netdev - Re: [RFC net-next v3 23/29] io_uring: allow to pass addr into sendzc

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Date:   Wed, 29 Jun 2022 10:53:36 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Stefan Metzmacher <metze@...ba.org>, io-uring@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jens Axboe <axboe@...nel.dk>, kernel-team@...com
Subject: Re: [RFC net-next v3 23/29] io_uring: allow to pass addr into sendzc

On 6/29/22 08:42, Stefan Metzmacher wrote:
> 
> Hi Pavel,
> 
>> +    if (zc->addr) {
>> +        ret = move_addr_to_kernel(zc->addr, zc->addr_len, &address);
>> +        if (unlikely(ret < 0))
>> +            return ret;
>> +        msg.msg_name = (struct sockaddr *)&address;
>> +        msg.msg_namelen = zc->addr_len;
>> +    }
>> +
> 
> Given that this fills in msg almost completely can we also have
> a version of SENDMSGZC, it would be very useful to also allow
> msg_control to be passed and as well as an iovec.
> 
> Would that be possible?

Right, I left it to follow ups as the series is already too long.

fwiw, I'm going to also add addr to IORING_OP_SEND.


> Do I understand it correctly, that the reason for the new opcode is,
> that IO_OP_SEND would already work with existing MSG_ZEROCOPY behavior, together
> with the recvmsg based completion?

Right, it should work with MSG_ZEROCOPY, but with a different notification
semantics, would need recvmsg from error queues, and with performance
implications.


> In addition I wondering if a completion based on msg_iocb->ki_complete() (indicated by EIOCBQUEUED)
> what have also worked, just deferring the whole sendmsg operation until all buffers are no longer used.
> That way it would be possible to buffers are acked by the remote end when it comes back to the application
> layer.

There is msg_iocb, but it's mostly unused by protocols, IIRC apart
from crypto sockets. And then we'd need to repeat the path of
ubuf_info to handle stuff like skb splitting and perhaps also
changing rules for ->ki_complete


> I'm also wondering if the ki_complete() based approach should always be provided to sock_sendmsg()
> triggered by io_uring (independend of the new zerocopy stuff), it would basically work very simular to
> the uring_cmd() completions, which are able to handle both true async operation indicated by EIOCBQUEUED
> as well as EAGAIN triggered path via io-wq.

Would be even more similar to how we has always been doing
read/write, and rw requests do pass in a msg_iocb, but again,
it's largely ignored internally.

-- 
Pavel Begunkov