From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: [PATCH] xen-netback: count number required slots for an skb more carefully Date: Wed, 4 Sep 2013 13:41:25 +0100 Message-ID: <1378298485.17510.109.camel@kazak.uk.xensource.com> References: <1378229390-12919-1-git-send-email-david.vrabel@citrix.com> <20130903215328.GA13465@zion.uk.xensource.com> <52271DFF.3070008@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Wei Liu , , "Konrad Rzeszutek Wilk" , Boris Ostrovsky , , , To: David Vrabel Return-path: Received: from smtp.eu.citrix.com ([46.33.159.39]:21914 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934439Ab3IDMl1 (ORCPT ); Wed, 4 Sep 2013 08:41:27 -0400 In-Reply-To: <52271DFF.3070008@citrix.com> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 2013-09-04 at 12:48 +0100, David Vrabel wrote: > On 03/09/13 22:53, Wei Liu wrote: > > On Tue, Sep 03, 2013 at 06:29:50PM +0100, David Vrabel wrote: > >> From: David Vrabel > >> > >> When a VM is providing an iSCSI target and the LUN is used by the > >> backend domain, the generated skbs for direct I/O writes to the disk > >> have large, multi-page skb->data but no frags. > >> > >> With some lengths and starting offsets, xen_netbk_count_skb_slots() > >> would be one short because the simple calculation of > >> DIV_ROUND_UP(skb_headlen(), PAGE_SIZE) was not accounting for the > >> decisions made by start_new_rx_buffer() which does not guarantee > >> responses are fully packed. > >> > >> For example, a skb with length < 2 pages but which spans 3 pages would > >> be counted as requiring 2 slots but would actually use 3 slots. > >> > >> skb->data: > >> > >> | 1111|222222222222|3333 | > >> > >> Fully packed, this would need 2 slots: > >> > >> |111122222222|22223333 | > >> > >> But because the 2nd page wholy fits into a slot it is not split across > >> slots and goes into a slot of its own: > >> > >> |1111 |222222222222|3333 | > >> > >> Miscounting the number of slots means netback may push more responses > >> than the number of available requests. This will cause the frontend > >> to get very confused and report "Too many frags/slots". The frontend > >> never recovers and will eventually BUG. > >> > >> Fix this by counting the number of required slots more carefully. In > >> xen_netbk_count_skb_slots(), more closely follow the algorithm used by > >> xen_netbk_gop_skb() by introducing xen_netbk_count_frag_slots() which > >> is the dry-run equivalent of netbk_gop_frag_copy(). > >> > > > > Phew! So this is backend miscounting bug. I thought it was a frontend > > bug so it didn't ring a bell when we had our face-to-face discussion, > > sorry. :-( > > > > This bug was discussed back in July among Annie, Matt, Ian and I. We > > finally agreed to take Matt's solution. Matt agreed to post final > > version within a week but obviously he's too busy to do so. I was away > > so I didn't follow closely. Eventually it fell through the crack. :-( > > I think I prefer fixing the counting for backporting to stable kernels. That's a good argument. I think we should take this patch, or something very like it, now and then rebase the more complex thing on top. > Xi's approach of packing the ring differently is a change in frontend > visible behaviour and seems more risky. e.g., possible performance > impact so I would like to see some performance analysis of that approach. Yes.