netdev - Re: [PATCH net-next 1/8] virtio_net: mergeable xdp: put old page immediately

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Message-ID: <1679535365.5410192-1-xuanzhuo@linux.alibaba.com>
Date:   Thu, 23 Mar 2023 09:36:05 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "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>,
        <virtualization@...ts.linux-foundation.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/8] virtio_net: mergeable xdp: put old page immediately

On Wed, 22 Mar 2023 16:22:18 +0800, Yunsheng Lin <linyunsheng@...wei.com> 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@...ux.alibaba.com>
> > ---
> >  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++) {
> >