netdev - Re: [PATCH v2 1/1] xen/netback: correctly calculate required slots of skb.

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Date:	Thu, 11 Jul 2013 10:47:58 +0100
From:	Ian Campbell <ian.campbell@...rix.com>
To:	annie li <annie.li@...cle.com>
CC:	<xen-devel@...ts.xensource.com>, <netdev@...r.kernel.org>,
	<wei.liu2@...rix.com>, <konrad.wilk@...cle.com>, <msw@...zon.com>
Subject: Re: [PATCH v2 1/1] xen/netback: correctly calculate required slots
 of skb.

On Thu, 2013-07-11 at 16:34 +0800, annie li wrote:
> On 2013-7-11 16:11, Ian Campbell wrote:
> > On Wed, 2013-07-10 at 17:15 +0800, Annie Li wrote:
> >> +static int netbk_count_slots(struct xenvif *vif, struct sk_buff *skb,
> >> +			    int *copy_off, unsigned long size,
> >> +			    unsigned long offset, int *head)
> >>   {
> >> -	unsigned int count;
> >> -	int i, copy_off;
> >> +	unsigned long bytes;
> >> +	int count = 0;
> >>   
> >> -	count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> >> +	offset &= ~PAGE_MASK;
> >>   
> >> -	copy_off = skb_headlen(skb) % PAGE_SIZE;
> >> +	while (size > 0) {
> >> +		BUG_ON(offset >= PAGE_SIZE);
> >> +		BUG_ON(*copy_off > MAX_BUFFER_OFFSET);
> >>   
> >> -	if (skb_shinfo(skb)->gso_size)
> >> -		count++;
> >> +		bytes = PAGE_SIZE - offset;
> >>   
> >> -	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> >> -		unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
> >> -		unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
> >> -		unsigned long bytes;
> >> +		if (bytes > size)
> >> +			bytes = size;
> >>   
> >> -		offset &= ~PAGE_MASK;
> >> +		if (start_new_rx_buffer(*copy_off, bytes, *head)) {
> >> +			count++;
> >> +			*copy_off = 0;
> >> +		}
> >>   
> >> -		while (size > 0) {
> >> -			BUG_ON(offset >= PAGE_SIZE);
> >> -			BUG_ON(copy_off > MAX_BUFFER_OFFSET);
> >> +		if (*copy_off + bytes > MAX_BUFFER_OFFSET)
> >> +			bytes = MAX_BUFFER_OFFSET - *copy_off;
> >>   
> >> -			bytes = PAGE_SIZE - offset;
> >> +		*copy_off += bytes;
> >>   
> >> -			if (bytes > size)
> >> -				bytes = size;
> >> +		offset += bytes;
> >> +		size -= bytes;
> >>   
> >> -			if (start_new_rx_buffer(copy_off, bytes, 0)) {
> >> -				count++;
> >> -				copy_off = 0;
> >> -			}
> >> +		/* Next frame */
> >> +		if (offset == PAGE_SIZE && size)
> >> +			offset = 0;
> >> +
> >> +		if (*head)
> >> +			count++;
> > This little bit corresponds to the "/* Leave a gap for the GSO
> > descriptor. */" in gop_frag_copy?
> 
> No, it does not correspond to this in gop_frag_copy.

So what does it correspond to?

>  The code here only 
> increase count for the first time. I thought to initialize the count in 
> xen_netbk_count_skb_slots with 1 to avoid this. But thinking of the 
> extreme case when the header size is zero(not sure whether this case 
> could be true), I increase the count here to keep safe in case header 
> size is zero.

netfront requires that the first slot always contains some data,
gop_frag_copy will BUG if that's not the case.

> 
> There is code correspond to that in gop_frag_copy in 
> xen_netbk_count_skb_slots, see following,
> 
> +	if (skb_shinfo(skb)->gso_size && !vif->gso_prefix)
> +		count++;
> (The original code does not have gso_prefix, I added it in this patch
> too based on Wei's suggestion)

OK.

It's be nice if the count and copy variants of this logic could be as
similar as possible but they already differ quite a bit. I have
considered whether we could combine the two by adding "dry-run"
functionality to gop_frag_copy but that seems like it would just ugly up
both versions.

Ian.

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