[$] The return of RWF_UNCACHED

Linux offers two broad ways of performing I/O to files. Buffered I/O,which is the usual way of accessing a file, stores a copy of thetransferred data in the kernel's page cache to speed future accesses.Direct I/O, instead, moves data directly between the storage device and auser-space buffer, avoiding the page cache. Both modes have theiradvantages and disadvantages. In 2019, Jens Axboe proposed an uncached buffered mode to get someof the advantages of both, but that effort stalled at the time. Now, uncached bufferedI/O is back with some impressive performance results behind it.