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

[Zoltan Kiss]

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 <zoltan.kiss@xxxxxxxxxx>
> >
> > ---
> >   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

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