Re: [RFC 7/7] io_uring,fs: introduce IORING_OP_GET_BUF

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

On Sun, Apr 30, 2023 at 10:35:29AM +0100, Pavel Begunkov wrote:
> There are several problems with splice requests, aka IORING_OP_SPLICE:
> 1) They are always executed by a worker thread, which is a slow path,
>    as we don't have any reliable way to execute it NOWAIT.
> 2) It can't easily poll for data, as there are 2 files it operates on.
>    It would either need to track what file to poll or poll both of them,
>    in both cases it'll be a mess and add lot of overhead.
> 3) It has to have pipes in the middle, which adds overhead and is not
>    great from the uapi design perspective when it goes for io_uring
>    requests.
> 4) We want to operate with spliced data as with a normal buffer, i.e.
>    write / send / etc. data as normally while it's zerocopy.
> 
> It can partially be solved, but the root cause is a suboptimal for
> io_uring design of IORING_OP_SPLICE. Introduce a new request type
> called IORING_OP_GET_BUF, inspired by splice(2) as well as other
> proposals like fused requests. The main idea is to use io_uring's
> registered buffers as the middle man instead of pipes. Once a buffer
> is fetched / spliced from a file using a new fops callback
> ->iou_get_buf, it's installed as a registered buffers and can be used
> by all operations supporting the feature.
> 
> Once the userspace releases the buffer, io_uring will wait for all
> requests using the buffer to complete and then use a file provided
> callback ->release() to return the buffer back. It operates on the

In the commit of "io_uring: add an example for buf-get op", I don't see
any code to release the buffer, can you explain it in details about how
to release the buffer in userspace? And add it in your example?

Here I guess the ->release() is called in the following code path:

io_buffer_unmap
    io_rsrc_buf_put
        io_rsrc_put_work
            io_rsrc_node_ref_zero
                io_put_rsrc_node

If it is true, what is counter-pair code for io_put_rsrc_node()?
So far, only see io_req_set_rsrc_node() is called from
io_file_get_fixed(), is it needed for consumer OP of the buffer?

Also io_buffer_unmap() is called after io_rsrc_node's reference drops
to zero, which means ->release() isn't called after all its consumer(s)
are done given io_rsrc_node is shared by in-flight requests. If it is
true, this way will increase buffer lifetime a lot.

ublk zero copy needs to call ->release() immediately after all
consumers are done, because the ublk disk request won't be completed
until the buffer is released(the buffer actually belongs to ublk block request).

Also the usage in liburing example needs two extra syscall(io_uring_enter) for
handling one IO, not take into account the "release OP". IMO, this way makes
application more complicated, also might perform worse:

1) for ublk zero copy, the original IO just needs one OP, but now it
takes three OPs, so application has to take coroutine for applying
3 stages batch submission(GET_BUF, IO, release buffer) since IO_LINK can't
or not suggested to be used. In case of low QD, batch size is reduced much,
and performance may hurt because IOs/syscall is 1/3 of fused command.

2) GET_BUF OP is separated from the consumer OP, this way may cause
more cache miss, and I know this way is for avoiding IO_LINK.

I'd understand the approach first before using it to implement ublk zero copy
and comparing its performance with fused command.


Thanks, 
Ming