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 <xuanzhuo@linux.alibaba.com>
Link: https://www.spinics.net/lists/netdev/msg731397.html
Co-Developed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 82e520d..0824e69 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -406,9 +406,13 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
        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) {

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 98775d7..b465f8f 100644 (file)
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -65,14 +65,18 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
        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.
@@ -102,14 +106,14 @@ retry:
                        }
 
                        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;
                }
        }