From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH v3] xen-netfront: pull on receive skb may need to happen earlier Date: Wed, 17 Jul 2013 12:52:05 -0700 (PDT) Message-ID: <20130717.125205.635721424198600555.davem@davemloft.net> References: <1373970815.4663.45.camel@kazak.uk.xensource.com> <51E65F5102000078000E57ED@nat28.tlf.novell.com> <1374049584.4663.64.camel@kazak.uk.xensource.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: JBeulich@suse.com, wei.liu2@citrix.com, g.w.kant@hunenet.nl, xen-devel@lists.xenproject.org, netdev@vger.kernel.org, stable@vger.kernel.org To: Ian.Campbell@citrix.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:52900 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757177Ab3GQTwH (ORCPT ); Wed, 17 Jul 2013 15:52:07 -0400 In-Reply-To: <1374049584.4663.64.camel@kazak.uk.xensource.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Ian Campbell 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 >> Acked-by: Wei Liu > > Acked-by: Ian Campbell Applied.