netdev - Re: [net-next-2.6 PATCH v2] net: consolidate netif_needs

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Date:	Sat, 27 Feb 2010 23:52:45 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	jeffrey.t.kirsher@...el.com, netdev@...r.kernel.org,
	gospo@...hat.com, john.r.fastabend@...el.com
Subject: Re: [net-next-2.6 PATCH v2] net: consolidate netif_needs_gso()
	checks

On Sat, Feb 27, 2010 at 03:27:25AM -0800, David Miller wrote:
>
> If we have ip_summed == CHECKSUM_PARTIAL might some classifier
> or packet scheduler action module require that the
> transport header is setup properly before the SKB gets into
> there?

I think this is OK as the transport header setting was only there
for backwards compatibility with certain drivers that relied on it.
Its use was very much isolated.

I just did a grep on net/sched and couldn't see anything obvious
that uses transport_header.

> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index eb7f1a4..626124d 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -1835,12 +1835,40 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
> >  {
> >  	const struct net_device_ops *ops = dev->netdev_ops;
> >  	int rc = NETDEV_TX_OK;
> > +	int need_gso = netif_needs_gso(dev, skb);
> > +
> > +	if (!need_gso) {
> > +		if (skb_has_frags(skb) &&
> > +		    !(dev->features & NETIF_F_FRAGLIST) &&
> > +		    __skb_linearize(skb))
> > +			goto out_kfree_skb;
> > +
> > +		/* Fragmented skb is linearized if device does not support SG,
> > +		 * or if at least one of fragments is in highmem and device
> > +		 * does not support DMA from it.
> > +		 */
> > +		if (skb_shinfo(skb)->nr_frags &&
> > +		    (!(dev->features & NETIF_F_SG) ||
> > +		      illegal_highdma(dev, skb)) &&
> > +		    __skb_linearize(skb))
> > +			goto out_kfree_skb;

Please use skb_needs_linearize.

> > +		/* If packet is not checksummed and device does not support
> > +		 * checksumming for this protocol, complete checksumming here.
> > +		 */
> > +		if (skb->ip_summed == CHECKSUM_PARTIAL) {
> > +			skb_set_transport_header(skb, skb->csum_start -
> > +				      skb_headroom(skb));
> > +			if (!dev_can_checksum(dev, skb) &&
> > +			     skb_checksum_help(skb))
> > +				goto out_kfree_skb;
> > +		}
> > +	}
> >  
> >  	if (likely(!skb->next)) {
> >  		if (!list_empty(&ptype_all))
> >  			dev_queue_xmit_nit(skb, dev);
> >  
> > -		if (netif_needs_gso(dev, skb)) {
> > +		if (need_gso) {
> >  			if (unlikely(dev_gso_segment(skb)))
> >  				goto out_kfree_skb;
> >  			if (skb->next)

That whole if block should be moved into an else clause here:

		if (netif_needs_gso(dev, skb)) {
			if (unlikely(dev_gso_segment(skb)))
				goto out_kfree_skb;
			if (skb->next)
				goto gso;
		} else {
			do your thing
		}

The reason is that the other paths only act on the fragments
generated by GSO, so logically with your change we shouldn't
apply any further software emulation to them, even if the device
setting changed.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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