netdev - Re: [PATCH net-next] xen-netfront: try linearizing SKB if it occupies too many slots

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Date:	Fri, 16 May 2014 09:47:09 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
Cc:	Wei Liu <wei.liu2@...rix.com>, netdev@...r.kernel.org,
	xen-devel@...ts.xen.org, David Vrabel <david.vrabel@...rix.com>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Stefan Bader <stefan.bader@...onical.com>
Subject: Re: [PATCH net-next] xen-netfront: try linearizing SKB if it
 occupies too many slots

On Fri, 2014-05-16 at 17:29 +0100, Zoltan Kiss wrote:
> On 16/05/14 16:34, Wei Liu wrote:
> > 
> > It works, at least in this Redis testcase. Could you explain a bit where
> > this 56000 magic number comes from? :-)
> > 
> > Presumably I can derive it from some constant in core network code?
> 
> I guess it just makes more unlikely to have packets with problematic layout. But the following packet would still fail:
> linear buffer : 80 bytes, on 2 pages
> 17 frags, 80 bytes each, each spanning over page boundary.

How would you build such skbs ? Its _very_ difficult, you have to be
very very smart to hit this.

Also reducing gso_max_size made sure order-5 allocations would not be
attempted in this unlikely case.

56000 + overhead is below 65536 -> order-4 allocations at most.





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