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

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Date:   Thu, 23 Mar 2023 11:38:34 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.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 2023/3/23 9:36, Xuan Zhuo wrote:
> 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.

For compatibility, it is still needed, right?

> 
> 
>>
>>
>>> +			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.

Isn't refoctor patch supposed to be self-contianed too, instead of
depending on follow-up patch?