Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

From: Eric Dumazet <edumazet@google.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	netdev <netdev@vger.kernel.org>,
	Alexander Duyck <alexanderduyck@fb.com>,
	Paolo Abeni <pabeni@redhat.com>, Greg Thelen <gthelen@google.com>,
	"David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	su-lifan@linux.alibaba.com, "dust.li" <dust.li@linux.alibaba.com>,
	Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs
Date: Thu, 1 Apr 2021 16:08:39 +0200	[thread overview]
Message-ID: <CANn89iJrbnuUGk3OAG+DRjxAz6YoUSyNUXEPsKVZRMxNFQbSBw@mail.gmail.com> (raw)
In-Reply-To: <20210401094908-mutt-send-email-mst@kernel.org>

On Thu, Apr 1, 2021 at 3:51 PM Michael S. Tsirkin <mst@redhat.com> wrote:

> So IIUC what you are saying is we should do more or less
>      if (hdr_len != 0)
>                         copy hdr_len
> ?

This part is not pulling bytes into skb->head, but into
skb_vnet_hdr(skb) (which is basically skb->cb)

I suggest the following patch

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index bb4ea9dbc16bcb19c5969fc8247478aa66c63fce..f5a3cecd18eada32694714ecb85c205af7108aae
100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -409,9 +409,13 @@ 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 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 6b5fcfa1e5553576b0e853ae31a2df655c04204b..2ee8f3ba76a548d54e0b21321a67da958c9984a0
100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -63,8 +63,12 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *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;
@@ -100,14 +104,14 @@ static inline int virtio_net_hdr_to_skb(struct
sk_buff *skb,
                        }

                        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;
                }
        }
