netdev - Re: [Xen-devel] [PATCH RFC] tcp: Allow sk_wmem_alloc to exceed sysctl_tcp_limit_output

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Message-ID: <1428933925.3834.6.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 13 Apr 2015 07:05:25 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Jonathan Davies <jonathan.davies@...rix.com>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	netdev@...r.kernel.org, James Morris <jmorris@...ei.org>,
	"David S. Miller" <davem@...emloft.net>,
	xen-devel@...ts.xenproject.org,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [Xen-devel] [PATCH RFC] tcp: Allow sk_wmem_alloc to exceed
 sysctl_tcp_limit_output_bytes

On Mon, 2015-04-13 at 14:46 +0100, David Vrabel wrote:

> > And the proof-of-concept patch for idea (b) I used was:
> > 
> > @@ -552,6 +552,8 @@ static int xennet_start_xmit(struct sk_buff *skb,
> > struct net_device *dev)
> >              goto drop;
> >      }
> > 
> > +    skb_orphan(skb);
> > +
> >      page = virt_to_page(skb->data);
> >      offset = offset_in_page(skb->data);
> >      len = skb_headlen(skb);
> 
> No. This a bunch of allocations and  a full memcpy of all the frags.

skb_orphan() does nothing like that.

But the main concern here is it basically breaks back pressure.

And you do not want this, unless there is no other choice.


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