netdev - Re: [PATCH net-next v8 2/3] sock: add MSG_ZEROCOPY notification mechanism based on msg

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Message-ID: <66aab8b37157d_21c08c2941@willemb.c.googlers.com.notmuch>
Date: Wed, 31 Jul 2024 18:20:35 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: zijianzhang@...edance.com, 
 netdev@...r.kernel.org
Cc: linux-api@...r.kernel.org, 
 willemdebruijn.kernel@...il.com, 
 almasrymina@...gle.com, 
 edumazet@...gle.com, 
 davem@...emloft.net, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 dsahern@...nel.org, 
 axboe@...nel.dk, 
 shuah@...nel.org, 
 linux-kselftest@...r.kernel.org, 
 cong.wang@...edance.com, 
 xiaochun.lu@...edance.com, 
 Zijian Zhang <zijianzhang@...edance.com>
Subject: Re: [PATCH net-next v8 2/3] sock: add MSG_ZEROCOPY notification
 mechanism based on msg_control

zijianzhang@ wrote:
> From: Zijian Zhang <zijianzhang@...edance.com>
> 
> The MSG_ZEROCOPY flag enables copy avoidance for socket send calls.
> However, zerocopy is not a free lunch. Apart from the management of user
> pages, the combination of poll + recvmsg to receive notifications incurs
> unignorable overhead in the applications. We try to mitigate this overhead
> with a new notification mechanism based on msg_control. Leveraging the
> general framework to copy cmsgs to the user space, we copy zerocopy
> notifications to the user upon returning of sendmsgs.

May want to

- Explicitly state that receiving notifications on sendmsg is
  optional and existing recvmsg MSG_ERRQUEUE continues to work

- Include a very brief example of how this interface is used.
  Probably pseudo-code, as msghdr setup and CMSG processing are
  verbose operations

Btw patchwork shows red for patch 1/3 due to a new error or warning.
Not sure if it's a false positive, but take a look.
 
> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>

> +/*
> + * zc_info is the struct used for the SCM_ZC_NOTIFICATION control message.
> + */
> +struct zc_info {
> +	__u32 size; /* size of the zc_info_elem arr */

Size is ambiguous, could mean byte size. Perhaps length, or number of
elements in arr[].

> +	struct zc_info_elem arr[];
> +};