netdev - Re: [Xen-devel] [PATCH] xen/netfront: handle compound page fragments on transmit

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Date:	Tue, 20 Nov 2012 07:28:28 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Sander Eikelenboom <linux@...elenboom.it>,
	Eric Dumazet <edumazet@...gle.com>,
	KonradRzeszutekWilk <konrad@...nel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	ANNIE LI <annie.li@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/netfront: handle compound page
 fragments on transmit

On Tue, 2012-11-20 at 15:06 +0000, Ian Campbell wrote:

> In practice no because of the property that the number of pages backing
> the frags is <= MAX_SKB_FRAGS even if you are using compound pages as
> the frags.

Yes, but you can make this test trigger with some hacks from userland
(since the frag allocator is per task instead of per socket), so you
should remove the dump_stack() ?

Best way would be to count exact number of slots.

This could be something like 48 slots for a single skb

(if each frag is 4098 (1+4096+1)bytes, only the last one is around 4000
bytes)

MAX_SKB_FRAGS is really number of frags, while your driver needs a count
of 'order-0' 'frames'



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