[Crash-utility] x86 remap allocator in kernel 3.0

[Petr Tesarik]

Hi folks,

I've just discovered that the crash utility fails to initialize the vm 
subsystem properly on our latest SLES 32-bit kernels. It turns out that our 
kernels are compiled with CONFIG_DISCONTIGMEM=y, which causes pgdat structs to 
be allocated by the remap allocator (cf. arch/x86/mm/numa_32.c and also the 
code in setup_node_data).

If you don't know what the remap allocator is (like I didn't before I hit the 
bug), it's a very special early-boot allocator which remaps physical pages 
from low memory to high memory, giving them virtual addresses from the 
identity mapping. Looks a bit like this:

                        physical addr
                       +------------+
                       |            |
                       +------------+
                  +--> |  KVA RAM   |
                  |    +------------+
                  |    |            |
                  |    \/\/\/\/\/\/\/
                  |    /\/\/\/\/\/\/\
                  |    |            |
  virtual addr    |    |  highmem   |
 +------------+   |    |------------|
 |            | -----> |            |
 +------------+   |    +------------+
 |  remap va  | --+    |   KVA PG   | (unused)
 +------------+        +------------+
 |            |        |            |
 |            | -----> | RAM bottom |
 +------------+        +------------+

This breaks a very basic assumption that crash makes about low-memory virtual 
addresses.

The attached patch fixes the issue for me, but may not be the cleanest method 
to handle these mappings.

Ken'ichi Ohmichi, please note that makedumpfile is also affected by this 
deficiency. On my test system, it will fail to produce any output if I set 
dump level to anything greater than zero:

makedumpfile -c -d 31 -x vmlinux-3.0.13-0.5-pae.debug vmcore kdump.31
readmem: Can't convert a physical address(34a012b4) to offset.
readmem: type_addr: 0, addr:f4a012b4, size:4
get_mm_discontigmem: Can't get node_start_pfn.

makedumpfile Failed.

However, fixing this for makedumpfile is harder, and it will most likely 
require a few more lines in VMCOREINFO, because debug symbols may not be 
available at dump time, and I can't see any alternative method to locate the 
remapped regions.

Regards,
Petr Tesarik
SUSE Linux
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