Re: [PATCH mptcp-next] mptcp: preserve MSG_EOR semantics in sendmsg path

[Matthieu Baerts <matttbe@kernel.org>]

Hi Gang,

Thank you for the new version.

On 09/03/2026 03:54, Gang Yan wrote:
> From: Gang Yan <yangang@kylinos.cn>
> 
> Extend MPTCP's sendmsg handling to recognize and honor the MSG_EOR flag,
> which marks the end of a record for application-level message boundaries.
> 
> Data fragments tagged with MSG_EOR are explicitly marked in the
> mptcp_data_frag structure and skb context to prevent unintended
> coalescing with subsequent data chunks. This ensures the intent of
> applications using MSG_EOR is preserved across MPTCP subflows,
> maintaining consistent message segmentation behavior.
> 
> Signed-off-by: Gang Yan <yangang@kylinos.cn>
> ---
> 
> Notes:
>       - This patch incorporates feedback and suggestions from Paolo Abeni
>         and Geliang Tang, including memory alignment optimizations for the
>         mptcp_data_frag struct (shrinking overhead to u8 and using bitfield
>         for eor to avoid size increase) and compile-time checks with BUILD_BUG_ON.

Please mention why you shrank "overhead" to a u8 (not to increase the
struct size), and why it is OK to do so (u16 not needed because ...) +
explaining the BUILD_BUG_ON().

>       - Packetdrill test cases validating this feature are available at:
>         https://github.com/multipath-tcp/packetdrill/pull/189/changes/d6ce92a4786704fe749bbd848ced0c047632282e

Thank you, I just reviewed it.

Do you mind checking the AI review there please:


https://netdev-ai.bots.linux.dev/ai-review.html?id=22434689-7326-48c8-af75-273d99fbef55

I think it is valid, but better to double-check.

> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index 17e43aff4459..3e574c87301b 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c

(...)

> @@ -4621,6 +4638,9 @@ void __init mptcp_proto_init(void)
>  	inet_register_protosw(&mptcp_protosw);
>  
>  	BUILD_BUG_ON(sizeof(struct mptcp_skb_cb) > sizeof_field(struct sk_buff, cb));
> +	/* Compile-time check: ensure 'overhead' (alignment + struct size) fits in u8 */
> +	BUILD_BUG_ON(ALIGN(1, sizeof(long)) + sizeof(struct mptcp_data_frag) > U8_MAX);

Sorry, I'm not sure what you are checking here. Do you mind explaining
it please?

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