[$] The rapid growth of io_uring

One year ago, the io_uring subsystem didnot exist in the mainline kernel; it showed up in the 5.1 release in May2019. At its core, io_uring is a mechanism for performing asynchronousI/O, but it has been steadily growing beyond that use case and adding newcapabilities. Herein we catch up with the current state of io_uring, whereit is headed, and an interesting question or two that will come up alongthe way.