Date: Mon, 13 May 2024 13:22:55 -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: [External] Re: [PATCH net-next v3 2/3] sock: add MSG_ZEROCOPY
 notification mechanism based on msg_control

On 5/13/24 12:47 PM, Zijian Zhang wrote:
> On 5/12/24 5:58 PM, Willem de Bruijn wrote:
>> 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. 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@...edance.com>
>>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
>>
>>> +#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 128
>>> +
>>> +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 8d6e638b5426..15da609be026 100644
>>> --- a/net/core/sock.c
>>> +++ b/net/core/sock.c
>>> @@ -2842,6 +2842,74 @@ int __sock_cmsg_send(struct sock *sk, struct 
>>> cmsghdr *cmsg,
>>>       case SCM_RIGHTS:
>>>       case SCM_CREDENTIALS:
>>>           break;
>>> +    case SCM_ZC_NOTIFICATION: {
>>> +        int ret, i = 0;
>>> +        int cmsg_data_len, zc_info_elem_num;
>>> +        void __user    *usr_addr;
>>> +        struct zc_info_elem zc_info_kern[SOCK_ZC_INFO_MAX];
>>> +        unsigned long flags;
>>> +        struct sk_buff_head *q, local_q;
>>> +        struct sk_buff *skb, *tmp;
>>> +        struct sock_exterr_skb *serr;
>>
>> minor: reverse xmas tree
>>
> 
> Ack.
> 
>>> +
>>> +        if (!sock_flag(sk, SOCK_ZEROCOPY) || sk->sk_family == PF_RDS)
>>> +            return -EINVAL;
>>
>> Is this mechanism supported for PF_RDS?
>> The next patch fails on PF_RDS + '-n'
>>
> 
> Nice catch! This mechanism does not support PF_RDS, I will update the
> selftest code.
> 

PF_RDS does not use MSGERR queue to store the info, thus it is not
supported by this patch. I will leave it as "unsupported" in the 
"selftest -n" now.

If possible, I may leave the support for PF_RDS in another patch set in
the future.

>>> +
>>> +        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;
>>> +
>>> +        if (in_compat_syscall())
>>> +            usr_addr = compat_ptr(*(compat_uptr_t *)CMSG_DATA(cmsg));
>>> +        else
>>> +            usr_addr = (void __user *)*(void **)CMSG_DATA(cmsg);
>>
>> The main design issue with this series is this indirection, rather
>> than passing the array of notifications as cmsg.
>>
>> This trick circumvents having to deal with compat issues and having to
>> figure out copy_to_user in ____sys_sendmsg (as msg_control is an
>> in-kernel copy).
>>
>> This is quite hacky, from an API design PoV.
>>
>> As is passing a pointer, but expecting msg_controllen to hold the
>> length not of the pointer, but of the pointed to user buffer.
>>
>> I had also hoped for more significant savings. Especially with the
>> higher syscall overhead due to meltdown and spectre mitigations vs
>> when MSG_ZEROCOPY was introduced and I last tried this optimization.
>>
> Thanks for the summary, totally agree! It's a hard choice to design the
> API like this.
> 
>>> +        if (!access_ok(usr_addr, cmsg_data_len))
>>> +            return -EFAULT;
>>> +
>>> +        q = &sk->sk_error_queue;
>>> +        skb_queue_head_init(&local_q);
>>> +        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);
>>> +                __skb_queue_tail(&local_q, skb);
>>> +                i++;
>>> +            }
>>> +            skb = skb_next;
>>> +        }
>>> +        spin_unlock_irqrestore(&q->lock, flags);
>>> +
>>> +        ret = copy_to_user(usr_addr,
>>> +                   zc_info_kern,
>>> +                    i * sizeof(struct zc_info_elem));
>>> +
>>> +        if (unlikely(ret)) {
>>> +            spin_lock_irqsave(&q->lock, flags);
>>> +            skb_queue_reverse_walk_safe(&local_q, skb, tmp) {
>>> +                __skb_unlink(skb, &local_q);
>>> +                __skb_queue_head(q, skb);
>>> +            }
>>
>> Can just list_splice_init?
>>
> 
> Ack.
> 
>>> +            spin_unlock_irqrestore(&q->lock, flags);
>>> +            return -EFAULT;
>>> +        }
>>> +
>>> +        while ((skb = __skb_dequeue(&local_q)))
>>> +            consume_skb(skb);
>>> +        break;
>>> +    }
>>>       default:
>>>           return -EINVAL;
>>>       }
>>> -- 
>>> 2.20.1
>>>
>>
>>

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