netdev - Re: [RFC PATCH 02/13] net: dsa: implement a central TX reallocation procedure

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Date:   Sun, 18 Oct 2020 00:13:27 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        Christian Eggers <ceggers@...i.de>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: Re: [RFC PATCH 02/13] net: dsa: implement a central TX reallocation
 procedure

On Sun, Oct 18, 2020 at 01:31:20AM +0300, Vladimir Oltean wrote:
> On Sun, Oct 18, 2020 at 01:01:04AM +0300, Vladimir Oltean wrote:
> > > +	return pskb_expand_head(skb, headroom, tailroom, GFP_ATOMIC);
> > 	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 	err = pskb_expand_head(skb, headroom, tailroom, GFP_ATOMIC);
> > 	if (err < 0 || !padlen)
> > 		return err;
> > 
> > 	return __skb_put_padto(skb, padlen, false);
> 
> Oops, another one here. Should be:
> 
> 	return __skb_put_padto(skb, skb->len + padlen, false);
> > > +}

Last one for today. This should actually be correct now, and not
allocate double the needed headroom size.

static int dsa_realloc_skb(struct sk_buff *skb, struct net_device *dev)
{
	struct dsa_slave_priv *p = netdev_priv(dev);
	struct dsa_slave_stats *e;
	int needed_headroom;
	int needed_tailroom;
	int padlen = 0, err;

	needed_headroom = dev->needed_headroom;
	needed_tailroom = dev->needed_tailroom;
	/* For tail taggers, we need to pad short frames ourselves, to ensure
	 * that the tail tag does not fail at its role of being at the end of
	 * the packet, once the master interface pads the frame.
	 */
	if (unlikely(needed_tailroom && skb->len < ETH_ZLEN))
		padlen = ETH_ZLEN - skb->len;
	needed_tailroom += padlen;
	needed_headroom -= skb_headroom(skb);
	needed_tailroom -= skb_tailroom(skb);

	if (likely(needed_headroom <= 0 && needed_tailroom <= 0 &&
		   !skb_cloned(skb)))
		/* No reallocation needed, yay! */
		return 0;

	e = this_cpu_ptr(p->extra_stats);
	u64_stats_update_begin(&e->syncp);
	e->tx_reallocs++;
	u64_stats_update_end(&e->syncp);

	err = pskb_expand_head(skb, max(needed_headroom, 0),
			       max(needed_tailroom, 0), GFP_ATOMIC);
	if (err < 0 || !padlen)
		return err;

	return __skb_put_padto(skb, ETH_ZLEN, false);
}