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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Thu, 2 Dec 2021 16:25:42 -0500
Message-ID: <CA+FuTSfe63=SuuZeC=eZPLWstgOL6oFUrsL4o+J8=3BwHJSTVg@mail.gmail.com>
Subject: Re: [RFC 00/12] io_uring zerocopy send
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        io-uring@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>, Jens Axboe <axboe@kernel.dk>
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> > What if the ubuf pool can be found from the sk, and the index in that
> > pool is passed as a cmsg?
>
> It looks to me that ubufs are by nature is something that is not
> tightly bound to a socket (at least for io_uring API in the patchset),
> it'll be pretty ugly:
>
> 1) io_uring'd need to care to register the pool in the socket. Having
> multiple rings using the same socket would be horrible. It may be that
> it doesn't make much sense to send in parallel from multiple rings, but
> a per thread io_uring is a popular solution, and then someone would
> want to pass a socket from one thread to another and we'd need to support
> it.
>
> 2) And io_uring would also need to unregister it, so the pool would
> store a list of sockets where it's used, and so referencing sockets
> and then we need to bind it somehow to io_uring fixed files or
> register all that for tracking referencing circular dependencies.
>
> 3) IIRC, we can't add a cmsg entry from the kernel, right? May be wrong,
> but if so I don't like exposing basically io_uring's referencing through
> cmsg. And it sounds io_uring would need to parse cmsg then.
>
>
> A lot of nuances :) I'd really prefer to pass it on per-request basis,

Ok

> it's much cleaner, but still haven't got what's up with msghdr
> initialisation...

And passing the struct through multiple layers of functions.

> Maybe, it's better to add a flags field, which would include
> "msg_control_is_user : 1" and whether msghdr includes msg_iocb, msg_ubuf,
> and everything else that may be optional. Does it sound sane?

If sendmsg takes the argument, it will just have to be initialized, I think.

Other functions are not aware of its existence so it can remain
uninitialized there.