Re: [RFC v2] tcp: add support for read with offset when using MSG_PEEK

[Jon Maloy <jmaloy@redhat.com>]

(Added <passt-dev@passt.top> to distribution list.

I found that my first approach was too simplistic, since it only moved 
the reading
area forward in the receive buffer, but continue to fill in iov[0] with 
the indicated
length.
This commit exactly what we want: we indicate a NULL pointer in iov[0], but
want the actually read bytes to end up in the remaining entries, and also
the returned value to indicate the actually read length.

I look forward to feedback to this, then I can hopefully post it to the 
netdev
list next week.

///jon


On 2023-06-22 22:12, Jon Maloy wrote:
> When reading received messages with MSG_PEEK, we sometines have to read
> the leading bytes of the stream several times, only to reach the bytes
> we really want. This is clearly non-optimal.
>
> What we would want is something similar to pread/preadv(), but working
> even for tcp sockets. At the same time, we obviously don't want to add
> any new arguments to the recv/recvmsg() calls.
>
> In this commit, we allow the user to set iovec.iov_base in the first
> vector entry to NULL. This tells the socket to skip the first entry,
> hence making the iov_len field of that entry indicate the offset value.
> This way, there is no need to add any new arguments.
>
> This change is simple and non-intrusive, and should be safe addition to
> the socket API. We have measured it to give a throughput improvement of
> 8-10 % for the protocol splicer 'passst', which is used in KubeVirt
> containers.
>
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
>
> works with original msghdr
>
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>   net/ipv4/tcp.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 33f559f491c8..1d89337e89b6 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2428,6 +2428,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>   	struct tcp_sock *tp = tcp_sk(sk);
>   	int copied = 0;
>   	u32 peek_seq;
> +	u32 peek_offset;
>   	u32 *seq;
>   	unsigned long used;
>   	int err;
> @@ -2435,7 +2436,6 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>   	long timeo;
>   	struct sk_buff *skb, *last;
>   	u32 urg_hole = 0;
> -
>   	err = -ENOTCONN;
>   	if (sk->sk_state == TCP_LISTEN)
>   		goto out;
> @@ -2469,6 +2469,14 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>   	if (flags & MSG_PEEK) {
>   		peek_seq = tp->copied_seq;
>   		seq = &peek_seq;
> +		if (msg->msg_iter.iov[0].iov_base == NULL) {
> +			peek_offset = msg->msg_iter.iov[0].iov_len;
> +			msg->msg_iter.iov = &msg->msg_iter.iov[1];
> +			msg->msg_iter.nr_segs -= 1;
> +			msg->msg_iter.count -= peek_offset;
> +			len -= peek_offset;
> +			*seq += peek_offset;
> +		}
>   	}
>   
>   	target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);