Re: [RFC mptcp-next] mptcp: support MSG_EOR in mptcp_sendmsg

[Matthieu Baerts <matttbe@kernel.org>]

Hi Gang,

On 02/02/2026 05:09, Gang Yan wrote:
> From: Gang Yan <yangang@kylinos.cn>
> 
> This patch adds support for the MSG_EOR flag in MPTCP's sendmsg path,
> ensuring that data fragments marked with MSG_EOR are properly handled
> to prevent coalescing with subsequent data.
> 
> Key changes:
> 1. Added an 'eor' field to struct mptcp_data_frag to track MSG_EOR marking
> 2. Initialize the eor field to 0 in mptcp_carve_data_frag()
> 3. In mptcp_sendmsg_frag(), when sending the last chunk of a data fragment
>    that has MSG_EOR set, mark the corresponding skb with TCP_SKB_CB(skb)->eor = 1
>    to prevent coalescing with subsequent data
> 4. Modified mptcp_sendmsg() to:
>    - Preserve MSG_EOR flag in msg_flags filtering
>    - Mark the last pending data fragment with eor = 1 when MSG_EOR is set
>      in the message flags
> 
> This ensures that applications using MSG_EOR to indicate record boundaries
> have their intent preserved across MPTCP subflows, maintaining proper
> message segmentation semantics.

Thank you for looking at this.

> 
> Signed-off-by: Gang Yan <yangang@kylinos.cn>
> ---
> 
> Notes:
>     Hi Matt,
>     
>     I create two packetdrill scripts to test the 'MSG_EOR' in
>     single/multi subflow(s). The link is attached below:
>     https://github.com/multipath-tcp/packetdrill/compare/mptcp-net-next...Dwyane-Yan:packetdrill:mptcp-net-next
>     
>     Please ignore the indent problems, I will fix them when
>     the 'MSG_EOR' patch is accepted.

I think it is better to be able to review the tests at the same time as
the implementation. Can you then fix the indent problems and open a PR
please?

In the meantime, it looks like checkpatch is complaining here:

> WARNING: Prefer a maximum 75 chars per line (possible unwrapped commit description?)
> #14: 
>    that has MSG_EOR set, mark the corresponding skb with TCP_SKB_CB(skb)->eor = 1
> 
> WARNING: line length of 107 exceeds 80 columns
> #95: FILE: net/mptcp/protocol.h:270:
> +	u8 eor;					/* Is MSG_EOR marked? Prevents coalescing with next frag */
> 
> total: 0 errors, 2 warnings, 0 checks, 54 lines checked

Can you look at that, please?

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.