[$] 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 the
transferred data in the kernel's page cache to speed future accesses.
Direct I/O, instead, moves data directly between the storage device and a
user-space buffer, avoiding the page cache. Both modes have their
advantages and disadvantages. In 2019, Jens Axboe proposed an uncached buffered mode to get some
of the advantages of both, but that effort stalled at the time. Now, uncached buffered
I/O is back with some impressive performance results behind it.