From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756732AbcAYKDL (ORCPT ); Mon, 25 Jan 2016 05:03:11 -0500 Received: from e06smtp16.uk.ibm.com ([195.75.94.112]:50799 "EHLO e06smtp16.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755710AbcAYKCx (ORCPT ); Mon, 25 Jan 2016 05:02:53 -0500 X-IBM-Helo: d06dlp03.portsmouth.uk.ibm.com X-IBM-MailFrom: heiko.carstens@de.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Mon, 25 Jan 2016 11:02:48 +0100 From: Heiko Carstens To: Christian Borntraeger Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, Joonsoo Kim Subject: Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc Message-ID: <20160125100248.GB4298@osiris> References: <1453713588-119602-1-git-send-email-borntraeger@de.ibm.com> <20160125094132.GA4298@osiris> <56A5EECE.90607@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56A5EECE.90607@de.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16012510-0025-0000-0000-000008B61E14 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 ;)