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

[Feng Yang]

On Sat, 5 Jul 2025 08:16:40 +0100 David Laight <david.laight.linux@xxxxxxxxx> wrote:

> On Fri, 4 Jul 2025 17:50:42 +0200
> Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
> 
> > On 7/4/25 11:26 AM, Feng Yang wrote:
> > > Thu, 3 Jul 2025 12:44:53 +0100 david.laight.linux@xxxxxxxxx wrote:
> > >   
> > >> On Thu, 3 Jul 2025 10:48:40 +0200
> > >> Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
> > >>  
> > >>> On 6/30/25 9:10 AM, Feng Yang wrote:  
> > >>>> From: Feng Yang <yangfeng@xxxxxxxxxx>
> > >>>>
> > >>>> 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.
> 
> Correct, but the function is __skb_send_sock() - so you'd expect it to
> send the 'message' held in the skb to the socket.
> I don't think that the fact that the skb has fragments should make any
> difference to what is sent.
> In other words it ought to be valid for any code to 'linearize' the skb.
> 
> 	David

Okay, thank you for your explanations.

> > 
> > 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
> > 

So do I need to resend the v2 version again (https://lore.kernel.org/all/20250627094406.100919-1-yangfeng59949@xxxxxxx/), 
or is this version also inapplicable in some cases?