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

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Date:	Wed, 27 Jan 2016 08:59:11 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	David Rientjes <rientjes@...gle.com>
Cc:	Heiko Carstens <heiko.carstens@...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 01/27/2016 01:59 AM, Joonsoo Kim wrote:
> On Tue, Jan 26, 2016 at 04:36:11PM -0800, David Rientjes wrote:
>> 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.  
> 
> Yes, I also found it.
> 
>>
>> 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.
> 
> Yes, at least, non-architecture dependent code (vmalloc, SLAB, SLUB) should
> be changed first. If Christian doesn't mind, I will try to fix above 3
> things.

Ok. So I will change this as Heiko and Thomas suggested for s390 and x86 and 
resend these 3 patches. Feel free to work on the other areas.

Christian