From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932181AbcAYKHQ (ORCPT ); Mon, 25 Jan 2016 05:07:16 -0500 Received: from e06smtp13.uk.ibm.com ([195.75.94.109]:40200 "EHLO e06smtp13.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756053AbcAYKHL (ORCPT ); Mon, 25 Jan 2016 05:07:11 -0500 X-IBM-Helo: d06dlp02.portsmouth.uk.ibm.com X-IBM-MailFrom: borntraeger@de.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc To: Heiko Carstens References: <1453713588-119602-1-git-send-email-borntraeger@de.ibm.com> <20160125094132.GA4298@osiris> <56A5EECE.90607@de.ibm.com> <20160125100248.GB4298@osiris> Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, Joonsoo Kim From: Christian Borntraeger Message-ID: <56A5F3C8.4050202@de.ibm.com> Date: Mon, 25 Jan 2016 11:07:04 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <20160125100248.GB4298@osiris> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16012510-0013-0000-0000-000008650291 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/25/2016 11:02 AM, Heiko Carstens wrote: > On Mon, Jan 25, 2016 at 10:45:50AM +0100, Christian Borntraeger wrote: >>>> + By default this option will be almost for free and can be activated >>>> + in distribution kernels. The overhead and the debugging can be enabled >>>> + by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc command line >>>> + parameter. >>> >>> Sorry, but it's not almost for free and should not be used by distribution >>> kernels. If we have DEBUG_PAGEALLOC enabled, at least on s390 we will not >>> make use of 2GB and 1MB pagetable entries for the identy mapping anymore. >>> Instead we will only use 4K mappings. >> >> Hmmm, can we change these code areas to use debug_pagealloc_enabled? I guess >> this evaluated too late? > > Yes, that should be possible. "debug_pagealloc" is an early_param, which > will be evaluated before we call paging_init() (both in > arch/s390/kernel/setup.c). > > So it looks like this can be trivially changed. (replace the ifdefs in > arch/s390/mm/vmem.c with debug_pagealloc_enabled()). > >>> I assume this is true for all architectures since freeing pages can happen >>> in any context and therefore we can't allocate memory in order to split >>> page tables. >>> >>> So enabling this will cost memory and put more pressure on the TLB. >> >> So I will change the description and drop the "if unsure" statement. > > Well, given that we can change it like above... I don't care anymore ;) Ok, I will give it a try, and come back with a rewording or an s390 patch.