Subject: Re: [PATCH net, 3/3] net: mana: Fix oversized sge0 for GSO packets

From: Simon Horman <[email-masked]>
Date: Sat, 30 Sep 2023 20:19:46 +0200

On Fri, Sep 29, 2023 at 04:11:15PM +0000, Haiyang Zhang wrote:

...

> > > @@ -209,19 +281,6 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb,
> > struct net_device *ndev)
> > >  	pkg.wqe_req.client_data_unit = 0;
> > >
> > >  	pkg.wqe_req.num_sge = 1 + skb_shinfo(skb)->nr_frags;
> > > -	WARN_ON_ONCE(pkg.wqe_req.num_sge >
> > MAX_TX_WQE_SGL_ENTRIES);
> > > -
> > > -	if (pkg.wqe_req.num_sge <= ARRAY_SIZE(pkg.sgl_array)) {
> > > -		pkg.wqe_req.sgl = pkg.sgl_array;
> > > -	} else {
> > > -		pkg.sgl_ptr = kmalloc_array(pkg.wqe_req.num_sge,
> > > -					    sizeof(struct gdma_sge),
> > > -					    GFP_ATOMIC);
> > > -		if (!pkg.sgl_ptr)
> > > -			goto tx_drop_count;
> > > -
> > > -		pkg.wqe_req.sgl = pkg.sgl_ptr;
> > > -	}
> > 
> > It is unclear to me why this logic has moved from here to further
> > down in this function. Is it to avoid some cases where
> > alloation has to be unwond on error (when mana_fix_skb_head() fails) ?
> > If so, this feels more like an optimisation than a fix.
> mana_fix_skb_head() may add one more sge (success case) so the sgl 
> allocation should be done later. Otherwise, we need to free / re-allocate 
> the array later.

Understood, thanks for the clarification.

> > >  	if (skb->protocol == htons(ETH_P_IP))
> > >  		ipv4 = true;
> > > @@ -229,6 +288,23 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb,
> > struct net_device *ndev)
> > >  		ipv6 = true;
> > >
> > >  	if (skb_is_gso(skb)) {
> > > +		gso_hs = mana_get_gso_hs(skb);
> > > +
> > > +		if (mana_fix_skb_head(ndev, skb, gso_hs,
> > &pkg.wqe_req.num_sge))
> > > +			goto tx_drop_count;
> > > +
> > > +		if (skb->encapsulation) {
> > > +			u64_stats_update_begin(&tx_stats->syncp);
> > > +			tx_stats->tso_inner_packets++;
> > > +			tx_stats->tso_inner_bytes += skb->len - gso_hs;
> > > +			u64_stats_update_end(&tx_stats->syncp);
> > > +		} else {
> > > +			u64_stats_update_begin(&tx_stats->syncp);
> > > +			tx_stats->tso_packets++;
> > > +			tx_stats->tso_bytes += skb->len - gso_hs;
> > > +			u64_stats_update_end(&tx_stats->syncp);
> > > +		}
> > 
> > nit: I wonder if this could be slightly more succinctly written as:
> > 
> > 		u64_stats_update_begin(&tx_stats->syncp);
> > 		if (skb->encapsulation) {
> > 			tx_stats->tso_inner_packets++;
> > 			tx_stats->tso_inner_bytes += skb->len - gso_hs;
> > 		} else {
> > 			tx_stats->tso_packets++;
> > 			tx_stats->tso_bytes += skb->len - gso_hs;
> > 		}
> > 		u64_stats_update_end(&tx_stats->syncp);
> > 
> Yes it can be written this way:)
> 
> > Also, it is unclear to me why the stats logic is moved here from
> > futher down in the same block. It feels more like a clean-up than a fix
> > (as, btw, is my suggestion immediately above).
> Since we need to calculate the gso_hs and fix head earlier than the stats and 
> some other work, I move it immediately after skb_is_gso(skb).
> The gso_hs calculation was part of the tx_stats block, so the tx_stats is moved 
> together to remain close to the gso_hs calculation to keep readability.

I agree it is nice the way you have it.
I was mainly thinking that the diffstat could be made smaller,
which might be beneficial to a fix. But I have no strong feelings on that.

> > > +
> > >  		pkg.tx_oob.s_oob.is_outer_ipv4 = ipv4;
> > >  		pkg.tx_oob.s_oob.is_outer_ipv6 = ipv6;
> > >

...