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

[Eric Dumazet]

On Mon, Apr 12, 2021 at 11:12 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> 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 ...

I think the patch should be backported now, all issues have been sorted out ?

>
>
> > ---
> >  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
> >
> >
>