[PATCHSET 0/5] Support for RWF_UNCACHED

[Jens Axboe <axboe@kernel.dk>]

Recently someone asked me how io_uring buffered IO compares to mmaped
IO in terms of performance. So I ran some tests with buffered IO, and
found the experience to be somewhat painful. The test case is pretty
basic, random reads over a dataset that's 10x the size of RAM.
Performance starts out fine, and then the page cache fills up and we
hit a throughput cliff. CPU usage of the IO threads go up, and we have
kswapd spending 100% of a core trying to keep up. Seeing that, I was
reminded of the many complaints I here about buffered IO, and the fact
that most of the folks complaining will ultimately bite the bullet and
move to O_DIRECT to just get the kernel out of the way.

But I don't think it needs to be like that. Switching to O_DIRECT isn't
always easily doable. The buffers have different life times, size and
alignment constraints, etc. On top of that, mixing buffered and O_DIRECT
can be painful.

Seems to me that we have an opportunity to provide something that sits
somewhere in between buffered and O_DIRECT, and this is where
RWF_UNCACHED enters the picture. If this flag is set on IO, we get the
following behavior:

- If the data is in cache, it remains in cache and the copy (in or out)
  is served to/from that.

- If the data is NOT in cache, we add it while performing the IO. When
  the IO is done, we remove it again.

With this, I can do 100% smooth buffered reads or writes without pushing
the kernel to the state where kswapd is sweating bullets. In fact it
doesn't even register.

Comments appreciated! Patches are against current git (ish), and can
also be found here:

https://git.kernel.dk/cgit/linux-block/log/?h=buffered-uncached

 fs/ceph/file.c          |   2 +-
 fs/dax.c                |   2 +-
 fs/ext4/file.c          |   2 +-
 fs/iomap/apply.c        |   2 +-
 fs/iomap/buffered-io.c  |  75 +++++++++++++++++------
 fs/iomap/direct-io.c    |   3 +-
 fs/iomap/fiemap.c       |   5 +-
 fs/iomap/seek.c         |   6 +-
 fs/iomap/swapfile.c     |   2 +-
 fs/nfs/file.c           |   2 +-
 include/linux/fs.h      |   9 ++-
 include/linux/iomap.h   |   6 +-
 include/uapi/linux/fs.h |   5 +-
 mm/filemap.c            | 132 ++++++++++++++++++++++++++++++++++++----
 14 files changed, 208 insertions(+), 45 deletions(-)

-- 
Jens Axboe