[RFC v2 mptcp-next] mptcp: support MSG_EOR in mptcp_sendmsg

Gang Yan
2026-02-03T02:31:05Z

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.

Signed-off-by: Gang Yan <yangang@kylinos.cn>
---

Notes:
changelog:
v2:
- Fix code-style issues.
- Submitted a pull request to the mptcp_packetdrill repository:
https://github.com/multipath-tcp/packetdrill/pull/189

Hi Matt:

Thank you for your feedback on v1. I've addressed the suggestions in this
updated version. Please take a look when you have a moment.

Thanks
Gang

net/mptcp/protocol.c | 22 +++++++++++++++++++---
net/mptcp/protocol.h | 1 +
2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c88882062c40..b8200765506f 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1174,6 +1174,7 @@ mptcp_carve_data_frag(const struct mptcp_sock *msk, struct page_frag *pfrag,
dfrag->offset = offset + sizeof(struct mptcp_data_frag);
dfrag->already_sent = 0;
dfrag->page = pfrag->page;
+ dfrag->eor = 0;

return dfrag;
}
@@ -1434,6 +1435,13 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
mptcp_update_infinite_map(msk, ssk, mpext);
trace_mptcp_sendmsg_frag(mpext);
mptcp_subflow_ctx(ssk)->rel_write_seq += copy;
+
+ /* If this is the last chunk of a dfrag with MSG_EOR set
+ * mark the skb to prevent coalescing with subsequent data
+ */
+ if (dfrag->eor && info->sent + copy >= dfrag->data_len)
+ TCP_SKB_CB(skb)->eor = 1;
+
return copy;
}

@@ -1894,7 +1902,8 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
long timeo;

/* silently ignore everything else */
- msg->msg_flags &= MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL | MSG_FASTOPEN;
+ msg->msg_flags &= MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL |
+ MSG_FASTOPEN | MSG_EOR;

lock_sock(sk);

@@ -2001,9 +2010,16 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
goto do_error;
}

- if (copied)
- __mptcp_push_pending(sk, msg->msg_flags);
+ if (copied) {
+ /* Mark the last dfrag with EOR if MSG_EOR was set */
+ if (msg->msg_flags & MSG_EOR) {
+ struct mptcp_data_frag *dfrag = mptcp_pending_tail(sk);

+ if (dfrag)
+ dfrag->eor = 1;
+ }
+ __mptcp_push_pending(sk, msg->msg_flags);
+ }
out:
release_sock(sk);
return copied;
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index cd5266099993..8b243062009c 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -267,6 +267,7 @@ struct mptcp_data_frag {
u16 overhead;
u16 already_sent;
struct page *page;
+ u8 eor; /* Is MSG_EOR marked? Prevents coalescing with next frag */
};

/* Arbitrary compromise between as low as possible to react timely to subflow
--
2.43.0