From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f49.google.com (mail-wm0-f49.google.com [74.125.82.49]) by kanga.kvack.org (Postfix) with ESMTP id 330A26B0005 for ; Mon, 25 Jan 2016 04:41:38 -0500 (EST) Received: by mail-wm0-f49.google.com with SMTP id l65so56150040wmf.1 for ; Mon, 25 Jan 2016 01:41:38 -0800 (PST) Received: from e06smtp06.uk.ibm.com (e06smtp06.uk.ibm.com. [195.75.94.102]) by mx.google.com with ESMTPS id 65si23332202wmg.21.2016.01.25.01.41.36 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 25 Jan 2016 01:41:36 -0800 (PST) Received: from localhost by e06smtp06.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 25 Jan 2016 09:41:36 -0000 Received: from b06cxnps4076.portsmouth.uk.ibm.com (d06relay13.portsmouth.uk.ibm.com [9.149.109.198]) by d06dlp03.portsmouth.uk.ibm.com (Postfix) with ESMTP id 18BE01B08070 for ; Mon, 25 Jan 2016 09:41:41 +0000 (GMT) Received: from d06av02.portsmouth.uk.ibm.com (d06av02.portsmouth.uk.ibm.com [9.149.37.228]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u0P9fYAq39780534 for ; Mon, 25 Jan 2016 09:41:34 GMT Received: from d06av02.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av02.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u0P9fW5f014833 for ; Mon, 25 Jan 2016 02:41:33 -0700 Date: Mon, 25 Jan 2016 10:41:32 +0100 From: Heiko Carstens Subject: Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc Message-ID: <20160125094132.GA4298@osiris> References: <1453713588-119602-1-git-send-email-borntraeger@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1453713588-119602-1-git-send-email-borntraeger@de.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Christian Borntraeger Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, Joonsoo Kim 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. 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. -- 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