Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751856AbaKGKoV (ORCPT ); Fri, 7 Nov 2014 05:44:21 -0500 Received: from smtp02.citrix.com ([66.165.176.63]:47515 "EHLO SMTP02.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751425AbaKGKoS (ORCPT ); Fri, 7 Nov 2014 05:44:18 -0500 X-IronPort-AV: E=Sophos;i="5.07,331,1413244800"; d="scan'208";a="190513031" Message-ID: <545CA27F.4070400@citrix.com> Date: Fri, 7 Nov 2014 10:44:15 +0000 From: David Vrabel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Icedove/24.5.0 MIME-Version: 1.0 To: , "David S. Miller" , Konrad Rzeszutek Wilk , Boris Ostrovsky , Stefan Bader , Jay Vosburgh , , Subject: Re: BUG in xennet_make_frags with paged skb data References: <20141106214940.GD44162@ubuntu-hedt> In-Reply-To: <20141106214940.GD44162@ubuntu-hedt> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-DLP: MIA1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/11/14 21:49, Seth Forshee wrote: > We've had several reports of hitting the following BUG_ON in > xennet_make_frags with 3.2 and 3.13 kernels (I'm currently awaiting > results of testing with 3.17): > > /* Grant backend access to each skb fragment page. */ > for (i = 0; i < frags; i++) { > skb_frag_t *frag = skb_shinfo(skb)->frags + i; > struct page *page = skb_frag_page(frag); > > len = skb_frag_size(frag); > offset = frag->page_offset; > > /* Data must not cross a page boundary. */ > BUG_ON(len + offset > PAGE_SIZE< > When this happens the page in question is a "middle" page in a compound > page (i.e. it's a tail page but not the last tail page), and the data is > fully contained within the compound page. The data does however cross > the hardware page boundary, and since compound_order evaluates to 0 for > tail pages the check fails. > > In going over this I've been unable to determine whether the BUG_ON in > xennet_make_frags is incorrect or the paged skb data is wrong. I can't > find that it's documented anywhere, and the networking code itself is a > bit ambiguous when it comes to compound pages. On the one hand > __skb_fill_page_desc specifically handles adding tail pages as paged > data, but on the other hand skb_copy_bits kmaps frag->page.p which could > fail with data that extends into another page. netfront will safely handle this case so you can remove this BUG_ON() (and the one later on). But it would be better to find out were these funny-looking skbs are coming from and (if necessary) fixing the bug there. David -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/