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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

zijianzhang@ wrote:
> From: Zijian Zhang <zijianzhang@bytedance.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@bytedance.com>
> Signed-off-by: Xiaochun Lu <xiaochun.lu@bytedance.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[];
> +};