From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Durrant Subject: RE: [Xen-devel] [PATCH net-next] xen-netback: fix xenvif_count_skb_slots() Date: Mon, 7 Oct 2013 10:37:31 +0000 Message-ID: <9AAE0902D5BC7E449B7C8E4E778ABCD0129EE1@AMSPEX01CL01.citrite.net> References: <1380903983-27429-1-git-send-email-paul.durrant@citrix.com> <525283D4.7010504@citrix.com> <9AAE0902D5BC7E449B7C8E4E778ABCD0129E7A@AMSPEX01CL01.citrite.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT Cc: Wei Liu , Ian Campbell , "netdev@vger.kernel.org" , "xen-devel@lists.xen.org" , Annie Li , Matt Wilson , Xi Xiong To: Paul Durrant , David Vrabel Return-path: Received: from smtp02.citrix.com ([66.165.176.63]:19358 "EHLO SMTP02.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752777Ab3JGKhd convert rfc822-to-8bit (ORCPT ); Mon, 7 Oct 2013 06:37:33 -0400 In-Reply-To: <9AAE0902D5BC7E449B7C8E4E778ABCD0129E7A@AMSPEX01CL01.citrite.net> Content-Language: en-US Sender: netdev-owner@vger.kernel.org List-ID: > -----Original Message----- > From: xen-devel-bounces@lists.xen.org [mailto:xen-devel- > bounces@lists.xen.org] On Behalf Of Paul Durrant > Sent: 07 October 2013 11:24 > To: David Vrabel > Cc: Wei Liu; Ian Campbell; netdev@vger.kernel.org; xen-devel@lists.xen.org; > Annie Li; Matt Wilson; Xi Xiong > Subject: Re: [Xen-devel] [PATCH net-next] xen-netback: fix > xenvif_count_skb_slots() > > > -----Original Message----- > > From: David Vrabel > > Sent: 07 October 2013 10:50 > > To: Paul Durrant > > Cc: xen-devel@lists.xen.org; netdev@vger.kernel.org; Wei Liu; Ian > Campbell; > > Annie Li; Matt Wilson; Xi Xiong > > Subject: Re: [Xen-devel] [PATCH net-next] xen-netback: fix > > xenvif_count_skb_slots() > > > > On 04/10/13 17:26, Paul Durrant wrote: > > > Commit 4f0581d25827d5e864bcf07b05d73d0d12a20a5c introduced an > error > > into > > > xenvif_count_skb_slots() for skbs with a linear area spanning a page > > > boundary. The alignment of skb->data needs to be taken into account, > not > > > just the head length. This patch fixes the issue by dry-running the code > > > from xenvif_gop_skb() (and adjusting the comment above the function > to > > note > > > that). > > > > If 4f0581d2582 is causing the skb->data to be fully packed into a > > minimal number of slots then the simple > > DIV_ROUND_UP(skb_headlen(skb)) > > is correct. > > > > I think this change will miscount in the number of slots, > > over-estimating the count which I think will eventually cause netback to > > think the ring has no space when it has some. > > > > Is the problem here not the miscounting of slots but running out of > > space in the grant table op array because we know use more copy ops? > > > > Essentially yes. Netback is built on the assumption of no more than two grant > copies per ring slot. > To be clear; I believe that, with the packing change, a third grant copy may be used for the initial slot and that is why we blow the array. Paul > > I didn't think there was any real merit in the problematic commit (or at > > least there was no evidence that it was better) so I would suggest just > > reverting it instead of trying to fix it up. > > > > I'd be happy with a reversion. > > > If we do want to change how netback fills the ring then netback needs > > some redesign (i.e., change it so it doesn't have to this counting in > > advance) to make it much less fragile to changes in this area. > > > > Yes, that would be much better. > > Paul > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel