linux-kernel - [POC RFC 0/3] splice(2) support for io

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Date:   Wed, 22 Jan 2020 03:05:16 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     Alexander Viro <viro@...iv.linux.org.uk>
Subject: [POC RFC 0/3] splice(2) support for io_uring

It works well for basic cases, but there is still work to be done. E.g.
it misses @hash_reg_file checks for the second (output) file. Anyway,
there are some questions I want to discuss:

- why sqe->len is __u32? Splice uses size_t, and I think it's better
to have something wider (e.g. u64) for fututre use. That's the story
behind added sqe->splice_len.

- it requires 2 fds, and it's painful. Currently file managing is done
by common path (e.g. io_req_set_file(), __io_req_aux_free()). I'm
thinking to make each opcode function handle file grabbing/putting
themself with some helpers, as it's done in the patch for splice's
out-file.
    1. Opcode handler knows, whether it have/needs a file, and thus
       doesn't need extra checks done in common path.
    2. It will be more consistent with splice.
Objections? Ideas?

- do we need offset pointers with fallback to file->f_pos? Or is it
enough to have offset value. Jens, I remember you added the first
option somewhere, could you tell the reasoning?


Pavel Begunkov (3):
  splice: make do_splice public
  io_uring: add interface for getting files
  io_uring: add splice(2) support

 fs/io_uring.c                 | 152 ++++++++++++++++++++++++++++------
 fs/splice.c                   |   6 +-
 include/linux/splice.h        |   3 +
 include/uapi/linux/io_uring.h |  16 +++-
 4 files changed, 147 insertions(+), 30 deletions(-)

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2.24.0