Comment # 3 on bug 1023755 from Jiri Bohac

Hi,

(In reply to Richard Palethorpe from comment #0)
> Error messages vary, but with 256MB,high:
> Cannot reserve 256MB crashkernel low memory, please try smaller size

If you're puzzled why the kernel is trying to reserve low memory when you
specified 256MB,high, then it's because the crash kernel cannot be entirely in
the high memory above 4G. So if the region found is above 4G, it will try to
allocate an extra low region of 256MB.

Your system does not have much low memory; after e820__reserve_setup_data it's
basically these three blocks:
0x0000000000100000-0x000000001cb75017        458 MB @          1 MB 
0x000000001cb9f658-0x000000002a6a0fff        219 MB @        459 MB 
0x000000002c994000-0x00000000376a8fff        173 MB @        713 MB 

And, I just found a probable bug in the low reservation code, where 
it can't allocate 256 MB starting under 256 MB; just reported upstream:
https://marc.info/?l=linux-kernel&m=149762977921771
This would cause reserve_crashkernel_low to fail.

But I don't understand why the 256 MB were not allocated under 4G before
calling reserve_crashkernel_low and why your other tries failed with "no
suitable memory could be found".

Could you try booting with memblock=debug and post the full dmesg output?
This should print out memory blocks being allocated prior to te crashkernel
reservation, allowing us to see exactly what blocks remain available.

Thanks!