Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753667AbaLANhB (ORCPT ); Mon, 1 Dec 2014 08:37:01 -0500 Received: from smtp.citrix.com ([66.165.176.89]:21627 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752404AbaLANg7 (ORCPT ); Mon, 1 Dec 2014 08:36:59 -0500 X-IronPort-AV: E=Sophos;i="5.07,494,1413244800"; d="scan'208";a="198060225" Message-ID: <547C6EF6.8020604@citrix.com> Date: Mon, 1 Dec 2014 13:36:54 +0000 From: David Vrabel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.2.0 MIME-Version: 1.0 To: Stefan Bader , Zoltan Kiss , Konrad Rzeszutek Wilk , Boris Ostrovsky , David Vrabel CC: Wei Liu , Ian Campbell , , , Paul Durrant , Subject: Re: [Xen-devel] [PATCH] xen-netfront: Fix handling packets on compound pages with skb_linearize References: <1407778343-13622-1-git-send-email-zoltan.kiss@citrix.com> <547C2CFC.7060908@canonical.com> In-Reply-To: <547C2CFC.7060908@canonical.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 8bit X-DLP: MIA2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/12/14 08:55, Stefan Bader wrote: > On 11.08.2014 19:32, Zoltan Kiss wrote: >> There is a long known problem with the netfront/netback interface: if the guest >> tries to send a packet which constitues more than MAX_SKB_FRAGS + 1 ring slots, >> it gets dropped. The reason is that netback maps these slots to a frag in the >> frags array, which is limited by size. Having so many slots can occur since >> compound pages were introduced, as the ring protocol slice them up into >> individual (non-compound) page aligned slots. The theoretical worst case >> scenario looks like this (note, skbs are limited to 64 Kb here): >> linear buffer: at most PAGE_SIZE - 17 * 2 bytes, overlapping page boundary, >> using 2 slots >> first 15 frags: 1 + PAGE_SIZE + 1 bytes long, first and last bytes are at the >> end and the beginning of a page, therefore they use 3 * 15 = 45 slots >> last 2 frags: 1 + 1 bytes, overlapping page boundary, 2 * 2 = 4 slots >> Although I don't think this 51 slots skb can really happen, we need a solution >> which can deal with every scenario. In real life there is only a few slots >> overdue, but usually it causes the TCP stream to be blocked, as the retry will >> most likely have the same buffer layout. >> This patch solves this problem by linearizing the packet. This is not the >> fastest way, and it can fail much easier as it tries to allocate a big linear >> area for the whole packet, but probably easier by an order of magnitude than >> anything else. Probably this code path is not touched very frequently anyway. >> >> Signed-off-by: Zoltan Kiss >> Cc: Wei Liu >> Cc: Ian Campbell >> Cc: Paul Durrant >> Cc: netdev@vger.kernel.org >> Cc: linux-kernel@vger.kernel.org >> Cc: xen-devel@lists.xenproject.org > > This does not seem to be marked explicitly as stable. Has someone already asked > David Miller to put it on his stable queue? IMO it qualifies quite well and the > actual change should be simple to pick/backport. I think it's a candidate, yes. Can you expand on the user visible impact of the bug this patch fixes? I think it results in certain types of traffic not working (because the domU always generates skb's with the problematic frag layout), but I can't remember the details. 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/