On 2023/3/22 11:03, Xuan Zhuo wrote:
> In the xdp implementation of virtio-net mergeable, it always checks
> whether two page is used and a page is selected to release. This is
> complicated for the processing of action, and be careful.
>
> In the entire process, we have such principles:
> * If xdp_page is used (PASS, TX, Redirect), then we release the old
> page.
> * If it is a drop case, we will release two. The old page obtained from
> buf is release inside err_xdp, and xdp_page needs be relased by us.
>
> But in fact, when we allocate a new page, we can release the old page
> immediately. Then just one is using, we just need to release the new
> page for drop case. On the drop path, err_xdp will release the variable
> "page", so we only need to let "page" point to the new xdp_page in
> advance.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> ---
> drivers/net/virtio_net.c | 15 ++++++---------
> 1 file changed, 6 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index e2560b6f7980..4d2bf1ce0730 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1245,6 +1245,9 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> if (!xdp_page)
> goto err_xdp;
> offset = VIRTIO_XDP_HEADROOM;
> +
> + put_page(page);
the error handling of xdp_linearize_page() does not seems self contained.
Does it not seem better：
1. if xdp_linearize_page() succesed, call put_page() for first buffer just
as put_page() is call for other buffer
2. or call virtqueue_get_buf() and put_page() for all the buffer of the packet
so the error handling is not needed outside the virtqueue_get_buf().
In that case, it seems we can just do below without xdp_page:
page = xdp_linearize_page(rq, num_buf, page, ...);
> + page = xdp_page;
> } else if (unlikely(headroom < virtnet_get_headroom(vi))) {
> xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
> sizeof(struct skb_shared_info));
> @@ -1259,6 +1262,9 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> page_address(page) + offset, len);
> frame_sz = PAGE_SIZE;
> offset = VIRTIO_XDP_HEADROOM;
> +
> + put_page(page);
> + page = xdp_page;
It seems we can limit the scope of xdp_page in this "else if" block.
> } else {
> xdp_page = page;
> }
It seems the above else block is not needed anymore.
> @@ -1278,8 +1284,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> if (unlikely(!head_skb))
> goto err_xdp_frags;
>
> - if (unlikely(xdp_page != page))
> - put_page(page);
> rcu_read_unlock();
> return head_skb;
> case XDP_TX:
> @@ -1297,8 +1301,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> goto err_xdp_frags;
> }
> *xdp_xmit |= VIRTIO_XDP_TX;
> - if (unlikely(xdp_page != page))
> - put_page(page);
> rcu_read_unlock();
> goto xdp_xmit;
> case XDP_REDIRECT:
> @@ -1307,8 +1309,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> if (err)
> goto err_xdp_frags;
> *xdp_xmit |= VIRTIO_XDP_REDIR;
> - if (unlikely(xdp_page != page))
> - put_page(page);
> rcu_read_unlock();
> goto xdp_xmit;
> default:
> @@ -1321,9 +1321,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> goto err_xdp_frags;
> }
> err_xdp_frags:
> - if (unlikely(xdp_page != page))
> - __free_pages(xdp_page, 0);
It seems __free_pages() and put_page() is used interchangeably here.
Perhaps using __free_pages() have performance reason? As the comment below:
https://elixir.bootlin.com/linux/v6.3-rc3/source/net/core/page_pool.c#L500
> -
> if (xdp_buff_has_frags(&xdp)) {
> shinfo = xdp_get_shared_info_from_buff(&xdp);
> for (i = 0; i < shinfo->nr_frags; i++) {
>
