RE: [PATCH v2 2/2] crash_core, vmcoreinfo: Append 'MAX_PHYSMEM_BITS' to vmcoreinfo

[Kazuhito Hagio]

Hi Bhupesh,

-----Original Message-----
> Right now user-space tools like 'makedumpfile' and 'crash' need to rely
> on a best-guess method of determining value of 'MAX_PHYSMEM_BITS'
> supported by underlying kernel.
> 
> This value is used in user-space code to calculate the bit-space
> required to store a section for SPARESMEM (similar to the existing
> calculation method used in the kernel implementation):
> 
>   #define SECTIONS_SHIFT    (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
> 
> Now, regressions have been reported in user-space utilities
> like 'makedumpfile' and 'crash' on arm64, with the recently added
> kernel support for 52-bit physical address space, as there is
> no clear method of determining this value in user-space
> (other than reading kernel CONFIG flags).
> 
> As per suggestion from makedumpfile maintainer (Kazu), it makes more
> sense to append 'MAX_PHYSMEM_BITS' to vmcoreinfo in the core code itself
> rather than in arch-specific code, so that the user-space code for other
> archs can also benefit from this addition to the vmcoreinfo and use it
> as a standard way of determining 'SECTIONS_SHIFT' value in user-land.
> 
> A reference 'makedumpfile' implementation which reads the
> 'MAX_PHYSMEM_BITS' value from vmcoreinfo in a arch-independent fashion
> is available here:
> 
> [0]. https://github.com/bhupesh-sharma/makedumpfile/blob/remove-max-phys-mem-bit-v1/arch/ppc64.c#L471
> 
> Cc: Boris Petkov <bp@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Dave Anderson <anderson@xxxxxxxxxx>
> Cc: Kazuhito Hagio <k-hagio@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> ---
>  kernel/crash_core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 093c9f917ed0..44b90368e183 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -467,6 +467,7 @@ static int __init crash_save_vmcoreinfo_init(void)
>  #define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
>  	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
>  #endif
> +	VMCOREINFO_NUMBER(MAX_PHYSMEM_BITS);

Some architectures define MAX_PHYSMEM_BITS only with CONFIG_SPARSEMEM,
so we need to move this to the #ifdef section that exports some
mem_section things.

Thanks!
Kazu

> 
>  	arch_crash_save_vmcoreinfo();
>  	update_vmcoreinfo_note();
> --
> 2.7.4
>