netdev - Re: [PATCH v3] xen-netfront: pull on receive skb may need to happen earlier

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Message-Id: <20130717.125205.635721424198600555.davem@davemloft.net>
Date:	Wed, 17 Jul 2013 12:52:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	Ian.Campbell@...rix.com
Cc:	JBeulich@...e.com, wei.liu2@...rix.com, g.w.kant@...enet.nl,
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v3] xen-netfront: pull on receive skb may need to
 happen earlier

From: Ian Campbell <Ian.Campbell@...rix.com>
Date: Wed, 17 Jul 2013 09:26:24 +0100

> On Wed, 2013-07-17 at 08:09 +0100, Jan Beulich wrote:
>> Due to commit 3683243b ("xen-netfront: use __pskb_pull_tail to ensure
>> linear area is big enough on RX") xennet_fill_frags() may end up
>> filling MAX_SKB_FRAGS + 1 fragments in a receive skb, and only reduce
>> the fragment count subsequently via __pskb_pull_tail(). That's a
>> result of xennet_get_responses() allowing a maximum of one more slot to
>> be consumed (and intermediately transformed into a fragment) if the
>> head slot has a size less than or equal to RX_COPY_THRESHOLD.
>> 
>> Hence we need to adjust xennet_fill_frags() to pull earlier if we
>> reached the maximum fragment count - due to the described behavior of
>> xennet_get_responses() this guarantees that at least the first fragment
>> will get completely consumed, and hence the fragment count reduced.
>> 
>> In order to not needlessly call __pskb_pull_tail() twice, make the
>> original call conditional upon the pull target not having been reached
>> yet, and defer the newly added one as much as possible (an alternative
>> would have been to always call the function right before the call to
>> xennet_fill_frags(), but that would imply more frequent cases of
>> needing to call it twice).
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> Acked-by: Wei Liu <wei.liu2@...rix.com>
> 
> Acked-by: Ian Campbell <ian.campbell@...rix.com>

Applied.
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