netdev - Re: compound skb frag pages appearing in start

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Message-ID: <1349793630.21847.208.camel@zakaz.uk.xensource.com>
Date:	Tue, 9 Oct 2012 15:40:30 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Eric Dumazet <edumazet@...gle.com>,
	xen-devel <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	Sander Eikelenboom <linux@...elenboom.it>
Subject: Re: compound skb frag pages appearing in start_xmit

On Tue, 2012-10-09 at 15:27 +0100, Eric Dumazet wrote:
> On Tue, 2012-10-09 at 15:17 +0100, Ian Campbell wrote:
> 
> > Does the higher order pages effectively reduce the number of frags which
> > are in use? e.g if MAX_SKB_FRAGS is 16, then for order-0 pages you could
> > have 64K worth of frag data.
> > 
> > If we switch to order-3 pages everywhere then can the skb contain 512K
> > of data, or does the effective maximum number of frags in an skb reduce
> > to 2?
> 
> effective number of frags reduce to 2 or 3
> 
> (We still limit GSO packets to ~63536 bytes)

Great! Then I think the fix is more/less trivial...

As an aside, when the skb head is < 4096 bytes is that necessarily a
compound page or might it just be a large kmalloc area?

Only really relevant since it impacts the possibility for code sharing
between the head and the frags sending.

Ian


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