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

[gang.yan@linux.dev]

March 27, 2026 at 12:42 AM, "Matthieu Baerts" <matttbe@kernel.org mailto:matttbe@kernel.org?to=%22Matthieu%20Baerts%22%20%3Cmatttbe%40kernel.org%3E > wrote:


> 
> 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().

The ‘u8’ is one of Paolo's suggestions[1]. I think 'u16' is not needed because:
 - 'offset = ALIGN(orig_offset, sizeof(long));'
 - 'dfrag->offset = offset - origin_offset + sizeof(struct mptcp_data_frag);',
the max value of offset is 7, and sizeof(struct mptcp_data_frag)) is
usually 40, so the overhead is 47, far less than 255.

Another suggestion from Paolo[1] is a build time check on the max 'overhead'
value. So I use 'ALIGN(1, sizeof(long)) + sizeof(struct mptcp_data_frag)' to
represent the max_val of 'overhead'.

But Paolo also mention it's probably too conservative. WDYT?

[1] https://patchwork.kernel.org/project/mptcp/patch/20260203023029.855434-1-gang.yan@linux.dev/

> 
> > 
> > - 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.

Thanks， I'll try to fix them.

> 
> 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.

Yes, I think it's a good catch, and we should fix it as follows:

@@ -1032,7 +1032,8 @@ static bool mptcp_frag_can_collapse_to(const struct mptcp_sock *msk,
                                       const struct page_frag *pfrag,
                                       const struct mptcp_data_frag *df)
 {
-       return df && pfrag->page == df->page &&
+       return df && !df->eor &&
+               pfrag->page == df->page &&
                pfrag->size - pfrag->offset > 0 &&
                pfrag->offset == (df->offset + df->data_len) &&
                df->data_seq + df->data_len == msk->write_seq;
If OK, I'll apply it when sending v2.

> 
> > 
> > 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?
> 

The 'BUILD_BUG_ON' is explained at the beginning of the reply, thanks.

Cheers,
Gang

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