Subject: Re: [PATCH net v2 0/2] virtio-net: fix for build_skb()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 2 Aug 2021 12:58:18 -0400

On Fri, Jul 30, 2021 at 05:16:43AM -0700, Jakub Kicinski wrote:
> On Tue, 1 Jun 2021 07:06:43 -0400 Michael S. Tsirkin wrote:
> > On Tue, Jun 01, 2021 at 02:39:58PM +0800, Xuan Zhuo wrote:
> > > #1 Fixed a serious error.
> > > #2 Fixed a logical error, but this error did not cause any serious consequences.
> > > 
> > > The logic of this piece is really messy. Fortunately, my refactored patch can be
> > > completed with a small amount of testing.  
> > 
> > Looks good, thanks!
> > Also needed for stable I think.
> > 
> > Acked-by: Michael S. Tsirkin <[email-masked]>
> 
> Just a heads up folks, looks like we ended up merging both this and the
> net-next version of the patch set:
> 
> 8fb7da9e9907 virtio_net: get build_skb() buf by data ptr
> 5c37711d9f27 virtio-net: fix for unable to handle page fault for address
> 
> and
> 
> 7bf64460e3b2 virtio-net: get build_skb() buf by data ptr
> 6c66c147b9a4 virtio-net: fix for unable to handle page fault for address
> 
> Are you okay with the code as is or should we commit something like:

Yes:
Acked-by: Michael S. Tsirkin <[email-masked]>

Will you post this one then?

> ---
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 56c3f8519093..74482a52f076 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -380,7 +380,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  				   struct page *page, unsigned int offset,
>  				   unsigned int len, unsigned int truesize,
>  				   bool hdr_valid, unsigned int metasize,
> -				   bool whole_page)
> +				   unsigned int headroom)
>  {
>  	struct sk_buff *skb;
>  	struct virtio_net_hdr_mrg_rxbuf *hdr;
> @@ -398,28 +398,16 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  	else
>  		hdr_padded_len = sizeof(struct padded_vnet_hdr);
>  
> -	/* If whole_page, there is an offset between the beginning of the
> +	/* 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 (whole_page) {
> -		/* Buffers with whole_page use PAGE_SIZE as alloc size,
> -		 * see add_recvbuf_mergeable() + get_mergeable_buf_len()
> -		 */
> -		truesize = PAGE_SIZE;
> -
> -		/* page maybe head page, so we should get the buf by p, not the
> -		 * page
> -		 */
> -		tailroom = truesize - len - offset_in_page(p);
> -		buf = (char *)((unsigned long)p & PAGE_MASK);
> -	} 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;
> @@ -978,7 +966,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  				put_page(page);
>  				head_skb = page_to_skb(vi, rq, xdp_page, offset,
>  						       len, PAGE_SIZE, false,
> -						       metasize, true);
> +						       metasize,
> +						       VIRTIO_XDP_HEADROOM);
>  				return head_skb;
>  			}
>  			break;
> @@ -1029,7 +1018,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  	rcu_read_unlock();
>  
>  	head_skb = page_to_skb(vi, rq, page, offset, len, truesize, !xdp_prog,
> -			       metasize, !!headroom);
> +			       metasize, headroom);
>  	curr_skb = head_skb;
>  
>  	if (unlikely(!curr_skb))