Subject: Re: [PATCH V3] xen/netfront: handle compound page fragments on transmit

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Wed, 21 Nov 2012 10:16:27 -0500

On Wed, Nov 21, 2012 at 12:02:16PM +0000, 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]>

Acked-by: Konrad Rzeszutek Wilk <[email-masked]>

David, would you like me to send it to Linus via my tree or are you
OK sending him a git pull with this patch (and hopefully some other
ones?)

Thank you.

> Cc: ANNIE LI <[email-masked]>
> Cc: Sander Eikelenboom <[email-masked]>
> Cc: Stefan Bader <[email-masked]>
> ---
> v3: limit to 80-characters. Use net_alert_ratelimited.
> v2: check we have enough room in the ring and that the other end can
>     cope with the number of slots in a single frame
> ---
>  drivers/net/xen-netfront.c |   98 ++++++++++++++++++++++++++++++++++---------
>  1 files changed, 77 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index caa0110..fc24eb9 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -452,29 +452,85 @@ 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);
>  
> -		tx->flags |= XEN_NETTXF_more_data;
> +		len = skb_frag_size(frag);
> +		offset = frag->page_offset;
>  
> -		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);
> +		/* Data must not cross a page boundary. */
> +		BUG_ON(len + offset > PAGE_SIZE<<compound_order(page));
>  
> -		mfn = pfn_to_mfn(page_to_pfn(skb_frag_page(frag)));
> -		gnttab_grant_foreign_access_ref(ref, np->xbdev->otherend_id,
> -						mfn, GNTMAP_readonly);
> +		/* Skip unused frames from start of page */
> +		page += offset >> PAGE_SHIFT;
> +		offset &= ~PAGE_MASK;
>  
> -		tx->gref = np->grant_tx_ref[id] = ref;
> -		tx->offset = frag->page_offset;
> -		tx->size = skb_frag_size(frag);
> -		tx->flags = 0;
> +		while (len > 0) {
> +			unsigned long bytes;
> +
> +			BUG_ON(offset >= PAGE_SIZE);
> +
> +			bytes = PAGE_SIZE - offset;
> +			if (bytes > len)
> +				bytes = len;
> +
> +			tx->flags |= XEN_NETTXF_more_data;
> +
> +			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);
> +
> +			mfn = pfn_to_mfn(page_to_pfn(page));
> +			gnttab_grant_foreign_access_ref(ref,
> +							np->xbdev->otherend_id,
> +							mfn, GNTMAP_readonly);
> +
> +			tx->gref = np->grant_tx_ref[id] = ref;
> +			tx->offset = offset;
> +			tx->size = bytes;
> +			tx->flags = 0;
> +
> +			offset += bytes;
> +			len -= bytes;
> +
> +			/* Next frame */
> +			if (offset == PAGE_SIZE && len) {
> +				BUG_ON(!PageCompound(page));
> +				page++;
> +				offset = 0;
> +			}
> +		}
>  	}
>  
>  	np->tx.req_prod_pvt = prod;
>  }
>  
> +/*
> + * Count how many ring slots are required to send the frags of this
> + * skb. Each frag might be a compound page.
> + */
> +static int xennet_count_skb_frag_slots(struct sk_buff *skb)
> +{
> +	int i, frags = skb_shinfo(skb)->nr_frags;
> +	int pages = 0;
> +
> +	for (i = 0; i < frags; i++) {
> +		skb_frag_t *frag = skb_shinfo(skb)->frags + i;
> +		unsigned long size = skb_frag_size(frag);
> +		unsigned long offset = frag->page_offset;
> +
> +		/* Skip unused frames from start of page */
> +		offset &= ~PAGE_MASK;
> +
> +		pages += PFN_UP(offset + size);
> +	}
> +
> +	return pages;
> +}
> +
>  static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	unsigned short id;
> @@ -487,23 +543,23 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	grant_ref_t ref;
>  	unsigned long mfn;
>  	int notify;
> -	int frags = skb_shinfo(skb)->nr_frags;
> +	int slots;
>  	unsigned int offset = offset_in_page(data);
>  	unsigned int len = skb_headlen(skb);
>  	unsigned long flags;
>  
> -	frags += DIV_ROUND_UP(offset + len, PAGE_SIZE);
> -	if (unlikely(frags > MAX_SKB_FRAGS + 1)) {
> -		printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n",
> -		       frags);
> -		dump_stack();
> +	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
> +		xennet_count_skb_frag_slots(skb);
> +	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> +		net_alert_ratelimited(
> +			"xennet: skb rides the rocket: %d slots\n", slots);
>  		goto drop;
>  	}
>  
>  	spin_lock_irqsave(&np->tx_lock, flags);
>  
>  	if (unlikely(!netif_carrier_ok(dev) ||
> -		     (frags > 1 && !xennet_can_sg(dev)) ||
> +		     (slots > 1 && !xennet_can_sg(dev)) ||
>  		     netif_needs_gso(skb, netif_skb_features(skb)))) {
>  		spin_unlock_irqrestore(&np->tx_lock, flags);
>  		goto drop;
> -- 
> 1.7.2.5
>