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

From: annie li <[email-masked]>
Date: Wed, 10 Jul 2013 10:31:49 +0800

On 2013-7-10 0:16, Wei Liu wrote:
> On Tue, Jul 09, 2013 at 02:15:20PM +0800, Annie Li wrote:
>> When counting required slots for skb, netback directly uses DIV_ROUND_UP to get
>> slots required by header data. This is wrong when offset in the page of header
>> data is not zero, and is also inconsistent with following calculation for
>> required slot in netbk_gop_skb.
>>
>> In netbk_gop_skb, required slots are calculated based on offset and len in page
>> of header data. It is possible that required slots here is larger than the one
>> calculated in earlier netbk_count_requests. This inconsistency directly results
>> in rx_req_cons_peek and xen_netbk_rx_ring_full judgement are wrong.
>>
>> Then it comes to situation the ring is actually full, but netback thinks it is
>> not and continues to create responses. This results in response overlaps request
>> in the ring, then grantcopy gets wrong grant reference and throws out error,
>> for example "(XEN) grant_table.c:1763:d0 Bad grant reference 2949120", the
>> grant reference is invalid value here. Netback returns XEN_NETIF_RSP_ERROR(-1)
>> to netfront when grant copy status is error, then netfront gets rx->status
>> (the status is -1, not really data size now), and throws out error,
>> "kernel: net eth1: rx->offset: 0, size: 4294967295". This issue can be reproduced
>> by doing gzip/gunzip in nfs share with mtu = 9000, the guest would panic after
>> running such test for a while.
>>
>> This patch is based on 3.10-rc7.
>>
>> Signed-off-by: Annie Li <[email-masked]>
>> ---
>>   drivers/net/xen-netback/netback.c |   97 +++++++++++++++++++++++++-----------
>>   1 files changed, 67 insertions(+), 30 deletions(-)
>>
>> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
>> index 8c20935..7ff9333 100644
>> --- a/drivers/net/xen-netback/netback.c
>> +++ b/drivers/net/xen-netback/netback.c
>> @@ -359,51 +359,88 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
>>    * the guest. This function is essentially a dry run of
>>    * netbk_gop_frag_copy.
>>    */
> Just to be clear, now the above comment applies to your new function,
> right?
>
>> -unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
>> +static void netbk_get_slots(struct xenvif *vif, struct sk_buff *skb,
> A better name would be netbk_count_slots because it doesn't actually
> acquire any slots.
>
> And you can make it return the slot count and adds that to the total
> number of slots in the caller, as this function doesn't seem to rely on
> previous count so there is not much point passing in *count.
>
>> +			    struct page *page, int *copy_off,
>> +			    unsigned long size, unsigned long offset,
>> +			    int *head, int *count)
>>   {
>> -	unsigned int count;
>> -	int i, copy_off;
>> +	unsigned long bytes;
>>   
>> -	count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
>> +	/* Data must not cross a page boundary. */
>> +	BUG_ON(size + offset > PAGE_SIZE<<compound_order(page));
>>   
>> -	copy_off = skb_headlen(skb) % PAGE_SIZE;
>> +	/* Skip unused frames from start of page */
>> +	page += offset >> PAGE_SHIFT;
>> +	offset &= ~PAGE_MASK;
>>   
>> -	if (skb_shinfo(skb)->gso_size)
>> -		count++;
>> +	while (size > 0) {
>> +		BUG_ON(offset >= PAGE_SIZE);
>> +		BUG_ON(*copy_off > MAX_BUFFER_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;
>> +		bytes = PAGE_SIZE - offset;
>>   
>> -		offset &= ~PAGE_MASK;
>> +		if (bytes > size)
>> +			bytes = size;
>>   
>> -		while (size > 0) {
>> -			BUG_ON(offset >= PAGE_SIZE);
>> -			BUG_ON(copy_off > MAX_BUFFER_OFFSET);
>> +		if (start_new_rx_buffer(*copy_off, bytes, *head)) {
>> +			*count = *count + 1;
>> +			*copy_off = 0;
>> +		}
>>   
>> -			bytes = PAGE_SIZE - offset;
>> +		if (*copy_off + bytes > MAX_BUFFER_OFFSET)
>> +			bytes = MAX_BUFFER_OFFSET - *copy_off;
>>   
>> -			if (bytes > size)
>> -				bytes = size;
>> +		*copy_off += bytes;
>>   
>> -			if (start_new_rx_buffer(copy_off, bytes, 0)) {
>> -				count++;
>> -				copy_off = 0;
>> -			}
>> +		offset += bytes;
>> +		size -= bytes;
>>   
>> -			if (copy_off + bytes > MAX_BUFFER_OFFSET)
>> -				bytes = MAX_BUFFER_OFFSET - copy_off;
>> +		/* Next frame */
>> +		if (offset == PAGE_SIZE && size) {
>> +			BUG_ON(!PageCompound(page));
>> +			page++;
>> +			offset = 0;
>> +		}
>>   
>> -			copy_off += bytes;
>> +		if (*head)
> This seems to differ from netbk_gop_frag_copy. In that function it's
> like
>
>      if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)

I mixed netbk_gop_frag_copy and xen_netbk_count_skb_slots just now, so 
please ignore the comments here in my last email.
In xen_netbk_count_skb_slots, we have following code already, so it is 
not necessary to add gso condition here.

         if (skb_shinfo(skb)->gso_size)
                 count++;

Thanks
Annie
>
> Wei.
>
>> +			*count = *count + 1;
>> +		*head = 0; /* There must be something in this buffer now. */
>> +	}
>> +}
>> +
>> +unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
>> +{
>> +	int i, copy_off = 0;
>> +	int nr_frags = skb_shinfo(skb)->nr_frags;
>> +	unsigned char *data;
>> +	int head = 1;
>> +	unsigned int count = 0;
>>   
>> -			offset += bytes;
>> -			size -= bytes;
>> +	if (skb_shinfo(skb)->gso_size)
>> +		count++;
>>   
>> -			if (offset == PAGE_SIZE)
>> -				offset = 0;
>> -		}
>> +	data = skb->data;
>> +	while (data < skb_tail_pointer(skb)) {
>> +		unsigned int offset = offset_in_page(data);
>> +		unsigned int len = PAGE_SIZE - offset;
>> +
>> +		if (data + len > skb_tail_pointer(skb))
>> +			len = skb_tail_pointer(skb) - data;
>> +
>> +		netbk_get_slots(vif, skb, virt_to_page(data), &copy_off,
>> +				len, offset, &head, &count);
>> +		data += len;
>> +	}
>> +
>> +	for (i = 0; i < nr_frags; i++) {
>> +		netbk_get_slots(vif, skb,
>> +				skb_frag_page(&skb_shinfo(skb)->frags[i]),
>> +				&copy_off,
>> +				skb_frag_size(&skb_shinfo(skb)->frags[i]),
>> +				skb_shinfo(skb)->frags[i].page_offset,
>> +				&head, &count);
>>   	}
>> +
>>   	return count;
>>   }
>>   
>> -- 
>> 1.7.3.4
>>
>> --
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