[PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting

Christian Borntraeger borntraeger at de.ibm.com
Wed Feb 3 08:53:36 AEDT 2016

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.  
>> the current state is very confusing about what is being done and what 
>> isn't.
> Ping?

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