netdev - Re: [PATCH net-next v7 1/3] sock: support copying cmsgs to the user space in sendmsg

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Message-ID: <d53adec9-10d5-41e2-8065-3826029f6134@bytedance.com>
Date: Wed, 24 Jul 2024 18:11:06 -0700
From: Zijian Zhang <zijianzhang@...edance.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: edumazet@...gle.com, cong.wang@...edance.com, xiaochun.lu@...edance.com
Subject: Re: [PATCH net-next v7 1/3] sock: support copying cmsgs to the user
 space in sendmsg

On 7/9/24 9:40 AM, Willem de Bruijn wrote:
> zijianzhang@ wrote:
>> From: Zijian Zhang <zijianzhang@...edance.com>
>>
>> Users can pass msg_control as a placeholder to recvmsg, and get some info
>> from the kernel upon returning of it, but it's not available for sendmsg.
>> Recvmsg uses put_cmsg to copy info back to the user, while ____sys_sendmsg
>> creates a kernel copy of msg_control and passes that to the callees,
>> put_cmsg in sendmsg path will write into this kernel buffer.
>>
>> If users want to get info after returning of sendmsg, they typically have
>> to call recvmsg on the ERRMSG_QUEUE of the socket, incurring extra system
> 
> nit: error queue or MSG_ERRQUEUE
> 
>> call overhead. This commit supports copying cmsg from the kernel space to
>> the user space upon returning of sendmsg to mitigate this overhead.
>>
>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
> 
> Overall this approach follows what I had in mind, thanks.
> 
> Looking forward to the discussion with a wider audience at netdevconf
> next week.


After wider exposure to netdev, besides the comments in this email
series, I want to align the next step with you :)

Shall I also make this a config and add conditional compilation in the
hot path？

>> ---
>>   include/linux/socket.h |  6 +++++
>>   include/net/sock.h     |  2 +-
>>   net/core/sock.c        |  6 +++--
>>   net/ipv4/ip_sockglue.c |  2 +-
>>   net/ipv6/datagram.c    |  2 +-
>>   net/socket.c           | 54 ++++++++++++++++++++++++++++++++++++++----
>>   6 files changed, 62 insertions(+), 10 deletions(-)
>>