[$] 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 with some impressive performance results behind it.