netdev - Re: Possible bugfix for AF_UNIX, SOCK

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Date:	Wed, 15 Feb 2012 13:42:07 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Piergiorgio Beruto <piergiorgio.beruto@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: Possible bugfix for AF_UNIX, SOCK_SEQPACKET sockets

Le mercredi 15 février 2012 à 11:43 +0100, Piergiorgio Beruto a écrit :

> Yes, there's nothing that "doesn't work", it's a matter of performance
> (I am working on strong embedded so I'm quite concerned about both
> memory usage and "speed").
> 
> The problem is that when the socket queue is filled with short sized
> packets, dequeue operation would allocate a lot of "big" chunks of
> memory, progressively smaller (first one the size of the queue, second
> one = queue size - first packet size and so on).
> 
> Besides the waste of memory, you get less perfromance as malloc()
> would use the heap or mmap() depending on the size of the chunk
> (usually 64 bytes) and  the use of mmap is more memory efficient but
> quite slower.

I see

> Ok, that's why I asked :) But if you agree with my objection regarding
> performance, what about adding a brand new (linux only) ioctl which
> implements the other behaviour? So the code would look something like
> this:
> 
> case SIOCINQ:
> case SIOCPSZ:     //// new packet size ioctl
>        {
>                struct sk_buff *skb;
> 
>                if (sk->sk_state == TCP_LISTEN) {
>                        err = -EINVAL;
>                        break;
>                }
> 
>                spin_lock(&sk->sk_receive_queue.lock);
>                if ((sk->sk_type == SOCK_STREAM ||
>                    sk->sk_type == SOCK_SEQPACKET) &&
>                    ioctl_code != SIOCPSZ) {       //// have SIOCPSZ
> behave as for datagram sockets
>                        skb_queue_walk(&sk->sk_receive_queue, skb)
>                                amount += skb->len;
>                } else {
>                        skb = skb_peek(&sk->sk_receive_queue);
>                        if (skb)
>                                amount = skb->len;
>                }
>                spin_unlock(&sk->sk_receive_queue.lock);
>                err = put_user(amount, (int __user *)arg);
>                break;
>        }

ioctl() are deprecated, and such a change has ramification (for example
on strace tool)

You could use a recvmsg( ... MSG_PEEK|MSG_TRUNC) to get size of next
packet (passing a small buffer)

Ah... it seems af_unix doesnt handle MSG_TRUNC semantic as
UDP/RAW/NETLINK sockets do.


diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 85d3bb7..70d9414 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1824,7 +1824,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 		if (UNIXCB(skb).fp)
 			siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);
 	}
-	err = size;
+	err = (flags & MSG_TRUNC) ? skb->len : size;
 
 	scm_recv(sock, msg, siocb->scm, flags);
 


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