linux-kernel - Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug

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Date:	Mon, 25 Jan 2016 10:45:50 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of
 debug_pagealloc

On 01/25/2016 10:41 AM, Heiko Carstens wrote:
> On Mon, Jan 25, 2016 at 10:19:48AM +0100, Christian Borntraeger wrote:
>> since commit 031bc5743f158 ("mm/debug-pagealloc: make debug-pagealloc
>> boottime configurable") CONFIG_DEBUG_PAGEALLOC is by default a no-op.
>>
>> This resulted in several unnoticed bugs, e.g.
>>
>> https://lkml.kernel.org/g/<569F5E29.3090107@...ibm.com>
>> or
>> https://lkml.kernel.org/g/<56A20F30.4050705@...ibm.com>
>>
>> as this behaviour change was not even documented in Kconfig.
>>
>> Let's provide a new Kconfig symbol that allows to change the default
>> back to enabled, e.g. for debug kernels. This also makes the change
>> obvious to kernel packagers.
>>
>> Let's also change the Kconfig description for CONFIG_DEBUG_PAGEALLOC,
>> to indicate that it is ok to enable this by default.
>>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
>> ---
>>  mm/Kconfig.debug | 17 +++++++++++++++++
>>  mm/page_alloc.c  |  6 +++++-
>>  2 files changed, 22 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
>> index 957d3da..4cf1212 100644
>> --- a/mm/Kconfig.debug
>> +++ b/mm/Kconfig.debug
>> @@ -26,5 +26,22 @@ config DEBUG_PAGEALLOC
>>  	  that would result in incorrect warnings of memory corruption after
>>  	  a resume because free pages are not saved to the suspend image.
>>
>> +	  By default this option will be almost for free and can be activated
>> +	  in distribution kernels. The overhead and the debugging can be enabled
>> +	  by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc command line
>> +	  parameter.
> 
> Sorry, but it's not almost for free and should not be used by distribution
> kernels. If we have DEBUG_PAGEALLOC enabled, at least on s390 we will not
> make use of 2GB and 1MB pagetable entries for the identy mapping anymore.
> Instead we will only use 4K mappings.

Hmmm, can we change these code areas to use debug_pagealloc_enabled? I guess
this evaluated too late?


> I assume this is true for all architectures since freeing pages can happen
> in any context and therefore we can't allocate memory in order to split
> page tables.
> 
> So enabling this will cost memory and put more pressure on the TLB.

So I will change the description and drop the "if unsure" statement.

Christian