linux-kernel - Re: [PATCH/RFC] mm/debug_pagealloc: enable it by default

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Message-ID: <56A67CDC.6080906@de.ibm.com>
Date:	Mon, 25 Jan 2016 20:51:56 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Dave Jones <davej@...emonkey.org.uk>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	heiko.carstens@...ibm.com, akpm@...ux-foundation.org,
	linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH/RFC] mm/debug_pagealloc: enable it by default

On 01/25/2016 08:38 PM, Dave Jones wrote:
>  Content preview:  On Mon, Jan 25, 2016 at 02:22:10PM -0500, Dave Jones wrote:
>     > > 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. [...] 
> 
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> On Mon, Jan 25, 2016 at 02:22:10PM -0500, Dave Jones wrote:
> 
>  >  > 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.
> 
> Looking at this closer, shouldn't the original diff to make this runtime
> have also changed the x86 implementation details in arch/x86/mm/pageattr.c ?
> Even when we don't boot with the enable flag, we're doing the "split 2M page into 4Ks"
> thing.  It looks like a s/debug_pagealloc/debug_pagealloc_enabled()/ might
> do the right thing maybe ?

I have some patches ready, that fix the identity mapping for s390 and x86. sparc
is hard to do as it allocates the TSB very early. Will send around what I have soon.