Re: [PATCH V6 17/17] block: ublk_drv: apply io_uring FUSED_CMD for supporting zero copy

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On 3/30/23 13:36, Ming Lei wrote:
> Apply io_uring fused command for supporting zero copy:
> 1) init the fused cmd buffer(io_mapped_buf) in ublk_map_io(), and deinit it
> in ublk_unmap_io(), and this buffer is immutable, so it is just fine to
> retrieve it from concurrent fused command.
> 1) add sub-command opcode of UBLK_IO_FUSED_SUBMIT_IO for retrieving this
> fused cmd(zero copy) buffer
> 2) call io_fused_cmd_start_secondary_req() to provide buffer to secondary
> request and submit secondary request; meantime setup complete callback via
> this API, once secondary request is completed, the complete callback is
> called for freeing the buffer and completing the fused command
> Also request reference is held during fused command lifetime, and this way
> guarantees that request buffer won't be freed until all inflight fused
> commands are completed.
> userspace(only implement sqe128 fused command):
> liburing test(only implement normal sqe fused command: two 64byte SQEs)
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
>   Documentation/block/ublk.rst  | 126 ++++++++++++++++++++--
>   drivers/block/ublk_drv.c      | 192 ++++++++++++++++++++++++++++++++--
>   include/uapi/linux/ublk_cmd.h |   6 +-
>   3 files changed, 303 insertions(+), 21 deletions(-)
> diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst
> index 1713b2890abb..7b7aa24e9729 100644
> --- a/Documentation/block/ublk.rst
> +++ b/Documentation/block/ublk.rst
> @@ -297,18 +297,126 @@ with specified IO tag in the command data:
>     ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the server, ublkdrv needs to copy
>     the server buffer (pages) read to the IO request pages.
> -Future development
> -==================
> +
> +  Used for implementing zero copy feature.
> +
> +  It has to been the primary command of io_uring fused command. This command

s/been/be/ ?

> +  submits the generic secondary IO request with io buffer provided by our primary
> +  command, and won't be completed until the secondary request is done.
> +
> +  The provided buffer is represented as ``io_uring_bvec_buf``, which is
> +  actually ublk request buffer's reference, and the reference is shared &
> +  read-only, so the generic secondary request can retrieve any part of the buffer
> +  by passing buffer offset & length.
>   Zero copy
> ----------
> +=========
> +
> +What is zero copy?
> +------------------
> +
> +When application submits IO to ``/dev/ublkb*``, userspace buffer(direct io)
> +or page cache buffer(buffered io) or kernel buffer(meta io often) is used
> +for submitting data to ublk driver, and all kinds of these buffers are
> +represented by bio/bvecs(ublk request buffer) finally. Before supporting
> +zero copy, data in these buffers has to be copied to ublk server userspace
> +buffer before handling WRITE IO, or after handing READ IO, so that ublk
> +server can handle IO for ``/dev/ublkb*`` with the copied data.
> +
> +The extra copy between ublk request buffer and ublk server userspace buffer
> +not only increases CPU utilization(such as pinning pages, copy data), but
> +also consumes memory bandwidth, and the cost could be very big when IO size
> +is big. It is observed that ublk-null IOPS may be increased to ~5X if the
> +extra copy can be avoided.
> +
> +So zero copy is very important for supporting high performance block device
> +in userspace.
> +
> +Technical requirements
> +----------------------
> +
> +- ublk request buffer use
> +
> +ublk request buffer is represented by bio/bvec, which is immutable, so do
> +not try to change bvec via buffer reference; data can be read from or
> +written to the buffer according to buffer direction, but bvec can't be
> +changed
> +
> +- buffer lifetime
> +
> +Ublk server borrows ublk request buffer for handling ublk IO, ublk request
> +buffer reference is used. Reference can't outlive the referent buffer. That
> +means all request buffer references have to be released by ublk server
> +before ublk driver completes this request, when request buffer ownership
> +is transferred to upper layer(FS, application, ...).
> +
> +Also after ublk request is completed, any page belonging to this ublk
> +request can not be written or read any more from ublk server since it is
> +one block device from kernel viewpoint.
> +
> +- buffer direction
> +
> +For ublk WRITE request, ublk request buffer should only be accessed as data
> +source, and the buffer can't be written by ublk server
> +
> +For ublk READ request, ublk request buffer should only be accessed as data
> +destination, and the buffer can't be read by ublk server, otherwise kernel
> +data is leaked to ublk server, which can be unprivileged application.
> +
> +- arbitrary size sub-buffer needs to be retrieved from ublk server
> +
> +ublk is one generic framework for implementing block device in userspace,
> +and typical requirements include logical volume manager(mirror, stripped, ...),
> +distributed network storage, compressed target, ...
> +
> +ublk server needs to retrieve arbitrary size sub-buffer of ublk request, and
> +ublk server needs to submit IOs with these sub-buffer(s). That also means
> +arbitrary size sub-buffer(s) can be used to submit IO multiple times.
> +
> +Any sub-buffer is actually one reference of ublk request buffer, which
> +ownership can't be transferred to upper layer if any reference is held
> +by ublk server.
> +
> +Why slice isn't good for ublk zero copy


> +---------------------------------------
> +
> +- spliced page from ->splice_read() can't be written
> +
> +ublk READ request can't be handled because spliced page can't be written to, and
> +extending splice for ublk zero copy isn't one good solution [#splice_extend]_
> +
> +- it is very hard to meet above requirements  wrt. request buffer lifetime
> +
> +splice/pipe focuses on page reference lifetime, but ublk zero copy pays more
> +attention to ublk request buffer lifetime. If is very inefficient to respect
> +request buffer lifetime by using all pipe buffer's ->release() which requires
> +all pipe buffers and pipe to be kept when ublk server handles IO. That means
> +one single dedicated ``pipe_inode_info`` has to be allocated runtime for each
> +provided buffer, and the pipe needs to be populated with pages in ublk request
> +buffer.
> +
> +
> +io_uring fused command based zero copy
> +--------------------------------------
> +
> +io_uring fused command includes one primary command(uring command) and one
> +generic secondary request. The primary command is responsible for submitting
> +secondary request with provided buffer from ublk request, and primary command
> +won't be completed until the secondary request is completed.
> +
> +Typical ublk IO handling includes network and FS IO, so it is usual enough
> +for io_uring net & fs to support IO with provided buffer from primary command.
> -Zero copy is a generic requirement for nbd, fuse or similar drivers. A

Maybe replace 'requirement' with "very useful"? It works without zc, 
just with limited performance.

> -problem [#xiaoguang]_ Xiaoguang mentioned is that pages mapped to userspace
> -can't be remapped any more in kernel with existing mm interfaces. This can
> -occurs when destining direct IO to ``/dev/ublkb*``. Also, he reported that
> -big requests (IO size >= 256 KB) may benefit a lot from zero copy.
> +Once primary command is submitted successfully, ublk driver guarantees that
> +the ublk request buffer won't be gone away since secondary request actually
> +grabs the buffer's reference. This way also guarantees that multiple
> +concurrent fused commands associated with same request buffer works fine,
> +as the provided buffer reference is shared & read-only.
> +Also buffer usage direction flag is passed to primary command from userspace,
> +so ublk driver can validate if it is legal to use buffer with requested
> +direction.
>   References
>   ==========
> @@ -323,4 +431,4 @@ References
>   .. [#stefan]
> -.. [#xiaoguang]
> +.. [#splice_extend]

Any chance you could add a few code path into this document? For example 
as in Documentation/filesystems/fuse.rst,
section "Kernel - userspace interface"

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