Re: [PATCH v2] mm/hotplug, x86: Disable ARCH_MEMORY_PROBE by default

From: Hush Bensen
Date: Wed Jul 24 2013 - 20:45:00 EST

On 07/25/2013 12:02 AM, Toshi Kani wrote:
On Wed, 2013-07-24 at 08:18 +0800, Hush Bensen wrote:
On 07/24/2013 04:45 AM, Toshi Kani wrote:
On Tue, 2013-07-23 at 10:01 +0200, Ingo Molnar wrote:
* Toshi Kani <toshi.kani@xxxxxx> 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.
Could you explain different methods to check the RAM is present if the
system supports hotplkug or not?
e820 and UEFI memory descriptor tables are the boot-time interfaces.
These interfaces are not required to reflect any run-time changes.

ACPI memory device objects can be used at both boot-time and run-time,
which reflect any run-time changes. But they are optional to implement.
They typically are not implemented unless the system supports hotplug.

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.
This is hot-added part, could you also explain how ACPI memory hotplug
works for hot-remove?
Sure. Here is high-level.

1. ACPI sends a hotplug event to an ACPI memory device object that is
requested to hot-remove.
2. The kernel is notified, and verifies if the memory device object is
attached by a handler.
3. The memory handler is called (which is being attached), and obtains
its memory range.
4. The memory handler calls remove_memory() with the address range.
5. The kernel calls eject method of the ACPI memory device object.

Could you give me the calltrace of add_memory and remove_memory? I don't have machine support hotplug, but I hope to investigate how ACPI part works for memory hotplug. ;-)


Thanks,
-Toshi



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