[PATCHSET v2 0/5] Support for RWF_UNCACHED

[Jens Axboe <axboe@kernel.dk>]

ecently 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! This should work on any standard file system,
using either the generic helpers or iomap. Patches are against current
git, 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  |  89 +++++++++++++++++++------
 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      |  11 +++-
 include/linux/iomap.h   |   6 +-
 include/uapi/linux/fs.h |   5 +-
 mm/filemap.c            | 139 ++++++++++++++++++++++++++++++++++++----
 14 files changed, 230 insertions(+), 46 deletions(-)

Changes since v1:
- Switch to pagevecs for write_drop_cached_pages()
- Use page_offset() instead of manual shift
- Ensure we hold a reference on the page between calling ->write_end()
  and checking the mapping on the locked page
- Fix XFS multi-page streamed writes, we'd drop the UNCACHED flag after
  the first page

-- 
Jens Axboe