernel thread).