On Thu, Jun 26, 2025 at 12:10 AM Bui Quang Minh
<minhquangbu...@gmail.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 <minhquangbu...@gmail.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
>
