[RFC PATCH 0/4] io_uring/splice: extend splice for supporting ublk zero copy

[Ming Lei <ming.lei@redhat.com>]

Hello Guys,

This patch extends io_uring/splice by adding two flags(SPLICE_F_DIRECT &
SPLICE_F_READ_TO_READ) for supporting ublk zero copy, and fuse could benefit
from the change too.

- SPLICE_F_DIRECT is for using do_splice_direct() to support zero copy

- SPLICE_F_READ_TO_READ is for supporting ublk READ zero copy, the plain
splice can support WRITE zero copy by:

	splice(ublkc_fd, ublkc_pos, pipe_wr_fd, NULL, len, flags)
	splice(pipe_rd_fd, NULL, backing_fd, backing_off, len, flags)

but can't support READ zc. Extend splice to allow to splice from the 1st
->splice_read()(producer) to the 2nd ->splice_read()(consumer), then READ
request pages reference can be transferred to backing IO code path.

The initial idea is suggested by Miklos Szeredi & Stefan Hajnoczi.

The patchset has been verified basically by ublk builtin tests(loop/008,
loop/009, generic/003), and basic mount, git clone, kernel building, umount
tests on ublk-loop[1] which is created by 'ublk add -t loop -f $backing -z'.

The next step is to allow io_uring to run do_splice_direct*()
in async style like normal async RW instead of offloading to
iowq context, so that top performance can be reached, and that
depends on current work.

Any comments are welcome.

[1] https://github.com/ming1/ubdsrv/commits/splice-zc


Ming Lei (4):
  io_uring/splice: support do_splice_direct
  fs/splice: add helper of splice_dismiss_pipe()
  io_uring/splice: support splice from ->splice_read to ->splice_read
  ublk_drv: support splice based read/write zero copy

 drivers/block/ublk_drv.c      | 151 +++++++++++++++++++++++++-
 fs/read_write.c               |   5 +-
 fs/splice.c                   | 193 +++++++++++++++++++++++++++++-----
 include/linux/fs.h            |   2 +
 include/linux/pipe_fs_i.h     |   9 ++
 include/linux/splice.h        |  14 +++
 include/uapi/linux/ublk_cmd.h |  34 +++++-
 io_uring/splice.c             |  16 ++-
 8 files changed, 392 insertions(+), 32 deletions(-)

-- 
2.31.1