Re: [RFC 00/12] io_uring zerocopy send

[Pavel Begunkov]

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.


> > 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;

--
Pavel Begunkov