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

[Eric Dumazet <edumazet@google.com>]

On Wed, Mar 31, 2021 at 10:46 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Mar 31, 2021 at 10:36 AM Eric Dumazet <edumazet@google.com> wrote:
> >

> >
> > I was looking at this code (page_to_skb())  a few minutes ago ;)
> >
> > pulling payload would make sense only if can pull of of it (to free the page)
> > (This is what some drivers implement and call copybreak)
> >
> > Even if we do not have an accurate knowledge of header sizes,
> > it would be better to pull only the Ethernet header and let GRO do the
> > rest during its dissection.
> >
> > Once fixed, virtio_net will reduce by 2x number of frags per skb,
> > compared to the situation before "net: avoid 32 x truesize
> > under-estimation for tiny skbs"
>
> Ie I suspect the simple way to fix this would be :
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index bb4ea9dbc16bcb19c5969fc8247478aa66c63fce..a5500bf6ac01051be949edf9fead934a90335f4f
> 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -409,9 +409,7 @@ 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 = min_t(int, len, ETH_HLEN);
>         skb_put_data(skb, p, copy);
>
>         if (metasize) {

A  'copybreak' aware version would be :

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index bb4ea9dbc16bcb19c5969fc8247478aa66c63fce..dd58b075ca53643231bc1795c7283fcd8609547b
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 = min_t(int, len, ETH_HLEN);
        skb_put_data(skb, p, copy);

        if (metasize) {