block: Allow REQ_FUA|REQ_READ

FUA is also allowed with reads, not just writes.

The specified behaviour is:
 - If the location being read from in the drive cache is dirty, it's
   flushed
 - Read is serviced from media, not cache

It's documented in the NVME specification, and the nvme driver already
passes through REQ_FUA for reads, not just writes, so there's no reason
for the block layer to be disallowing it.

To validate behaviour, a simple test was run on a variety of hardware
that checks latency of repeated reads to the same location (cached
reads), random reads (uncached), and FUA reads to the same location.

Data:
 - Samsung consumer SSDs
   Reads appear to not be cached
 - Seagate SCSI hard drives (ST20000NM002D)
   Reads are cached, and FUA reads appear to work correctly

Link: https://lore.kernel.org/linux-block/20250311133517.3095878-1-kent.overstreet@linux.dev/
Link: https://lore.kernel.org/linux-bcachefs/26585.34711.506258.318405@quad.stoffel.home/T/#m5fffbc0e1c68cf0479c94b9f4ac1bef297333782
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

0 files changed, 0 insertions, 0 deletions