Date:	Tue, 23 Jul 2013 14:45:01 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, x86@...nel.org, dave@...1.net,
	kosaki.motohiro@...il.com, isimatu.yasuaki@...fujitsu.com,
	tangchen@...fujitsu.com, vasilis.liaskovitis@...fitbricks.com
Subject: Re: [PATCH v2] mm/hotplug, x86: Disable ARCH_MEMORY_PROBE by default

On Tue, 2013-07-23 at 10:01 +0200, Ingo Molnar wrote:
> * Toshi Kani <toshi.kani@...com> wrote:
> 
> > > Could we please also fix it to never crash the kernel, even if stupid 
> > > ranges are provided?
> > 
> > Yes, this probe interface can be enhanced to verify the firmware 
> > information before adding a given memory address.  However, such change 
> > would interfere its test use of "fake" hotplug, which is only the known 
> > use-case of this interface on x86.
> 
> Not crashing the kernel is not a novel concept even for test interfaces...

Agreed.

> Where does the possible crash come from - from using invalid RAM ranges, 
> right? I.e. on x86 to fix the crash we need to check the RAM is present in 
> the e820 maps, is marked RAM there, and is not already registered with the 
> kernel, or so?

Yes, the crash comes from using invalid RAM ranges.  How to check if the
RAM is present is different if the system supports hotplug or not.

> > In order to verify if a given memory address is enabled at run-time (as 
> > opposed to boot-time), we need to check with ACPI memory device objects 
> > on x86.  However, system vendors tend to not implement memory device 
> > objects unless their systems support memory hotplug.  Dave Hansen is 
> > using this interface for his testing as a way to fake a hotplug event on 
> > a system that does not support memory hotplug.
> 
> All vendors implement e820 maps for the memory present at boot time.

Yes for boot time.  At run-time, e820 is not guaranteed to represent a
new memory added.  Here is a quote from ACPI spec.

===
15.1 INT 15H, E820H - Query System Address Map
 :
The memory map conveyed by this interface is not required to reflect any
changes in available physical memory that have occurred after the BIOS
has initially passed control to the operating system. For example, if
memory is added dynamically, this interface is not required to reflect
the new system memory configuration.
===

By definition, the "probe" interface is used for the kernel to recognize
a new memory added at run-time.  So, it should check ACPI memory device
objects (which represents run-time state) for the verification.  On x86,
however, ACPI also sends a hotplug event to the kernel, which triggers
the kernel to recognize the new physical memory properly.  Hence, users
do not need this "probe" interface.

> How is the testing done by Dave Hansen? If it's done by booting with less 
> RAM than available (via say the mem=1g boot parameter), and then 
> hot-adding some of the missing RAM, then this could be made safe via the 
> e820 maps and by consultig the physical memory maps (to avoid double 
> registry), right?

If we focus on this test scenario on a system that does not support
hotplug, yes, I agree that we can check with e820 since it is safe to
assume that the system has no change after boot.  IOW, it is unsafe to
check with e820 if the system supports hotplug, but there is no use in
this interface for testing if the system supports hotplug.  So, this may
be a good idea.

Dave, is this how you are testing?  Do you always specify a valid memory
address for your testing?

> How does the hotplug event based approach solve double adds? Relies on the 
> hardware not sending a hot-add event twice for the same memory area or for 
> an invalid memory area, or does it include fail-safes and double checks as 
> well to avoid double adds and adding invalid memory? If yes then that 
> could be utilized here as well.

In high-level, here is how ACPI memory hotplug works:

1. ACPI sends a hotplug event to a new ACPI memory device object that is
hot-added.
2. The kernel is notified, and verifies if the new memory device object
has not been attached by any handler yet.
3. The memory handler is called, and obtains a new memory range from the
ACPI memory device object. 
4. The memory handler calls add_memory() with the new address range.

The above step 1-4 proceeds automatically within the kernel.  No user
input (nor sysfs interface) is necessary.  Step 2 prevents double adds
and step 3 gets a valid address range from the firmware directly.  Step
4 is basically the same as the "probe" interface, but with all the
verification up front, this step is safe.

Thanks,
-Toshi


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