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

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Message-ID: <1353426878.13542.59.camel@zakaz.uk.xensource.com>
Date:	Tue, 20 Nov 2012 15:54:38 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.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 15:28 +0000, Eric Dumazet wrote:
> 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'

The use of MAX_SKB_FRAGS is a bit misleading here, it's really the max
number of slots which the other end will be willing to receive as a
single frame (in the Ethernet sense), as defined by the PV protocol. It
happens to be the same as MAX_SKB_FRAGS (or it is at least
MAX_SKB_FRAGS, I'm not too sure).

I'll nuke the dump_stack() though -- it's not clear what sort of useful
context it would contain anyway.

Ian.



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