linux-kernel - Re: [RFC PATCH v3 1/4] virtio/vsock: don't use skbuff state to account credit

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Message-ID: <20230309162726.lzkacyg3lfow4cfg@sgarzare-redhat>
Date:   Thu, 9 Mar 2023 17:27:26 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v3 1/4] virtio/vsock: don't use skbuff state to
 account credit

On Thu, Mar 09, 2023 at 01:11:46PM +0300, Arseniy Krasnov wrote:
>This replaces use of skbuff state to calculate new 'rx_bytes'/'fwd_cnt'
>values with explicit value as input argument. This makes code more
>simple, because it is not needed to change skbuff state before each
>call to update 'rx_bytes'/'fwd_cnt'.

I think we should also describe the issues you found that we are fixinig
now, for example the wrong calculation in virtio_transport_dec_rx_pkt().

Something like this:

   `skb->len` can vary when we partially read the data, this complicates
   the calculation of credit to be updated in
   virtio_transport_inc_rx_pkt()/virtio_transport_dec_rx_pkt().

   Also in virtio_transport_dec_rx_pkt() we were miscalculating the
   credit since `skb->len` was redundant.

   For these reasons, let's replace the use ...
   (continue with what is written in this commit message)

And we should add the Fixes tag:

Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")

>
>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> net/vmw_vsock/virtio_transport_common.c | 23 +++++++++++------------
> 1 file changed, 11 insertions(+), 12 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index a1581c77cf84..618680fd9906 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -241,21 +241,18 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> }
>
> static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
>-					struct sk_buff *skb)
>+					u32 len)
> {
>-	if (vvs->rx_bytes + skb->len > vvs->buf_alloc)
>+	if (vvs->rx_bytes + len > vvs->buf_alloc)
> 		return false;
>
>-	vvs->rx_bytes += skb->len;
>+	vvs->rx_bytes += len;
> 	return true;
> }
>
> static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs,
>-					struct sk_buff *skb)
>+					u32 len)
> {
>-	int len;
>-
>-	len = skb_headroom(skb) - sizeof(struct virtio_vsock_hdr) - skb->len;
> 	vvs->rx_bytes -= len;
> 	vvs->fwd_cnt += len;
> }
>@@ -388,7 +385,9 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 		skb_pull(skb, bytes);
>
> 		if (skb->len == 0) {
>-			virtio_transport_dec_rx_pkt(vvs, skb);
>+			u32 pkt_len = le32_to_cpu(virtio_vsock_hdr(skb)->len);

Good catch! In my proposal I used `bytes` wrongly!

The rest LGTM!

Stefano

>+
>+			virtio_transport_dec_rx_pkt(vvs, pkt_len);
> 			consume_skb(skb);
> 		} else {
> 			__skb_queue_head(&vvs->rx_queue, skb);
>@@ -437,17 +436,17 @@ static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
>
> 	while (!msg_ready) {
> 		struct virtio_vsock_hdr *hdr;
>+		size_t pkt_len;
>
> 		skb = __skb_dequeue(&vvs->rx_queue);
> 		if (!skb)
> 			break;
> 		hdr = virtio_vsock_hdr(skb);
>+		pkt_len = (size_t)le32_to_cpu(hdr->len);
>
> 		if (dequeued_len >= 0) {
>-			size_t pkt_len;
> 			size_t bytes_to_copy;
>
>-			pkt_len = (size_t)le32_to_cpu(hdr->len);
> 			bytes_to_copy = min(user_buf_len, pkt_len);
>
> 			if (bytes_to_copy) {
>@@ -484,7 +483,7 @@ static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
> 				msg->msg_flags |= MSG_EOR;
> 		}
>
>-		virtio_transport_dec_rx_pkt(vvs, skb);
>+		virtio_transport_dec_rx_pkt(vvs, pkt_len);
> 		kfree_skb(skb);
> 	}
>
>@@ -1040,7 +1039,7 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
>
> 	spin_lock_bh(&vvs->rx_lock);
>
>-	can_enqueue = virtio_transport_inc_rx_pkt(vvs, skb);
>+	can_enqueue = virtio_transport_inc_rx_pkt(vvs, len);
> 	if (!can_enqueue) {
> 		free_pkt = true;
> 		goto out;
>-- 
>2.25.1
>