[Crash-utility] Why module's global symbol cannot be displayed in crash? [ARM]

[Per Fransson]

Hi all,

>
> This is actually a known issue on ARM (just remembered that). When the crash
> happens it identity maps the whole address space of the running process. This
> has been fixed by upstream commit:
>
> commit 2c8951ab0c337cb198236df07ad55f9dd4892c26
> Author: Will Deacon <will.deacon at arm.com>
> Date:   Wed Jun 8 15:53:34 2011 +0100
>
>     ARM: idmap: use idmap_pgd when setting up mm for reboot
>
>     For soft-rebooting a system, it is necessary to map the MMU-off code
>     with an identity mapping so that execution can continue safely once the
>     MMU has been switched off.
>
>     Currently, switch_mm_for_reboot takes out a 1:1 mapping from 0x0 to
>     TASK_SIZE during reboot in the hope that the reset code lives at a
>     physical address corresponding to a userspace virtual address.
>
>     This patch modifies the code so that we switch to the idmap_pgd tables,
>     which contain a 1:1 mapping of the cpu_reset code. This has the
>     advantage of only remapping the code that we need and also means we
>     don't need to worry about allocating a pgd from an atomic context in the
>     case that the physical address of the cpu_reset code aliases with the
>     virtual space used by the kernel.
>

So, it actually allocates a 16k L1 page table just for this? But why
be so picky about which code is identity mapped by using the
.idmap.text section? Couldn't we just identity map all the kernel code
in that case?

I suggested a more selective and temporary modification of the current
mapping at one point:

http://thread.gmane.org/gmane.linux.kernel.kexec/4612

I guess 16k isn't worth making a fuzz about, but it just seems a
little bit wasteful...

By the way, as far as I can tell, there's no identity mapping of
'relocate_new_kernel'. Does the 'isb' instruction in cpu_v7_reset
guarantee that we're in the realm of physical addresses by then?

Regards,
Per