Subject: Re: [PATCH] xen/netfront: handle compound page fragments on transmit

From: ANNIE LI <[email-masked]>
Date: Wed, 21 Nov 2012 10:52:39 +0800

On 2012-11-20 19:40, Ian Campbell wrote:
> An SKB paged fragment can consist of a compound page with order>  0.
> However the netchannel protocol deals only in PAGE_SIZE frames.
>
> Handle this in xennet_make_frags by iterating over the frames which
> make up the page.
>
> This is the netfront equivalent to 6a8ed462f16b for netback.
>
> Signed-off-by: Ian Campbell<[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: Eric Dumazet<[email-masked]>
> Cc: Konrad Rzeszutek Wilk<[email-masked]>
> Cc: ANNIE LI<[email-masked]>
> Cc: Sander Eikelenboom<[email-masked]>
> Cc: Stefan Bader<[email-masked]>
> ---
>   drivers/net/xen-netfront.c |   58 +++++++++++++++++++++++++++++++++----------
>   1 files changed, 44 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index caa0110..a12b99a 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -452,24 +452,54 @@ static void xennet_make_frags(struct sk_buff *skb, struct net_device *dev,
>   	/* Grant backend access to each skb fragment page. */
>   	for (i = 0; i<  frags; i++) {
>   		skb_frag_t *frag = skb_shinfo(skb)->frags + i;
> +		struct page *page = skb_frag_page(frag);
> +		unsigned long size = skb_frag_size(frag);
> +		unsigned long offset = frag->page_offset;

There are following definitions at the beginning of xennet_make_frags,

         unsigned int offset = offset_in_page(data);
         unsigned int len = skb_headlen(skb);

Is it better to reuse those definitions, and not define new size and 
offset again in this for loop? And unsigned int is enough here, right?

>
> -		tx->flags |= XEN_NETTXF_more_data;
> +		/* Data must not cross a page boundary. */
> +		BUG_ON(size + offset>  PAGE_SIZE<<compound_order(page));
>
> -		id = get_id_from_freelist(&np->tx_skb_freelist, np->tx_skbs);
> -		np->tx_skbs[id].skb = skb_get(skb);
> -		tx = RING_GET_REQUEST(&np->tx, prod++);
> -		tx->id = id;
> -		ref = gnttab_claim_grant_reference(&np->gref_tx_head);
> -		BUG_ON((signed short)ref<  0);
> +		/* Skip unused frames from start of page */
> +		page += offset>>  PAGE_SHIFT;
> +		offset&= ~PAGE_MASK;
>
> -		mfn = pfn_to_mfn(page_to_pfn(skb_frag_page(frag)));
> -		gnttab_grant_foreign_access_ref(ref, np->xbdev->otherend_id,
> -						mfn, GNTMAP_readonly);
> +		while (size>  0) {
> +			unsigned long bytes;
>
> -		tx->gref = np->grant_tx_ref[id] = ref;
> -		tx->offset = frag->page_offset;
> -		tx->size = skb_frag_size(frag);
> -		tx->flags = 0;
> +			BUG_ON(offset>= PAGE_SIZE);
> +
> +			bytes = PAGE_SIZE - offset;
> +			if (bytes>  size)
> +				bytes = size;
> +
> +			tx->flags |= XEN_NETTXF_more_data;
> +
> +			id = get_id_from_freelist(&np->tx_skb_freelist, np->tx_skbs);
Over 80 characters?
> +			np->tx_skbs[id].skb = skb_get(skb);
> +			tx = RING_GET_REQUEST(&np->tx, prod++);
> +			tx->id = id;
> +			ref = gnttab_claim_grant_reference(&np->gref_tx_head);
> +			BUG_ON((signed short)ref<  0);
> +
> +			mfn = pfn_to_mfn(page_to_pfn(page));
> +			gnttab_grant_foreign_access_ref(ref, np->xbdev->otherend_id,
> +							mfn, GNTMAP_readonly);
Over 80 characters?

Thanks
Annie
> +
> +			tx->gref = np->grant_tx_ref[id] = ref;
> +			tx->offset = offset;
> +			tx->size = bytes;
> +			tx->flags = 0;
> +
> +			offset += bytes;
> +			size -= bytes;
> +
> +			/* Next frame */
> +			if (offset == PAGE_SIZE&&  size) {
> +				BUG_ON(!PageCompound(page));
> +				page++;
> +				offset = 0;
> +			}
> +		}
>   	}
>
>   	np->tx.req_prod_pvt = prod;