From: Madalin-Cristian Bucur <madalin.bucur@freescale.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
CC: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [RFC,v3 02/10] dpaa_eth: add support for DPAA Ethernet
Thread-Topic: [RFC,v3 02/10] dpaa_eth: add support for DPAA Ethernet
Thread-Index: AQHQgoy67USkhZ9S4U2K7B2IZnjQHp2mSFKAgAd4rHA=
Date: Mon, 15 Jun 2015 13:40:46 +0000
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> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
> 
> On Wed, 2015-04-29 at 17:56 +0300, Madalin Bucur wrote:
> > This introduces the Freescale Data Path Acceleration Architecture
> > (DPAA) Ethernet driver (dpaa_eth) that builds upon the DPAA QMan,
> > BMan, PAMU and FMan drivers to deliver Ethernet connectivity on
> > the Freescale DPAA QorIQ platforms.
> ...
> 
> > +	/* We're going to store the skb backpointer at the beginning
> > +	 * of the data buffer, so we need a privately owned skb
> > +	 */
> > +
> > +	/* Code borrowed from skb_unshare(). */
> > +	if (skb_cloned(skb)) {
> > +		struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
> > +
> > +		/* Finally, create a contig FD from this skb */
> > +		skb_to_contig_fd(priv, skb, &fd, countptr, &offset);
> > +
> > +		kfree_skb(skb);
> > +		skb = nskb;
> > +		/* skb_copy() has now linearized the skbuff. */
> > +	}
> > +
> 
> You know that TCP packets are clones, right ?
> This code is killing performance.
> 
> TCP allows the headers part being modified by a driver if needed.
> 
> You should use skb_header_cloned() instead.

Thank you, I'll address this. I plan to do something like this:

+       if (!nonlinear) {
+               /* We're going to store the skb backpointer at the beginning
+                * of the data buffer, so we need a privately owned skb
+                */
+
+               /* make sure skb is not shared, skb_cow_head() assumes it's not */
+               skb = skb_share_check(skb, GFP_ATOMIC);
+               if (!skb)
+                       goto enomem;
+
+               /* verify the skb head is not cloned */
+               if (skb_cow_head(skb, priv->tx_headroom))
+                       goto enomem;
+
+               nonlinear = skb_is_nonlinear(skb);
+       }

but I'm not sure the skb_share_check() is required on tx.
I'm also a bit puzzled by the aliasing between shared and cloned terms
(i.e. skb_unshare() could be named something like skb_unclone();
 the skb_share_check() not only checks but also unshares an skb so
 the already taken skb_unshare() name would probably fit too).

Madalin
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