netdev - Re: [PATCH] xfrm: don't segment UFO packets

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Message-ID: <20160317094115.GA11706@midget.suse.cz>
Date:	Thu, 17 Mar 2016 10:41:15 +0100
From:	Jiri Bohac <jbohac@...e.cz>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Steffen Klassert <steffen.klassert@...unet.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] xfrm: don't segment UFO packets

On Thu, Mar 17, 2016 at 01:03:59PM +0800, Herbert Xu wrote:
> On Wed, Mar 16, 2016 at 05:00:26PM +0100, Jiri Bohac wrote:
> > Prevent xfrm_output() from segmenting UFO packets so that they will be
> > fragmented after the xfrm transforms.
> 
> Fair enough.  But I wonder if this is enough.  Wouldn't UDP notice
> that we're doing IPsec and prefragment the packet anyway? So I think
> this check may also be needed in the UDP output path.

Fixes my broken case. Ftracing a sendmsg call that sends ~8k of
data over the UDP socket, I see a single 8k skb travel through the
stack all the way to xfrm_output(). MTU is 1500.
That's the whole poing of fragmentation offloading, to pass all
the data at once as far as we can, isn't it? What UDP code did
you think would "notice and prefragment"?

Thanks,

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, SUSE CZ