linux-kernel - Re: [PATCH v3 2/3] x86: query dynamic DEBUG

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Date:	Tue, 2 Feb 2016 22:53:36 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, x86@...nel.org,
	linuxppc-dev@...ts.ozlabs.org, davem@...emloft.net,
	Joonsoo Kim <iamjoonsoo.kim@....com>, davej@...emonkey.org.uk
Subject: Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting

On 02/02/2016 10:51 PM, David Rientjes wrote:
> On Thu, 28 Jan 2016, David Rientjes wrote:
> 
>> On Thu, 28 Jan 2016, Christian Borntraeger wrote:
>>
>>> Indeed, I only touched the identity mapping and dump stack.
>>> The question is do we really want to change free_init_pages as well?
>>> The unmapping during runtime causes significant overhead, but the
>>> unmapping after init imposes almost no runtime overhead. Of course,
>>> things get fishy now as what is enabled and what not.
>>>
>>> Kconfig after my patch "mm/debug_pagealloc: Ask users for default setting of debug_pagealloc"
>>> (in mm) now states
>>> ----snip----
>>> By default this option will have a small overhead, e.g. by not
>>> allowing the kernel mapping to be backed by large pages on some
>>> architectures. Even bigger overhead comes when the debugging is
>>> enabled by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc
>>> command line parameter.
>>> ----snip----
>>>
>>> So I am tempted to NOT change free_init_pages, but the x86 maintainers
>>> can certainly decide differently. Ingo, Thomas, H. Peter, please advise.
>>>
>>
>> I'm sorry, but I thought the discussion of the previous version of the 
>> patchset led to deciding that all CONFIG_DEBUG_PAGEALLOC behavior would be 
>> controlled by being enabled on the commandline and checked with 
>> debug_pagealloc_enabled().
>>
>> I don't think we should have a CONFIG_DEBUG_PAGEALLOC that does some stuff 
>> and then a commandline parameter or CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT 
>> to enable more stuff.  It should either be all enabled by the commandline 
>> (or config option) or split into a separate entity.  
>> CONFIG_DEBUG_PAGEALLOC_LIGHT and CONFIG_DEBUG_PAGEALLOC would be fine, but 
>> the current state is very confusing about what is being done and what 
>> isn't.
>>
> 
> Ping?
> 
https://lkml.org/lkml/2016/1/29/266 
?