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

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Date:	Wed, 17 Jul 2013 09:26:24 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	<davem@...emloft.net>, Wei Liu <wei.liu2@...rix.com>,
	Dion Kant <g.w.kant@...enet.nl>,
	xen-devel <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

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>

Ian.

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