Message-ID: <e79a9cf6-b315-d4a5-a4a8-1071b5046c6e@gmail.com>
Date:   Fri, 3 Dec 2021 16:19:28 +0000
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.3.2
Subject: Re: [RFC 00/12] io_uring zerocopy send
Content-Language: en-US
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     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>
References: <cover.1638282789.git.asml.silence@gmail.com>
 <CA+FuTSf-N08d6pcbie2=zFcQJf3_e2dBJRUZuop4pOhNfSANUA@mail.gmail.com>
 <0d82f4e2-730f-4888-ec82-2354ffa9c2d8@gmail.com>
 <CA+FuTSf1dk-ZCN_=oFcYo31XdkLLAaHJHHNfHwJKe01CVq3X+A@mail.gmail.com>
 <6e07fb0c-075b-4072-273b-f9d55ba1e1dd@gmail.com>
 <CA+FuTSfe63=SuuZeC=eZPLWstgOL6oFUrsL4o+J8=3BwHJSTVg@mail.gmail.com>
From:   Pavel Begunkov <asml.silence@gmail.com>
In-Reply-To: <CA+FuTSfe63=SuuZeC=eZPLWstgOL6oFUrsL4o+J8=3BwHJSTVg@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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