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 14:11:45 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.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>,
	Zoltan Kiss <zoltan.kiss@...rix.com>
Subject: Re: [PATCH net-next] xen-netfront: try linearizing SKB if it
 occupies too many slots

On Fri, May 16, 2014 at 06:04:34AM -0700, Eric Dumazet wrote:
> On Fri, 2014-05-16 at 12:08 +0100, Wei Liu wrote:
> > Some workload, such as Redis can generate SKBs which make use of
> > compound pages. Netfront doesn't quite like that because it doesn't want
> > to send packet that occupies exessive slots to the backend as backend
> > might deem it malicious. On the flip side these packets are actually
> > legit, the size check at the beginning of xennet_start_xmit ensures that
> > packet size is below 64K.
> > 
> > So we linearize SKB if it occupies too many slots. If the linearization
> > fails then the SKB is dropped.
> > 
> > Signed-off-by: Wei Liu <wei.liu2@...rix.com>
> > Cc: David Vrabel <david.vrabel@...rix.com>
> > Cc: Konrad Wilk <konrad.wilk@...cle.com>
> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > Cc: Stefan Bader <stefan.bader@...onical.com>
> > Cc: Zoltan Kiss <zoltan.kiss@...rix.com>
> > ---
> >  drivers/net/xen-netfront.c |   17 ++++++++++++++---
> >  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> This is likely to fail on typical host.
> 

It's not that common to trigger this, I only saw a few reports. In fact
Stefan's report is the first one that comes with a method to reproduce
it.

I tested with redis-benchmark on a guest with 256MB RAM and only saw a
few "failed to linearize", never saw a single one with 1GB guest.

> What about adding a smart helper trying to aggregate consecutive
> smallest fragments into a single frag ?
> 

Ideally this is a better apporach, but I'm afraid I won't be able to
look into this until early / mid June.

Wei.

> This would be needed for bnx2x for example as well.
> 
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