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

[gang.yan@linux.dev]

March 30, 2026 at 5:50 PM, "Matthieu Baerts" <matttbe@kernel.org mailto:matttbe@kernel.org?to=%22Matthieu%20Baerts%22%20%3Cmatttbe%40kernel.org%3E > wrote:


> 
> Hi Gang,
> 
> On 30/03/2026 10:19, gang.yan@linux.dev wrote:
> 
> > 
> > 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.
> > 
> Thank you for the explanation. Can you then mention in the commit
> message that it is fine to reduce overhead to a 'u8', and add the above
> explanation, please?
> 
> If 'offset' max value is 7, it could also be reduced from a u16 to a u8
> then, no?

Hi, Matt:

Sorry, there was an error in the explanation. The maximum value of
(offset - origin_offset) is 7, so the 'offset' variable should use u16.

> 
> > 
> > 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'.
> > 
> It might be good to add a comment here too, at least to explain that
> "ALIGN(1, sizeof(long))" represents 'offset' maximum size.

Good idea, I'll apply your suggestions in v2.

Thanks
Gang
> 
> > 
> > But Paolo also mention it's probably too conservative. WDYT?
> > 
> Maybe, but it doesn't hurt I suppose. As long as this check is clearly
> linked to different fields from the mptcp_data_frag structure → having a
> comment explaining that.
> 
> Cheers,
> Matt
> -- 
> Sponsored by the NGI0 Core fund.
>