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

From	David Rientjes <rientjes@google.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting
Date	2016-02-02 23:00 +0100
Message-ID	<qXR9o-S7-9@gated-at.bofh.it> (permalink)
References	<qVvmF-1Z6-3@gated-at.bofh.it> <qVvmG-1Z6-19@gated-at.bofh.it> <qVGBt-20k-45@gated-at.bofh.it> <qVRnd-1sZ-33@gated-at.bofh.it> <qW3Ro-28Y-37@gated-at.bofh.it>
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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.
> 

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Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting David Rientjes <rientjes@google.com> - 2016-01-27 23:20 +0100
  Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting Christian Borntraeger <borntraeger@de.ibm.com> - 2016-01-28 10:50 +0100
    Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting David Rientjes <rientjes@google.com> - 2016-01-29 00:10 +0100
      Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting Christian Borntraeger <borntraeger@de.ibm.com> - 2016-02-02 23:00 +0100
        Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting Andrew Morton <akpm@linux-foundation.org> - 2016-02-02 23:30 +0100
          Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting Christian Borntraeger <borntraeger@de.ibm.com> - 2016-02-02 23:40 +0100
            Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting Andrew Morton <akpm@linux-foundation.org> - 2016-02-03 00:10 +0100
              Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting Stephen Rothwell <sfr@canb.auug.org.au> - 2016-02-03 01:20 +0100
      Re: [PATCH v3 2/3] x86: query dynamic DEBUG_PAGEALLOC setting David Rientjes <rientjes@google.com> - 2016-02-02 23:00 +0100

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