From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758343AbcAYTWW (ORCPT ); Mon, 25 Jan 2016 14:22:22 -0500 Received: from arcturus.aphlor.org ([188.246.204.175]:48986 "EHLO arcturus.aphlor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758324AbcAYTWS (ORCPT ); Mon, 25 Jan 2016 14:22:18 -0500 Date: Mon, 25 Jan 2016 14:22:10 -0500 From: Dave Jones To: Christian Borntraeger Cc: Joonsoo Kim , linux-kernel@vger.kernel.org, peterz@infradead.org, heiko.carstens@de.ibm.com, akpm@linux-foundation.org, linux-s390 Subject: Re: [PATCH/RFC] mm/debug_pagealloc: enable it by default Message-ID: <20160125192210.GA20919@codemonkey.org.uk> Mail-Followup-To: Dave Jones , Christian Borntraeger , Joonsoo Kim , linux-kernel@vger.kernel.org, peterz@infradead.org, heiko.carstens@de.ibm.com, akpm@linux-foundation.org, linux-s390 References: <1453463080-84320-1-git-send-email-borntraeger@de.ibm.com> <20160125064151.GA22557@js1304-P5Q-DELUXE> <56A5DB21.2090300@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56A5DB21.2090300@de.ibm.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Spam-Score: -2.9 (--) X-Spam-Report: Spam detection software, running on the system "arcturus.aphlor.org", has NOT identified this incoming email as spam. The original message has been attached to this so you can view it or label similar future email. If you have any questions, see the administrator of that system for details. Content preview: On Mon, Jan 25, 2016 at 09:21:53AM +0100, Christian Borntraeger wrote: > >> Let's go back to the original default and reverse the command > >> line parameter to allow disabling the feature. > > > > Default disable is requested by Andrew and I agreed that. > > > > https://lkml.org/lkml/2014/11/27/3 > > Well, Andrew's assumption was "I suspect the number of "old users" is one" > and this is wrong. I can at least claim, that most of the s390 test were > conducted with CONFIG_DEBUG_PAGEALLOC enabled (at least regularly) - and > about a year ago this was changed to "no longer useful". I have posted 2 > real bugs that I triggered pretty quickly after I enabled things on the > command line. [...] Content analysis details: (-2.9 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP -1.9 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 25, 2016 at 09:21:53AM +0100, Christian Borntraeger wrote: > >> Let's go back to the original default and reverse the command > >> line parameter to allow disabling the feature. > > > > Default disable is requested by Andrew and I agreed that. > > > > https://lkml.org/lkml/2014/11/27/3 > > Well, Andrew's assumption was "I suspect the number of "old users" is one" > and this is wrong. I can at least claim, that most of the s390 test were > conducted with CONFIG_DEBUG_PAGEALLOC enabled (at least regularly) - and > about a year ago this was changed to "no longer useful". I have posted 2 > real bugs that I triggered pretty quickly after I enabled things on the > command line. Nearly all the fuzz-test runs I've done over the last few years have had this enabled most the time. I was wondering why things got faster last year. > > I think that default disable is good thing because we can use > > the kernel compiled with CONFIG_DEBUG_PAGEALLOC even for production. > > Unless boot param is provided, it's runtime cost is nearly zero. > > This could help to debug in some situations and make this debugging > > feature more useful. But, yes, old users can be surprised > > by this change. How about changing Kconfig to ask user for default mode? > > A default mode would be ok for me as it makes things obvious. Will send > a patch. The messaging around changing a default like this was really poor. When we do behaviour changes like this, we typically rename the CONFIG option to make it obvious that things aren't as they used to be. ho-hum. Dave