netdev - Re: [RFC PATCH v3 03/13] af_vsock: implement SEQPACKET rx loop

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Date:   Thu, 28 Jan 2021 17:55:18 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseny Krasnov <arseny.krasnov@...persky.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Andra Paraschiv <andraprs@...zon.com>,
        Colin Ian King <colin.king@...onical.com>,
        Jeff Vander Stoep <jeffv@...gle.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, stsp2@...dex.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v3 03/13] af_vsock: implement SEQPACKET rx loop

On Mon, Jan 25, 2021 at 02:12:36PM +0300, Arseny Krasnov wrote:
>This adds receive loop for SEQPACKET. It looks like receive loop for
>SEQPACKET, but there is a little bit difference:
>1) It doesn't call notify callbacks.
>2) It doesn't care about 'SO_SNDLOWAT' and 'SO_RCVLOWAT' values, because
>   there is no sense for these values in SEQPACKET case.
>3) It waits until whole record is received or error is found during
>   receiving.
>4) It processes and sets 'MSG_TRUNC' flag.
>
>So to avoid extra conditions for two types of socket inside one loop, two
>independent functions were created.
>
>Signed-off-by: Arseny Krasnov <arseny.krasnov@...persky.com>
>---
> include/net/af_vsock.h   |   5 ++
> net/vmw_vsock/af_vsock.c | 102 ++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 106 insertions(+), 1 deletion(-)
>
>diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
>index b1c717286993..46073842d489 100644
>--- a/include/net/af_vsock.h
>+++ b/include/net/af_vsock.h
>@@ -135,6 +135,11 @@ struct vsock_transport {
> 	bool (*stream_is_active)(struct vsock_sock *);
> 	bool (*stream_allow)(u32 cid, u32 port);
>
>+	/* SEQ_PACKET. */
>+	size_t (*seqpacket_seq_get_len)(struct vsock_sock *);
>+	ssize_t (*seqpacket_dequeue)(struct vsock_sock *, struct msghdr *,
>+				     size_t len, int flags);
>+
> 	/* Notification. */
> 	int (*notify_poll_in)(struct vsock_sock *, size_t, bool *);
> 	int (*notify_poll_out)(struct vsock_sock *, size_t, bool *);
>diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>index 524df8fc84cd..3b266880b7c8 100644
>--- a/net/vmw_vsock/af_vsock.c
>+++ b/net/vmw_vsock/af_vsock.c
>@@ -2006,7 +2006,107 @@ static int __vsock_stream_recvmsg(struct sock *sk, struct msghdr *msg,
> static int __vsock_seqpacket_recvmsg(struct sock *sk, struct msghdr *msg,
> 				     size_t len, int flags)
> {
>-	return -1;
>+	const struct vsock_transport *transport;
>+	const struct iovec *orig_iov;
>+	unsigned long orig_nr_segs;
>+	ssize_t dequeued_total = 0;
>+	struct vsock_sock *vsk;
>+	size_t record_len;
>+	long timeout;
>+	int err = 0;
>+	DEFINE_WAIT(wait);
>+
>+	vsk = vsock_sk(sk);
>+	transport = vsk->transport;
>+
>+	timeout = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
>+	msg->msg_flags &= ~MSG_EOR;

Maybe add a comment about why we need to clear MSG_EOR.

>+	orig_nr_segs = msg->msg_iter.nr_segs;
>+	orig_iov = msg->msg_iter.iov;
>+
>+	while (1) {
>+		ssize_t dequeued;
>+		s64 ready;
>+
>+		prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
>+		ready = vsock_stream_has_data(vsk);
>+
>+		if (ready == 0) {
>+			if (vsock_wait_data(sk, &wait, timeout, NULL, 0)) {
>+				/* In case of any loop break(timeout, signal
>+				 * interrupt or shutdown), we report user that
>+				 * nothing was copied.
>+				 */
>+				dequeued_total = 0;
>+				break;
>+			}
>+			continue;
>+		}
>+
>+		finish_wait(sk_sleep(sk), &wait);
>+
>+		if (ready < 0) {
>+			err = -ENOMEM;
>+			goto out;
>+		}
>+
>+		if (dequeued_total == 0) {
>+			record_len =
>+				transport->seqpacket_seq_get_len(vsk);
>+
>+			if (record_len == 0)
>+				continue;
>+		}
>+
>+		/* 'msg_iter.count' is number of unused bytes in iov.
>+		 * On every copy to iov iterator it is decremented at
>+		 * size of data.
>+		 */
>+		dequeued = transport->seqpacket_dequeue(vsk, msg,
>+					msg->msg_iter.count, flags);
                                         ^
                                         Is this needed or 'msg' can be 
                                         used in the transport?
>+
>+		if (dequeued < 0) {
>+			dequeued_total = 0;
>+
>+			if (dequeued == -EAGAIN) {
>+				iov_iter_init(&msg->msg_iter, READ,
>+					      orig_iov, orig_nr_segs,
>+					      len);
>+				msg->msg_flags &= ~MSG_EOR;
>+				continue;

Why we need to reset MSG_EOR here?

>+			}
>+
>+			err = -ENOMEM;
>+			break;
>+		}
>+
>+		dequeued_total += dequeued;
>+
>+		if (dequeued_total >= record_len)
>+			break;
>+	}

Maybe a new line here.

>+	if (sk->sk_err)
>+		err = -sk->sk_err;
>+	else if (sk->sk_shutdown & RCV_SHUTDOWN)
>+		err = 0;
>+
>+	if (dequeued_total > 0) {
>+		/* User sets MSG_TRUNC, so return real length of
>+		 * packet.
>+		 */
>+		if (flags & MSG_TRUNC)
>+			err = record_len;
>+		else
>+			err = len - msg->msg_iter.count;
>+
>+		/* Always set MSG_TRUNC if real length of packet is
>+		 * bigger that user buffer.

s/that/than

>+		 */
>+		if (record_len > len)
>+			msg->msg_flags |= MSG_TRUNC;
>+	}
>+out:
>+	return err;
> }
>
> static int
>-- 
>2.25.1
>