Re: [PATCH 5.4 042/111] virtio_net: Do not pull payload in skb->head

[Michael S. Tsirkin]

On Mon, Apr 12, 2021 at 10:40:20AM +0200, Greg Kroah-Hartman wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> [ Upstream commit 0f6925b3e8da0dbbb52447ca8a8b42b371aac7db ]
> 
> 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@xxxxxxxxxxxxxxxxx>
> Link: https://www.spinics.net/lists/netdev/msg731397.html
> Co-Developed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>


Note that an issue related to this patch was recently reported.
It's quite possible that the root cause is a bug elsewhere
in the kernel, but it probably makes sense to defer the backport
until we know more ...


> ---
>  drivers/net/virtio_net.c   | 10 +++++++---
>  include/linux/virtio_net.h | 14 +++++++++-----
>  2 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index b67460864b3c..d8ee001d8e8e 100644
> --- 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 98775d7fa696..b465f8f3e554 100644
> --- 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 @@ 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;
>  		}
>  	}
> -- 
> 2.30.2
> 
>