From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f50.google.com (mail-wm0-f50.google.com [74.125.82.50]) by kanga.kvack.org (Postfix) with ESMTP id 10D2F6B0254 for ; Mon, 25 Jan 2016 04:46:02 -0500 (EST) Received: by mail-wm0-f50.google.com with SMTP id b14so70325622wmb.1 for ; Mon, 25 Jan 2016 01:46:02 -0800 (PST) Received: from e06smtp13.uk.ibm.com (e06smtp13.uk.ibm.com. [195.75.94.109]) by mx.google.com with ESMTPS id r2si27348391wjz.116.2016.01.25.01.46.00 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 25 Jan 2016 01:46:01 -0800 (PST) Received: from localhost by e06smtp13.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 25 Jan 2016 09:46:00 -0000 Received: from b06cxnps4075.portsmouth.uk.ibm.com (d06relay12.portsmouth.uk.ibm.com [9.149.109.197]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id 647452190066 for ; Mon, 25 Jan 2016 09:45:39 +0000 (GMT) Received: from d06av03.portsmouth.uk.ibm.com (d06av03.portsmouth.uk.ibm.com [9.149.37.213]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u0P9jpWs8061336 for ; Mon, 25 Jan 2016 09:45:51 GMT Received: from d06av03.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av03.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u0P9joJ9001518 for ; Mon, 25 Jan 2016 02:45:51 -0700 Subject: Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc References: <1453713588-119602-1-git-send-email-borntraeger@de.ibm.com> <20160125094132.GA4298@osiris> From: Christian Borntraeger Message-ID: <56A5EECE.90607@de.ibm.com> Date: Mon, 25 Jan 2016 10:45:50 +0100 MIME-Version: 1.0 In-Reply-To: <20160125094132.GA4298@osiris> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Heiko Carstens Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, Joonsoo Kim On 01/25/2016 10:41 AM, Heiko Carstens wrote: > On Mon, Jan 25, 2016 at 10:19:48AM +0100, Christian Borntraeger wrote: >> since commit 031bc5743f158 ("mm/debug-pagealloc: make debug-pagealloc >> boottime configurable") CONFIG_DEBUG_PAGEALLOC is by default a no-op. >> >> This resulted in several unnoticed bugs, e.g. >> >> https://lkml.kernel.org/g/<569F5E29.3090107@de.ibm.com> >> or >> https://lkml.kernel.org/g/<56A20F30.4050705@de.ibm.com> >> >> as this behaviour change was not even documented in Kconfig. >> >> Let's provide a new Kconfig symbol that allows to change the default >> back to enabled, e.g. for debug kernels. This also makes the change >> obvious to kernel packagers. >> >> Let's also change the Kconfig description for CONFIG_DEBUG_PAGEALLOC, >> to indicate that it is ok to enable this by default. >> >> Cc: Joonsoo Kim >> Cc: Andrew Morton >> Signed-off-by: Christian Borntraeger >> --- >> mm/Kconfig.debug | 17 +++++++++++++++++ >> mm/page_alloc.c | 6 +++++- >> 2 files changed, 22 insertions(+), 1 deletion(-) >> >> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug >> index 957d3da..4cf1212 100644 >> --- a/mm/Kconfig.debug >> +++ b/mm/Kconfig.debug >> @@ -26,5 +26,22 @@ config DEBUG_PAGEALLOC >> that would result in incorrect warnings of memory corruption after >> a resume because free pages are not saved to the suspend image. >> >> + 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? > 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. Christian -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org