linux-kernel - Re: [RFC 00/12] io

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Date:   Fri, 3 Dec 2021 16:19:28 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     io-uring@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC 00/12] io_uring zerocopy send

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