From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755001AbaHALCx (ORCPT ); Fri, 1 Aug 2014 07:02:53 -0400 Received: from smtp.citrix.com ([66.165.176.89]:25309 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753793AbaHALCw (ORCPT ); Fri, 1 Aug 2014 07:02:52 -0400 X-IronPort-AV: E=Sophos;i="5.01,779,1400025600"; d="scan'208";a="157838310" Date: Fri, 1 Aug 2014 12:02:46 +0100 From: Wei Liu To: David Miller CC: , , , , , , , , , Subject: Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment Message-ID: <20140801110246.GB17947@zion.uk.xensource.com> References: <1406726730-17994-1-git-send-email-zoltan.kiss@citrix.com> <20140731.132520.1889577477638334755.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20140731.132520.1889577477638334755.davem@davemloft.net> User-Agent: Mutt/1.5.23 (2014-03-12) X-DLP: MIA2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 31, 2014 at 01:25:20PM -0700, David Miller wrote: > From: Zoltan Kiss > Date: Wed, 30 Jul 2014 14:25:30 +0100 [...] > Secondly, for something like UDP you can't just split the packet up > like this, or for any other datagram protocol for that matter. > > I know you're in a difficult situation, but I just can't see this > being an acceptable approach to solving the problem right now. > > Where does the MAX_SKB_FRAGS + 1 limit really come from, the size of > the TX queue? > It stems from the implicit transimit protocol since inception of netfront / netback. Sigh. > If you were to have a 64-slot TX queue, you ought to be able to handle > this theoretical 51 slot SKB. > There's two problems: 1. IIRC a single page ring has 256 slots, allowing 64 slots packet yields 4 in-flight packets in worst case. 2. Older netback could not handle this large number of slots and it's likely to deem the frontend malicious. For #1, we don't actually care that much if guest screws itself by generating 64 slot packets. #2 is more concerning. Wei. > And I don't think it's so theoretical, a carefully crafted sequence of > sendfile() calls during a TCP_CORK sequence should be able to do it. > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html