virtio_net: Do not pull payload in skb->head

Xuan Zhuo reported that commit 3226b158e67c ("net: avoid 32 x truesize
under-estimation for tiny skbs") brought a ~10% performance drop.

The reason for the performance drop was that GRO was forced
to chain sk_buff (using skb_shinfo(skb)->frag_list), which
uses more memory but also cause packet consumers to go over
a lot of overhead handling all the tiny skbs.

It turns out that virtio_net page_to_skb() has a wrong strategy :
It allocates skbs with GOOD_COPY_LEN (128) bytes in skb->head, then
copies 128 bytes from the page, before feeding the packet to GRO stack.

This was suboptimal before commit 3226b158e67c ("net: avoid 32 x truesize
under-estimation for tiny skbs") because GRO was using 2 frags per MSS,
meaning we were not packing MSS with 100% efficiency.

Fix is to pull only the ethernet header in page_to_skb()

Then, we change virtio_net_hdr_to_skb() to pull the missing
headers, instead of assuming they were already pulled by callers.

This fixes the performance regression, but could also allow virtio_net
to accept packets with more than 128bytes of headers.

Many thanks to Xuan Zhuo for his report, and his tests/help.

Fixes: 3226b158e67c ("net: avoid 32 x truesize under-estimation for tiny skbs")
Reported-by: Xuan Zhuo <[e-mail]>
Link: https://www.spinics.net/lists/netdev/msg731397.html
Co-Developed-by: Xuan Zhuo <[e-mail]>
Signed-off-by: Xuan Zhuo <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: Jason Wang <[e-mail]>
Cc: [e-mail]
Acked-by: Jason Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/virtio_net.c

···
 	offset += hdr_padded_len;
 	p += hdr_padded_len;
 
-	copy = len;
-	if (copy > skb_tailroom(skb))
-		copy = skb_tailroom(skb);
+	/* Copy all frame if it fits skb->head, otherwise
+	 * we let virtio_net_hdr_to_skb() and GRO pull headers as needed.
+	 */
+	if (len <= skb_tailroom(skb))
+		copy = len;
+	else
+		copy = ETH_HLEN + metasize;
 	skb_put_data(skb, p, copy);
 
 	if (metasize) {

include/linux/virtio_net.h

···
 	skb_reset_mac_header(skb);
 
 	if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
-		u16 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
-		u16 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
+		u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
+		u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
+		u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
+
+		if (!pskb_may_pull(skb, needed))
+			return -EINVAL;
 
 		if (!skb_partial_csum_set(skb, start, off))
 			return -EINVAL;
 
 		p_off = skb_transport_offset(skb) + thlen;
-		if (p_off > skb_headlen(skb))
+		if (!pskb_may_pull(skb, p_off))
 			return -EINVAL;
 	} else {
 		/* gso packets without NEEDS_CSUM do not set transport_offset.
···
 			}
 
 			p_off = keys.control.thoff + thlen;
-			if (p_off > skb_headlen(skb) ||
+			if (!pskb_may_pull(skb, p_off) ||
 			    keys.basic.ip_proto != ip_proto)
 				return -EINVAL;
 
 			skb_set_transport_header(skb, keys.control.thoff);
 		} else if (gso_type) {
 			p_off = thlen;
-			if (p_off > skb_headlen(skb))
+			if (!pskb_may_pull(skb, p_off))
 				return -EINVAL;
 		}
 	}