linux-kernel - Re: [PATCH/RFC 3/3] s390: query dynamic DEBUG

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Date:	Tue, 26 Jan 2016 16:36:11 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH/RFC 3/3] s390: query dynamic DEBUG_PAGEALLOC setting

On Wed, 27 Jan 2016, Joonsoo Kim wrote:

> > I'd agree if CONFIG_DEBUG_PAGEALLOC only did anything when 
> > debug_pagealloc_enabled() is true, but that doesn't seem to be the case.  
> > When CONFIG_DEBUG_SLAB is enabled, for instance, CONFIG_DEBUG_PAGEALLOC 
> > also enables stackinfo storing and poisoning and it's not guarded by 
> > debug_pagealloc_enabled().
> > 
> > It seems like CONFIG_DEBUG_PAGEALLOC enables debugging functionality 
> > outside the scope of the debug_pagealloc=on kernel parameter, so 
> > DEBUG_PAGEALLOC(disabled) actually does mean something.
> 
> Hello, David.
> 
> I tried to fix CONFIG_DEBUG_SLAB case on 04/16 of following patchset.
> 
> http://thread.gmane.org/gmane.linux.kernel.mm/144527
> 
> I found that there are more sites to fix but not so many.
> We can do it.
> 

For the slab case, sure, this can be fixed, but there is other code that 
uses CONFIG_DEBUG_PAGEALLOC to suggest debugging is always enabled and is 
indifferent to debug_pagealloc_enabled().  I find this in powerpc and 
sparc arch code as well as generic vmalloc code.  

If we can convert existing users that only check for 
CONFIG_DEBUG_PAGEALLOC to rather check for debug_pagealloc_enabled() and 
agree that it is only enabled for debug_pagealloc=on, then this would seem 
fine.  However, I think we should at least consult with those users before 
removing an artifact from the kernel log that could be useful in debugging 
why a particular BUG() happened.