Subject: Re: [PATCH net v2] net: pktgen: fix pkt_size

From: Paolo Abeni <[email-masked]>
Date: Sat, 01 Oct 2016 20:05:20 +0200

On Fri, 2016-09-30 at 08:39 -0700, Greg wrote:
> On Fri, 2016-09-30 at 16:56 +0200, Paolo Abeni wrote:
> > The commit 879c7220e828 ("net: pktgen: Observe needed_headroom
> > of the device") increased the 'pkt_overhead' field value by
> > LL_RESERVED_SPACE.
> > As a side effect the generated packet size, computed as:
> > 
> > 	/* Eth + IPh + UDPh + mpls */
> > 	datalen = pkt_dev->cur_pkt_size - 14 - 20 - 8 -
> > 		  pkt_dev->pkt_overhead;
> > 
> > is decreased by the same value.
> > The above changed slightly the behavior of existing pktgen users,
> > and made the procfs interface somewhat inconsistent.
> > Fix it by restoring the previous pkt_overhead value and using
> > LL_RESERVED_SPACE as extralen in skb allocation.
> > Also, change pktgen_alloc_skb() to only partially reserve
> > the headroom to allow the caller to prefetch from ll header
> > start.
> > 
> > v1 -> v2:
> >  - fixed some typos in the comments
> > 
> > Fixes: 879c7220e828 ("net: pktgen: Observe needed_headroom of the device")
> > Suggested-by: Ben Greear <[email-masked]>
> > Signed-off-by: Paolo Abeni <[email-masked]>
> > ---
> >  net/core/pktgen.c | 21 ++++++++++-----------
> >  1 file changed, 10 insertions(+), 11 deletions(-)
> > 
> > diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> > index bbd118b..5219a9e 100644
> > --- a/net/core/pktgen.c
> > +++ b/net/core/pktgen.c
> > @@ -2286,7 +2286,7 @@ out:
> >  
> >  static inline void set_pkt_overhead(struct pktgen_dev *pkt_dev)
> >  {
> > -	pkt_dev->pkt_overhead = LL_RESERVED_SPACE(pkt_dev->odev);
> > +	pkt_dev->pkt_overhead = 0;
> >  	pkt_dev->pkt_overhead += pkt_dev->nr_labels*sizeof(u32);
> >  	pkt_dev->pkt_overhead += VLAN_TAG_SIZE(pkt_dev);
> >  	pkt_dev->pkt_overhead += SVLAN_TAG_SIZE(pkt_dev);
> > @@ -2777,13 +2777,13 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
> >  }
> >  
> >  static struct sk_buff *pktgen_alloc_skb(struct net_device *dev,
> > -					struct pktgen_dev *pkt_dev,
> > -					unsigned int extralen)
> > +					struct pktgen_dev *pkt_dev)
> >  {
> > +	unsigned int extralen = LL_RESERVED_SPACE(dev);
> >  	struct sk_buff *skb = NULL;
> > -	unsigned int size = pkt_dev->cur_pkt_size + 64 + extralen +
> > -			    pkt_dev->pkt_overhead;
> > +	unsigned int size;
> >  
> > +	size = pkt_dev->cur_pkt_size + 64 + extralen + pkt_dev->pkt_overhead;
> >  	if (pkt_dev->flags & F_NODE) {
> >  		int node = pkt_dev->node >= 0 ? pkt_dev->node : numa_node_id();
> >  
> > @@ -2796,8 +2796,9 @@ static struct sk_buff *pktgen_alloc_skb(struct net_device *dev,
> >  		 skb = __netdev_alloc_skb(dev, size, GFP_NOWAIT);
> >  	}
> >  
> > +	/* the caller pre-fetches from skb->data and reserves for the mac hdr */
> >  	if (likely(skb))
> > -		skb_reserve(skb, LL_RESERVED_SPACE(dev));
> > +		skb_reserve(skb, extralen - 16);
> 
> Is the 16 here the same as HD_DATA_MOD?
> 
> Magic numbers...

This magic numbers comes from the current code (the extralen argument),
this patch just move it around. I can send a v3 using a define for it,
if really needed, but it seems more a separated clean-up. 

Thank you,

Paolo