linux-kernel - Re: [PATCH net 4/4] virtio-net: allow more allocated space for mergeable XDP

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Message-ID: <CACGkMEv-EgkZs6d4MHwxj0t_-pQvxMRLTdgguP7GUijbg-kEoA@mail.gmail.com>
Date: Thu, 26 Jun 2025 10:51:01 +0800
From: Jason Wang <jasowang@...hat.com>
To: Bui Quang Minh <minhquangbui99@...il.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, virtualization@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net 4/4] virtio-net: allow more allocated space for
 mergeable XDP

On Thu, Jun 26, 2025 at 12:10 AM Bui Quang Minh
<minhquangbui99@...il.com> wrote:
>
> When the mergeable receive buffer is prefilled before XDP is set, it
> does not reserve the space for XDP_PACKET_HEADROOM and skb_shared_info.
> So when XDP is set and this buffer is used to receive frame, we need to
> create a new buffer with reserved headroom, tailroom and copy the frame
> data over. Currently, the new buffer's size is restricted to PAGE_SIZE
> only. If the frame data's length + headroom + tailroom exceeds
> PAGE_SIZE, the frame is dropped.
>
> However, it seems like there is no restriction on the total size in XDP.
> So we can just increase the size of new buffer to 2 * PAGE_SIZE in that
> case and continue to process the frame.
>
> In my opinion, the current drop behavior is fine and expected so this
> commit is just an improvement not a bug fix.

Then this should go for net-next.

>
> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
> ---
>  drivers/net/virtio_net.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 844cb2a78be0..663cec686045 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2277,13 +2277,26 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
>                                               len);
>                 if (!xdp_page)
>                         return NULL;
> +
> +               *frame_sz = PAGE_SIZE;
>         } else {
> +               unsigned int total_len;
> +
>                 xdp_room = SKB_DATA_ALIGN(XDP_PACKET_HEADROOM +
>                                           sizeof(struct skb_shared_info));
> -               if (*len + xdp_room > PAGE_SIZE)
> +               total_len = *len + xdp_room;
> +
> +               /* This must never happen because len cannot exceed PAGE_SIZE */
> +               if (unlikely(total_len > 2 * PAGE_SIZE))
>                         return NULL;
>
> -               xdp_page = alloc_page(GFP_ATOMIC);
> +               if (total_len > PAGE_SIZE) {
> +                       xdp_page = alloc_pages(GFP_ATOMIC, 1);

I'm not sure it's worth optimizing the corner case here that may bring
burdens for maintenance.

And a good optimization here is to reduce the logic duplication by
reusing xdp_linearize_page().


> +                       *frame_sz = 2 * PAGE_SIZE;
> +               } else {
> +                       xdp_page = alloc_page(GFP_ATOMIC);
> +                       *frame_sz = PAGE_SIZE;
> +               }
>                 if (!xdp_page)
>                         return NULL;
>
> @@ -2291,8 +2304,6 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
>                        page_address(*page) + offset, *len);
>         }
>
> -       *frame_sz = PAGE_SIZE;
> -
>         put_page(*page);
>
>         *page = xdp_page;
> --

Thanks

> 2.43.0
>