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Message-ID: <ZqLKy8OqpMi-kPQ3@google.com>
Date: Thu, 25 Jul 2024 21:59:39 +0000
From: Mina Almasry <almasrymina@...gle.com>
To: zijianzhang@...edance.com
Cc: netdev@...r.kernel.org, edumazet@...gle.com,
willemdebruijn.kernel@...il.com, cong.wang@...edance.com,
xiaochun.lu@...edance.com
Subject: Re: [PATCH net-next v7 2/3] sock: add MSG_ZEROCOPY notification
mechanism based on msg_control
On Mon, Jul 08, 2024 at 09:04:04PM +0000, zijianzhang@...edance.com 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.
>
> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.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 | 13 ++++++++
> net/core/sock.c | 46 +++++++++++++++++++++++++++
> 8 files changed, 70 insertions(+), 1 deletion(-)
>
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index e94f621903fe..7c32d9dbe47f 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..3f7fade998cb 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..77f5bee0fdc9 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..eb44fc515b45 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 75461812a7a3..6f1b791e2de8 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -171,7 +171,7 @@ 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..02e9159c7944 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..ab361f30f3a6 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 +37,15 @@ struct __kernel_sockaddr_storage {
> #define SOCK_TXREHASH_DISABLED 0
> #define SOCK_TXREHASH_ENABLED 1
>
> +struct zc_info_elem {
> + __u32 lo;
> + __u32 hi;
> + __u8 zerocopy;
Some docs please on what each of these are, if possible. Sorry if the repeated
requests are annoying.
In particular I'm a bit confused why the zerocopy field is there. Looking at
the code, is this always set to 1?
> +};
> +
> +struct zc_info {
> + __u32 size;
> + struct zc_info_elem arr[];
> +};
> +
> #endif /* _UAPI_LINUX_SOCKET_H */
> diff --git a/net/core/sock.c b/net/core/sock.c
> index efb30668dac3..e0b5162233d3 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2863,6 +2863,52 @@ int __sock_cmsg_send(struct sock *sk, struct msghdr *msg, struct cmsghdr *cmsg,
> case SCM_RIGHTS:
> case SCM_CREDENTIALS:
> break;
> + case SCM_ZC_NOTIFICATION: {
> + struct zc_info *zc_info = CMSG_DATA(cmsg);
> + struct zc_info_elem *zc_info_arr;
> + struct sock_exterr_skb *serr;
> + int cmsg_data_len, i = 0;
> + struct sk_buff_head *q;
> + unsigned long flags;
> + struct sk_buff *skb;
> + u32 zc_info_size;
> +
> + 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))
> + return -EINVAL;
> +
> + zc_info_size = zc_info->size;
> + zc_info_arr = zc_info->arr;
Annoying nit: To be honest zc_info->size isn't much longer to type than
zc_info_size, so I would have not added local variables.
> + if (cmsg_data_len != sizeof(struct zc_info) +
> + zc_info_size * sizeof(struct zc_info_elem))
> + return -EINVAL;
> +
> + q = &sk->sk_error_queue;
> + spin_lock_irqsave(&q->lock, flags);
> + skb = skb_peek(q);
> + while (skb && i < zc_info_size) {
> + 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_arr[i].hi = serr->ee.ee_data;
> + zc_info_arr[i].lo = serr->ee.ee_info;
> + zc_info_arr[i].zerocopy = !(serr->ee.ee_code
> + & SO_EE_CODE_ZEROCOPY_COPIED);
> + __skb_unlink(skb, q);
> + consume_skb(skb);
> + i++;
> + }
> + skb = skb_next;
> + }
> + spin_unlock_irqrestore(&q->lock, flags);
I wonder if you should drop the spin lock in the middle of this loop somehow,
otherwise you may end up spinning for a very long time while the spinlock held
and irq disabled.
IIRC zc_info_size is user input, right? Maybe you should limit zc_info_size to
16 entries or something. So the user doesn't end up passing 100000 as
zc_info_size and making the kernel loop for a long time here.
> + zc_info->size = i;
> + break;
> + }
> default:
> return -EINVAL;
> }
> --
> 2.20.1
>
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