On Wed, 22 Mar 2023 16:22:18 +0800, Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> 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, ...);
This does look better.
In fact, we already have vq reset, we can load XDP based on vq reset.
In this way, we can run without xdp_linearize_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.
Yes, the follow-up patch has this optimization.
>
> > @@ -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
Yes, but now we don't seem to be very good to distinguish it. But I think
it doesn't matter. This logic is rare under actual situation.
Thanks.
>
> > -
> > if (xdp_buff_has_frags(&xdp)) {
> > shinfo = xdp_get_shared_info_from_buff(&xdp);
> > for (i = 0; i < shinfo->nr_frags; i++) {
> >
