On 7 Jan 2020, at 12:42, Christoph Hellwig wrote:

On Fri, Dec 13, 2019 at 01:32:10AM +0000, Chris Mason wrote:

quoted

They just have different tradeoffs.  O_DIRECT actively blows away 
caches
and can also force writes during reads, making RWF_UNCACHED a more
natural fit for some applications.  There are fewer surprises, and 
some
services are willing to pay for flexibility with a memcpy.  In 
general,
they still want to do some cache management because it reduces p90+
latencies across the board, and gives them more control over which 
pages
stay in cache.

We can always have a variant of O_DIRECT that doesn't do that and
instead check if data was in the cache and then also copy / from to
it in that case.  I need some time to actually look through this 
series,
so it might be pretty similar to the implementation, but if defined
the right way it could be concurrent for at least the fast path of no
cached pages.

Yeah, I really do think we can end up with a fairly unified solution 
through iomap:

* Allowing concurrent writes (xfs DIO does this now)
* Optionally doing zero copy if alignment is good (btrfs DIO does this 
now)
* Optionally tossing pages at the end (requires a separate syscall now)
* Supporting aio via io_uring

We could just call this O_DIRECT, but I like RWF_UNCACHED as a way to 
avoid surprises for people that know and love the existing O_DIRECT 
semantics.

-chris