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

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Date:	Tue, 26 Jan 2016 15:18:55 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Dave Jones <davej@...emonkey.org.uk>, 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 Mon, Jan 25, 2016 at 08:51:56PM +0100, Christian Borntraeger wrote:
> 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. [...] 
> > 
> >  Content analysis details:   (-2.9 points, 5.0 required)
> > 
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> > x-cbid: 16012519-0041-0000-0000-0000074865D5
> > X-IBM-ISS-SpamDetectors: Score=0.40962; FLB=0; FLI=0; BY=0; FL=0; FP=0; FZ=0;
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> >  19:38:24
> > X-TM-AS-MML: disable
> > 
> > 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.

Thanks for taking care of this.

I searched other sites using "#ifdef CONFIG_DEBUG_PAGEALLOC" and found
that we can do more. Maybe, powerpc and tile also can get benefit from such
a change. And, could you handle vmap_debug_free_range() and
get_freepointer_safe(), too?

And, when debug_pagealloc is disabled, it's better not to print
DEBUG_PAGEALLOC in some error logs.

Thanks.