visible memory seems wrong in kexec crash dump kernel

Chris Friesen cbf123 at
Fri Jul 12 17:08:38 EDT 2013

On 07/11/2013 07:21 PM, Michael Ellerman wrote:
> On Thu, Jul 11, 2013 at 03:22:49PM -0600, Chris Friesen wrote:
>> On 07/11/2013 02:55 PM, Chris Friesen wrote:
>>> Hi,
>>> I'm running 2.6.34 with kexec 2.0.1 on a Freescale p5020-based system
>>> with 8GB of memory.  (It's an embedded system and I can't do much
>>> about the fact that it's using older software.)
>> I should probably clarify this...I may be able to update kexec, I
>> can't update the kernel but I can backport more recent code if
>> necessary.
>> Looking at the version of kexec that I have, it seems like where x86
>> uses "memmap=" to specify the memory map usable by the capture
>> kernel, powerpc does something different.
>  From memory, it's been years, on powerpc we add properties to the memory
> nodes in the device tree that specify which memory is usable. The
> properties are called "linux,usable-memory", and in modern kernels they
> are read in early_init_dt_scan_memory().

I turned on the instrumentation in early_init_dt_scan_memory() and got 
the following when jumping to the capture kernel:

memory scan node memory, reg size 16, data: 0 0 2 0,
  - 0 ,  200000000

That 0x200000000 matches the fact that I'm seeing 8GB of memory 
available in the recovery kernel.

If I boot the original kernel with "crashkernel=224M at 32M", should I 
expect that only 224MB is marked as "linux,usable-memory" in the 
recovery kernel?


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