Re: [PATCH net-next v6 3/4] 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. The overhead of such sometimes
> might be more than the CPU savings from zerocopy. We try to solve this
> problem with a new notification mechanism based on msgcontrol.
> 
> This new mechanism aims to reduce the overhead associated with receiving
> notifications by embedding them directly into user arguments passed with
> each sendmsg control message. By doing so, we can significantly reduce
> the complexity and overhead for managing notifications. In an ideal
> pattern, the user will keep calling sendmsg with SCM_ZC_NOTIFICATION
> msg_control, and the notification will be delivered as soon as possible.
> 
> Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
> Signed-off-by: Xiaochun Lu <xiaochun.lu@bytedance.com>
> ---
>  arch/alpha/include/uapi/asm/socket.h  |  2 ++
>  arch/mips/include/uapi/asm/socket.h   |  2 ++
>  arch/parisc/include/uapi/asm/socket.h |  2 ++
>  arch/sparc/include/uapi/asm/socket.h  |  2 ++
>  include/linux/socket.h                |  2 +-
>  include/uapi/asm-generic/socket.h     |  2 ++
>  include/uapi/linux/socket.h           | 10 +++++++
>  net/core/sock.c                       | 42 +++++++++++++++++++++++++++
>  8 files changed, 63 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index e94f621903fe..7761a4e0ea2c 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -140,6 +140,8 @@
>  #define SO_PASSPIDFD		76
>  #define SO_PEERPIDFD		77
>  
> +#define SCM_ZC_NOTIFICATION 78
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 60ebaed28a4c..89edc51380f0 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -151,6 +151,8 @@
>  #define SO_PASSPIDFD		76
>  #define SO_PEERPIDFD		77
>  
> +#define SCM_ZC_NOTIFICATION 78
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index be264c2b1a11..2911b43e6a9d 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -132,6 +132,8 @@
>  #define SO_PASSPIDFD		0x404A
>  #define SO_PEERPIDFD		0x404B
>  
> +#define SCM_ZC_NOTIFICATION 0x404C
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 682da3714686..dc045e87cc8e 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -133,6 +133,8 @@
>  #define SO_PASSPIDFD             0x0055
>  #define SO_PEERPIDFD             0x0056
>  
> +#define SCM_ZC_NOTIFICATION 0x0057
> +
>  #if !defined(__KERNEL__)
>  
>  
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 35adc30c9db6..f2f013166525 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -170,7 +170,7 @@ static inline struct cmsghdr * cmsg_nxthdr (struct msghdr *__msg, struct cmsghdr
>  
>  static inline bool cmsg_copy_to_user(struct cmsghdr *__cmsg)
>  {
> -	return 0;
> +	return __cmsg->cmsg_type == SCM_ZC_NOTIFICATION;
>  }
>  
>  static inline size_t msg_data_left(struct msghdr *msg)
> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> index 8ce8a39a1e5f..7474c8a244bc 100644
> --- a/include/uapi/asm-generic/socket.h
> +++ b/include/uapi/asm-generic/socket.h
> @@ -135,6 +135,8 @@
>  #define SO_PASSPIDFD		76
>  #define SO_PEERPIDFD		77
>  
> +#define SCM_ZC_NOTIFICATION 78
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
> diff --git a/include/uapi/linux/socket.h b/include/uapi/linux/socket.h
> index d3fcd3b5ec53..26bee6291c6c 100644
> --- a/include/uapi/linux/socket.h
> +++ b/include/uapi/linux/socket.h
> @@ -2,6 +2,8 @@
>  #ifndef _UAPI_LINUX_SOCKET_H
>  #define _UAPI_LINUX_SOCKET_H
>  
> +#include <linux/types.h>
> +
>  /*
>   * Desired design of maximum size and alignment (see RFC2553)
>   */
> @@ -35,4 +37,12 @@ struct __kernel_sockaddr_storage {
>  #define SOCK_TXREHASH_DISABLED	0
>  #define SOCK_TXREHASH_ENABLED	1
>  
> +#define SOCK_ZC_INFO_MAX 16
> +
> +struct zc_info_elem {
> +	__u32 lo;
> +	__u32 hi;
> +	__u8 zerocopy;
> +};
> +
>  #endif /* _UAPI_LINUX_SOCKET_H */
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 4a766a91ff5c..1b2ce72e1338 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2863,6 +2863,48 @@ int __sock_cmsg_send(struct sock *sk, struct cmsghdr *cmsg,
>  	case SCM_RIGHTS:
>  	case SCM_CREDENTIALS:
>  		break;
> +	case SCM_ZC_NOTIFICATION: {
> +		struct zc_info_elem *zc_info_kern = CMSG_DATA(cmsg);
> +		int cmsg_data_len, zc_info_elem_num;
> +		struct sock_exterr_skb *serr;
> +		struct sk_buff_head *q;
> +		unsigned long flags;
> +		struct sk_buff *skb;
> +		int i = 0;
> +
> +		if (!sock_flag(sk, SOCK_ZEROCOPY) || sk->sk_family == PF_RDS)
> +			return -EINVAL;
> +
> +		cmsg_data_len = cmsg->cmsg_len - sizeof(struct cmsghdr);
> +		if (cmsg_data_len % sizeof(struct zc_info_elem))
> +			return -EINVAL;
> +
> +		zc_info_elem_num = cmsg_data_len / sizeof(struct zc_info_elem);
> +		if (!zc_info_elem_num || zc_info_elem_num > SOCK_ZC_INFO_MAX)
> +			return -EINVAL;
> +
> +		q = &sk->sk_error_queue;
> +		spin_lock_irqsave(&q->lock, flags);
> +		skb = skb_peek(q);
> +		while (skb && i < zc_info_elem_num) {
> +			struct sk_buff *skb_next = skb_peek_next(skb, q);
> +
> +			serr = SKB_EXT_ERR(skb);
> +			if (serr->ee.ee_errno == 0 &&
> +			    serr->ee.ee_origin == SO_EE_ORIGIN_ZEROCOPY) {
> +				zc_info_kern[i].hi = serr->ee.ee_data;
> +				zc_info_kern[i].lo = serr->ee.ee_info;
> +				zc_info_kern[i].zerocopy = !(serr->ee.ee_code
> +								& SO_EE_CODE_ZEROCOPY_COPIED);
> +				__skb_unlink(skb, q);
> +				consume_skb(skb);
> +			}
> +			skb = skb_next;
> +			i++;
> +		}

How will userspace know the number of entries written? Since the
cmsg_len is not updated, is the expectation that the CMSG_DATA is zero
initialized by the user and the list will be zero-element terminated?

> +		spin_unlock_irqrestore(&q->lock, flags);
> +		break;
> +	}
>  	default:
>  		return -EINVAL;
>  	}
> -- 
> 2.20.1
>