linux-kernel - Re: [PATCH v3] skbuff: Add MSG_MORE flag to optimize large packet transmission

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Message-ID: <1a24a603-b49f-4692-a116-f25605301af6@redhat.com>
Date: Fri, 4 Jul 2025 17:50:42 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Feng Yang <yangfeng59949@....com>, david.laight.linux@...il.com
Cc: aleksander.lobakin@...el.com, almasrymina@...gle.com,
 asml.silence@...il.com, davem@...emloft.net, ebiggers@...gle.com,
 edumazet@...gle.com, horms@...nel.org, kerneljasonxing@...il.com,
 kuba@...nel.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 stfomichev@...il.com, willemb@...gle.com, yangfeng@...inos.cn
Subject: Re: [PATCH v3] skbuff: Add MSG_MORE flag to optimize large packet
 transmission

On 7/4/25 11:26 AM, Feng Yang wrote:
> Thu, 3 Jul 2025 12:44:53 +0100 david.laight.linux@...il.com wrote:
> 
>> On Thu, 3 Jul 2025 10:48:40 +0200
>> Paolo Abeni <pabeni@...hat.com> wrote:
>>
>>> On 6/30/25 9:10 AM, Feng Yang wrote:
>>>> From: Feng Yang <yangfeng@...inos.cn>
>>>>
>>>> The "MSG_MORE" flag is added to improve the transmission performance of large packets.
>>>> The improvement is more significant for TCP, while there is a slight enhancement for UDP.  
>>>
>>> I'm sorry for the conflicting input, but i fear we can't do this for
>>> UDP: unconditionally changing the wire packet layout may break the
>>> application, and or at very least incur in unexpected fragmentation issues.
>>
>> Does the code currently work for UDP?
>>
>> I'd have thought the skb being sent was an entire datagram.
>> But each semdmsg() is going to send a separate datagram.
>> IIRC for UDP MSG_MORE indicates that the next send() will be
>> part of the same datagram - so the actual send can't be done
>> until the final fragment (without MSG_MORE) is sent.
> 
> If we add MSG_MORE, won't the entire skb be sent out all at once? Why doesn't this work for UDP?

Without MSG_MORE N sendmsg() calls will emit on the wire N (small) packets.

With MSG_MORE on the first N-1 calls, the stack will emit a single
packet with larger size.

UDP application may relay on packet size for protocol semantic. i.e. the
application level message size could be expected to be equal to the
(wire) packet size itself.

Unexpectedly aggregating the packets may break the application. Also it
can lead to IP fragmentation, which in turn could kill performances.

> If that's not feasible, would the v2 version of the code work for UDP?

My ask is to explicitly avoid MSG_MORE when the transport is UDP.

/P