linux-kernel - Re: [PATCH] mm/hotplug, x86: Disable ARCH_MEMORY

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Message-ID: <51E85016.2090403@intel.com>
Date:	Thu, 18 Jul 2013 13:29:10 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	Dave Hansen <dave@...1.net>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	isimatu.yasuaki@...fujitsu.com, tangchen@...fujitsu.com,
	vasilis.liaskovitis@...fitbricks.com
Subject: Re: [PATCH] mm/hotplug, x86: Disable ARCH_MEMORY_PROBE by default

On 07/18/2013 01:10 PM, Toshi Kani wrote:
> On Thu, 2013-07-18 at 11:34 -0700, Dave Hansen wrote:
> I do not think so.  Using echo command to write a value to /dev/sda is
> not how it is instructed to use in the document.  I am not saying that
> we need to protect from a privileged user doing something crazy.

If the document is the issue, then let's fix the document.

>> All that I'm asking is that you either leave it the way it is, or make a
>> Kconfig menu entry for it.
>>
>> But, really, what's the problem that you're solving?  Has this caused
>> you issues somehow?  It's been there for, what, 10 years?  Surely it's
>> part of the ABI.
> 
> The problem is that the probe interface is documented as one of the
> steps that may be necessary for memory hotplug.  A typical user may or
> may not know if his/her platform generates a hotplug notification to the
> kernel to decide if this step is necessary.

A typical user will never see any of this stuff.  It's buried deep under
the covers.

> If the user performs this
> step on x86, it will likely mess up the system.  Since we do not need it
> on x86, a prudent approach to protect such user is to disable or remove
> the interface on x86 and document it accordingly.  We have not seen this
> issue yet because we do not have many platforms that support memory
> hotplug today.  Once memory hotplug support in KVM gets merged into the
> mainline, anyone can start using this feature on their systems.  At that
> time, their choice of a stable kernel may be 3.12.x.  This interface has
> been there for while, but we need to fix it before the memory hotplug
> feature becomes available for everyone.

It sounds like you're arguing that anyone using memory hotplug on x86
might be confused by the probe file.  There's been a lot of hardware out
there that's supported memory hotplug for many, many years.  I've never
heard a complaint about it in practice.  Are KVM users more apt to be
confused than folks running on bare-metal? :)

> Does it make sense?  I understand that you are using this interface for
> your testing.  If I make a Kconfig menu entry, are you OK to disable
> this option by default?

I kinda wish you wouldn't mess with it.  But, sure, put it in the memory
debugging, and make sure it stays enabled on powerpc by default.

Another method would be to just change the default permissions of the
file on x86 instead of disabling it completely:

	# chmod u-w /sys/devices/system/memory/probe
	# echo x > /sys/devices/system/memory/probe
	bash: /sys/devices/system/memory/probe: Permission denied

That way folks can re-chmod it if they *really* want it back (me), and
they can still use it for testing.
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