Vitaly Mayatskikh <v.mayatskih@gmail.com> writes:

quoted text
> Patch applies to 2.6.34-rc5
>
> On x86 platform, even if hardware is 64-bit capable, kernel starts
> execution in 32-bit mode. When system is kdump-enabled, crashed kernel
> switches to 32 bit mode and jumps into new kernel. This automatically
> limits location of dump-capture kernel image and it's initrd by first
> 4Gb of memory. Switching to 32 bit mode is performed by purgatory
> code, which has relocations of type R_X86_64_32S (32-bit signed), and
> this cuts "good" address space for crash kernel down to 2 Gb. I/O
> regions may cut down this space further.
>
> When system has a lot of memory (hundreds of gigabytes), dump-capture
> kernel also needs relatively a lot of memory to account old kernel's
> pages. It may be impossible to reserve enough memory below 2 or even 4
> Gb. Simplest solution is it break dump-capture kernel's reserved
> memory region into two pieces: first (small) region for kernel and
> initrd images may be easily placed in "good" address space in the
> beginning of physical memory, and second region may be located
> anywhere.
>
> This serie of patches realizes this approach. It requires also changes
> in kexec utility to make this feature work, but is
> backward-compatible: old versions of kexec will work with new
> kernel. I will post patch to kexec-tools upstream separately.

Have you tried loading a 64bit vmlinux directly into a higher address
range?  There may be a bit or two missing but you should be able to
load a linux kernel above 4GB.  I tested the basics of that mechanism
when I made the 64bit relocatable kernel.

I don't buy the argument that there is a direct connection between
the amount of memory you have and how much memory it takes to dump it.
Even an indirect connections seems suspicious.

Eric
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