Subject: Re: [PATCH net-next v2 6/9] xen-netback: Handle guests with too many frags

From: Zoltan Kiss <[email-masked]>
Date: Mon, 16 Dec 2013 16:10:42 +0000

On 13/12/13 15:43, Wei Liu wrote:
> On Thu, Dec 12, 2013 at 11:48:14PM +0000, Zoltan Kiss wrote:
>> Xen network protocol had implicit dependency on MAX_SKB_FRAGS. Netback has to
>> handle guests sending up to XEN_NETBK_LEGACY_SLOTS_MAX slots. To achieve that:
>> - create a new skb
>> - map the leftover slots to its frags (no linear buffer here!)
>> - chain it to the previous through skb_shinfo(skb)->frag_list
>> - map them
>> - copy the whole stuff into a brand new skb and send it to the stack
>> - unmap the 2 old skb's pages
>>
>
> Do you see performance regression with this approach?
Well, it was pretty hard to reproduce that behaviour even with NFS. I 
don't think it happens often enough that it causes a noticable 
performance regression. Anyway, it would be just as slow as the current 
grant copy with coalescing, maybe a bit slower due to the unmapping. But 
at least we use a core network function to do the coalescing.
Or, if you mean the generic performance, if this problem doesn't appear, 
then no, I don't see performance regression.

>> Signed-off-by: Zoltan Kiss <[email-masked]>
>>
>> ---
>>   drivers/net/xen-netback/netback.c |   99 +++++++++++++++++++++++++++++++++++--
>>   1 file changed, 94 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
>> index e26cdda..f6ed1c8 100644
>> --- a/drivers/net/xen-netback/netback.c
>> +++ b/drivers/net/xen-netback/netback.c
>> @@ -906,11 +906,15 @@ static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,
>>   	u16 pending_idx = *((u16 *)skb->data);
>>   	int start;
>>   	pending_ring_idx_t index;
>> -	unsigned int nr_slots;
>> +	unsigned int nr_slots, frag_overflow = 0;
>>
>>   	/* At this point shinfo->nr_frags is in fact the number of
>>   	 * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
>>   	 */
>> +	if (shinfo->nr_frags > MAX_SKB_FRAGS) {
>> +		frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;
>> +		shinfo->nr_frags = MAX_SKB_FRAGS;
>> +	}
>>   	nr_slots = shinfo->nr_frags;
>>
>
> It is also probably better to check whether shinfo->nr_frags is too
> large which makes frag_overflow > MAX_SKB_FRAGS. I know skb should be
> already be valid at this point but it wouldn't hurt to be more careful.
Ok, I've added this:
	/* At this point shinfo->nr_frags is in fact the number of
	 * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
	 */
+	if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+		if (shinfo->nr_frags > XEN_NETBK_LEGACY_SLOTS_MAX) return NULL;
+		frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;

>
>>   	/* Skip first skb fragment if it is on same page as header fragment. */
>> @@ -926,6 +930,33 @@ static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,
>>
>>   	BUG_ON(shinfo->nr_frags > MAX_SKB_FRAGS);
>>
>> +	if (frag_overflow) {
>> +		struct sk_buff *nskb = alloc_skb(NET_SKB_PAD + NET_IP_ALIGN,
>> +				GFP_ATOMIC | __GFP_NOWARN);
>> +		if (unlikely(nskb == NULL)) {
>> +			netdev_err(vif->dev,
>> +				   "Can't allocate the frag_list skb.\n");
>> +			return NULL;
>> +		}
>> +
>> +		/* Packets passed to netif_rx() must have some headroom. */
>> +		skb_reserve(nskb, NET_SKB_PAD + NET_IP_ALIGN);
>> +
>
> The code to call alloc_skb and skb_reserve is copied from other
> location. I would like to have a dedicated function to allocate skb in
> netback if possible.
OK