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

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 22 Jun 2023 22:12:27 -0400
Jon Maloy <jmaloy@redhat.com> 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.

Ah-ha! I'm glad you found an acceptable way to pass a NULL pointer
there. :)

> 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

...it would be nice to also do a bit of profiling with perf(1) --
that's where I originally noticed we were wasting cycles on filling up
tcp_buf_discard. Plus, sure, there's also some value in dropping a
useless 16 MiB buffer.

If you need examples/inspiration: the pasta (automated) demo shows
that, skip at 9:20 in: https://passt.top/passt/about/#pasta_2 (the one
on the left)

and that's simply done like this:
  https://passt.top/passt/tree/test/demo/pasta#n163

> 8-10 % for the protocol splicer 'passst', which is used in KubeVirt

passt

> containers.

Those are virtual machines in containers -- but other than KubeVirt,
passt is generally supported in libvirt (>= 9.2) with QEMU (>= 7.2)

> 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;

Kernel networking code observes the reverse-Christmas tree notation (at
least for new additions), this would need to go after *tp:

	struct tcp_sock *tp = tcp_sk(sk);
	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;
> -

Probably not intended.

>  	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;

Do you also need to make sure that nr_segs > 1 if iov[0].iov_base is
NULL? I'm not sure if we need to check explicitly that msg_iter.iov[1]
is valid here (but I haven't followed the whole path from the syscall
handler).

> +			msg->msg_iter.count -= peek_offset;
> +			len -= peek_offset;
> +			*seq += peek_offset;
> +		}
>  	}
>  
>  	target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);

-- 
Stefano