[PATCH Makedumpfile 1/4] x86_64: Calculate page_offset from pt_load

[Pratyush Anand]

On Monday 24 October 2016 10:18 PM, Pratyush Anand wrote:
> page_offset can always be calculated as 'virtual - physical' for a direct
> mapping area on x86. Therefore, remove the version dependent calculation
> and use this method.
>
> Signed-off-by: Pratyush Anand <panand at redhat.com>
> ---
>  arch/x86_64.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86_64.c b/arch/x86_64.c
> index ddf7be6bc57b..a96fd8ae00a1 100644
> --- a/arch/x86_64.c
> +++ b/arch/x86_64.c
> @@ -44,6 +44,24 @@ get_xen_p2m_mfn(void)
>  	return NOT_FOUND_LONG_VALUE;
>  }
>
> +static int
> +get_page_offset_x86_64(void)
> +{
> +	int i;
> +	unsigned long long phys_start;
> +	unsigned long long virt_start;
> +
> +	for (i = 0; get_pt_load(i, &phys_start, NULL, &virt_start, NULL); i++) {
> +		if (virt_start >= __START_KERNEL_map) {

OK..So, this is the problem. We should have

if (virt_start < __START_KERNEL_map) {

Kernel text region lies above __START_KERNEL_map, which is linearly 
mapped however not a direct mapping. Direct mapping region lies below it 
instead. So, page_offset can only be calculated with a region which is 
below __START_KERNEL_map.

Thanks Baoquan for finding it.

~Pratyush

> +			info->page_offset = virt_start - phys_start;
> +			return TRUE;
> +		}
> +	}
> +
> +	ERRMSG("Can't get any pt_load to calculate page offset.\n");
> +	return FALSE;
> +}
> +
>  int
>  get_phys_base_x86_64(void)
>  {
> @@ -159,10 +177,8 @@ get_versiondep_info_x86_64(void)
>  	else
>  		info->max_physmem_bits  = _MAX_PHYSMEM_BITS_2_6_31;
>
> -	if (info->kernel_version < KERNEL_VERSION(2, 6, 27))
> -		info->page_offset = __PAGE_OFFSET_ORIG;
> -	else
> -		info->page_offset = __PAGE_OFFSET_2_6_27;
> +	if (!get_page_offset_x86_64())
> +		return FALSE;
>
>  	if (info->kernel_version < KERNEL_VERSION(2, 6, 31)) {
>  		info->vmalloc_start = VMALLOC_START_ORIG;
>