netdev - Re: [PATCH net v2 2/2] virtio_net: get build

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Date:   Tue, 1 Jun 2021 16:55:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, netdev@...r.kernel.org
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        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
Subject: Re: [PATCH net v2 2/2] virtio_net: get build_skb() buf by data ptr


在 2021/6/1 下午2:40, Xuan Zhuo 写道:
> In the case of merge, the page passed into page_to_skb() may be a head
> page, not the page where the current data is located. So when trying to
> get the buf where the data is located, we should get buf based on
> headroom instead of offset.
>
> This patch solves this problem. But if you don't use this patch, the
> original code can also run, because if the page is not the page of the
> current data, the calculated tailroom will be less than 0, and will not
> enter the logic of build_skb() . The significance of this patch is to
> modify this logical problem, allowing more situations to use
> build_skb().
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 17 ++++++-----------
>   1 file changed, 6 insertions(+), 11 deletions(-)


Acked-by: Jason Wang <jasowang@...hat.com>


>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6b929aca155a..fa407eb8b457 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -401,18 +401,13 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>   	/* If headroom is not 0, there is an offset between the beginning of the
>   	 * data and the allocated space, otherwise the data and the allocated
>   	 * space are aligned.
> +	 *
> +	 * Buffers with headroom use PAGE_SIZE as alloc size, see
> +	 * add_recvbuf_mergeable() + get_mergeable_buf_len()
>   	 */
> -	if (headroom) {
> -		/* Buffers with headroom use PAGE_SIZE as alloc size,
> -		 * see add_recvbuf_mergeable() + get_mergeable_buf_len()
> -		 */
> -		truesize = PAGE_SIZE;
> -		tailroom = truesize - len - offset;
> -		buf = page_address(page);
> -	} else {
> -		tailroom = truesize - len;
> -		buf = p;
> -	}
> +	truesize = headroom ? PAGE_SIZE : truesize;
> +	tailroom = truesize - len - headroom;
> +	buf = p - headroom;
>   
>   	len -= hdr_len;
>   	offset += hdr_padded_len;