Re: [RFC 00/12] io_uring zerocopy send

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Fri, Dec 3, 2021 at 11:19 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 12/2/21 21:25, Willem de Bruijn wrote:
> >>> 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.
>
> If you refer to ip_make_skb(ubuf) -> __ip_append_data(ubuf), I agree
> it's a bit messier, will see what can be done. If you're about
> msghdr::msg_ubuf, for me it's more like passing a callback,
> which sounds like a normal thing to do.

Thanks, I do mean the first.

Also, small nit now that it comes up again msghdr::msg_ubuf is not
plain C. I would avoid that pseudo C++ notation (in the subject line
of 3/12)
>
> >> 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.
>
> Got it, need to double check, but looks something like 1/12 should
> be as you outlined.
>
> And if there will be multiple optional fields that have to be
> initialised, we would be able to hide all the zeroing under a
> single bitmask. E.g. instead of
>
> msg->field1 = NULL;
> ...
> msg->fieldN = NULL;
>
> It may look like
>
> msg->mask = 0; // HAS_FIELD1 | HAS_FIELDN;

Makes sense to me. This patch series only adds one field, so you can
leave the optimization for a possible future separate patch series?