netdev - Re: [RFC PATCH v1 06/12] vsock/virtio: non-linear skb handling for TAP dev

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Date:   Thu, 16 Feb 2023 15:30:27 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <AVKrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Krasnov Arseniy <oxffffaa@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>
Subject: Re: [RFC PATCH v1 06/12] vsock/virtio: non-linear skb handling for
 TAP dev

On Mon, Feb 06, 2023 at 06:59:21AM +0000, Arseniy Krasnov wrote:
>For TAP device new skb is created and data from the current skb is
>copied to it. This adds copying data from non-linear skb to new
>the skb.
>
>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> net/vmw_vsock/virtio_transport_common.c | 43 +++++++++++++++++++++++--
> 1 file changed, 40 insertions(+), 3 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index a1581c77cf84..05ce97b967ad 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -101,6 +101,39 @@ virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
> 	return NULL;
> }
>
>+static void virtio_transport_copy_nonlinear_skb(struct sk_buff *skb,
>+						void *dst,
>+						size_t len)
>+{
>+	size_t rest_len = len;
>+
>+	while (rest_len && virtio_vsock_skb_has_frags(skb)) {
>+		struct bio_vec *curr_vec;
>+		size_t curr_vec_end;
>+		size_t to_copy;
>+		int curr_frag;
>+		int curr_offs;
>+
>+		curr_frag = VIRTIO_VSOCK_SKB_CB(skb)->curr_frag;
>+		curr_offs = VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
>+		curr_vec = &skb_shinfo(skb)->frags[curr_frag];
>+
>+		curr_vec_end = curr_vec->bv_offset + curr_vec->bv_len;
>+		to_copy = min(rest_len, (size_t)(curr_vec_end - curr_offs));
>+
>+		memcpy(dst, page_to_virt(curr_vec->bv_page) + curr_offs,
>+		       to_copy);
>+
>+		rest_len -= to_copy;
>+		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += to_copy;
>+
>+		if (VIRTIO_VSOCK_SKB_CB(skb)->frag_off == (curr_vec_end)) {
>+			VIRTIO_VSOCK_SKB_CB(skb)->curr_frag++;
>+			VIRTIO_VSOCK_SKB_CB(skb)->frag_off = 0;
>+		}
>+	}
>+}
>+
> /* Packet capture */
> static struct sk_buff *virtio_transport_build_skb(void *opaque)
> {
>@@ -109,7 +142,6 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
> 	struct af_vsockmon_hdr *hdr;
> 	struct sk_buff *skb;
> 	size_t payload_len;
>-	void *payload_buf;
>
> 	/* A packet could be split to fit the RX buffer, so we can retrieve
> 	 * the payload length from the header and the buffer pointer taking
>@@ -117,7 +149,6 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
> 	 */
> 	pkt_hdr = virtio_vsock_hdr(pkt);
> 	payload_len = pkt->len;
>-	payload_buf = pkt->data;
>
> 	skb = alloc_skb(sizeof(*hdr) + sizeof(*pkt_hdr) + payload_len,
> 			GFP_ATOMIC);
>@@ -160,7 +191,13 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
> 	skb_put_data(skb, pkt_hdr, sizeof(*pkt_hdr));
>
> 	if (payload_len) {
>-		skb_put_data(skb, payload_buf, payload_len);
>+		if (skb_is_nonlinear(skb)) {
>+			void *data = skb_put(skb, payload_len);
>+
>+			virtio_transport_copy_nonlinear_skb(skb, data, payload_len);
>+		} else {
>+			skb_put_data(skb, pkt->data, payload_len);
>+		}

Ehm I'm a bit confused. Maybe we need to rename the sk_buffs involved in
this function (pre-existing).

We have `pkt` that is the original sk_buff, and `skb` that it is 
allocated in this function, so IIUC we should check if `pkt` is 
nonlinear and copy its payload into `skb`, so we should do this 
(untested) chage:

@@ -367,10 +367,10 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
         skb_put_data(skb, pkt_hdr, sizeof(*pkt_hdr));

         if (payload_len) {
-               if (skb_is_nonlinear(skb)) {
+               if (skb_is_nonlinear(pkt)) {
                         void *data = skb_put(skb, payload_len);

-                       virtio_transport_copy_nonlinear_skb(skb, data, payload_len);
+                       virtio_transport_copy_nonlinear_skb(pkt, data, payload_len);
                 } else {
                         skb_put_data(skb, pkt->data, payload_len);
                 }

Thanks,
Stefano

> 	}
>
> 	return skb;
>-- 
>2.25.1