From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Greear Subject: Re: [PATCH net 1/2] tcp: Limit number of segments generated by GSO per skb Date: Mon, 30 Jul 2012 10:23:17 -0700 Message-ID: <5016C305.7080907@candelatech.com> References: <1343668498.2667.5.camel@bwh-desktop.uk.solarflarecom.com> <1343668602.2667.6.camel@bwh-desktop.uk.solarflarecom.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: David Miller , netdev@vger.kernel.org, linux-net-drivers@solarflare.com To: Ben Hutchings Return-path: Received: from mail.candelatech.com ([208.74.158.172]:50081 "EHLO ns3.lanforge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753487Ab2G3RX0 (ORCPT ); Mon, 30 Jul 2012 13:23:26 -0400 In-Reply-To: <1343668602.2667.6.camel@bwh-desktop.uk.solarflarecom.com> Sender: netdev-owner@vger.kernel.org List-ID: On 07/30/2012 10:16 AM, Ben Hutchings wrote: > A peer (or local user) may cause TCP to use a nominal MSS of as little > as 88 (actual MSS of 76 with timestamps). Given that we have a > sufficiently prodigious local sender and the peer ACKs quickly enough, > it is nevertheless possible to grow the window for such a connection > to the point that we will try to send just under 64K at once. This > results in a single skb that expands to 861 segments. > > In some drivers with TSO support, such an skb will require hundreds of > DMA descriptors; a substantial fraction of a TX ring or even more than > a full ring. The TX queue selected for the skb may stall and trigger > the TX watchdog repeatedly (since the problem skb will be retried > after the TX reset). This particularly affects sfc, for which the > issue is designated as CVE-2012-3412. However it may be that some > hardware or firmware also fails to handle such an extreme TSO request > correctly. > > Therefore, limit the number of segments per skb to 100. This should > make no difference to behaviour unless the actual MSS is less than > about 700. Please do not do this...or at least allow over-rides. We love the trick of seting very small MSS and making the NICs generate huge numbers of small TCP frames with efficient user-space logic. We use this for stateful TCP load testing when high numbers of tcp packets-per-second is desired. Intel NICs, including 10G, work just fine with minimal MSS in this scenario. Thanks, Ben -- Ben Greear Candela Technologies Inc http://www.candelatech.com