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

From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
To: Jason Wang <jasowang@redhat.com>
Cc: netdev@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	virtualization@lists.linux-foundation.org, bpf@vger.kernel.org
Subject: Re: [PATCH net-next 1/8] virtio_net: mergeable xdp: put old page immediately
Date: Mon, 3 Apr 2023 12:11:31 +0800	[thread overview]
Message-ID: <1680495091.0610447-1-xuanzhuo@linux.alibaba.com> (raw)
In-Reply-To: <CACGkMEseX1+oi+unZspnGzPduQc-tFkxpVg+9KfEhPSWWqCTrg@mail.gmail.com>

On Fri, 31 Mar 2023 17:01:54 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Tue, Mar 28, 2023 at 8:04 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> 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@linux.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);
> > +                       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;
> >                 } else {
> >                         xdp_page = page;
> >                 }
>
> While at this I would go a little further, just remove the above
> assignment then we can use:
>
>                 data = page_address(page) + offset;
>
> Which seems cleaner?

I will try.

Thanks.

>
> Thanks
>
> > @@ -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);
> > -
> >                 if (xdp_buff_has_frags(&xdp)) {
> >                         shinfo = xdp_get_shared_info_from_buff(&xdp);
> >                         for (i = 0; i < shinfo->nr_frags; i++) {
> > --
> > 2.32.0.3.g01195cf9f
> >
>