Re: [PATCH V14] mm/debug: Add tests validating architecture page table helpers

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 02/27/2020 09:33 AM, Andrew Morton wrote:
> On Thu, 27 Feb 2020 08:04:05 +0530 Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>>> Must be something wrong with the following in debug_vm_pgtable()
>>>
>>>     paddr = __pa(&start_kernel);
>>>
>>> Is there any explaination why start_kernel() is not in linear memory on ARM64 ?
>>
>>
>> Cc: + James Morse <james.morse@arm.com>
>>
>> This warning gets exposed with DEBUG_VIRTUAL due to __pa() on a kernel symbol
>> i.e 'start_kernel' which might be outside the linear map. This happens due to
>> kernel mapping position randomization with KASLR. Adding James here in case he
>> might like to add more.
>>
>> __pa_symbol() should have been used instead, for accessing the physical address
>> here. On arm64 __pa() does check for linear address with __is_lm_address() and
>> switch accordingly if it is a kernel text symbol. Nevertheless, its much better
>> to use __pa_symbol() here rather than __pa().
>>
>> Rather than respining the patch once more, will just send a fix replacing this
>> helper __pa() with __pa_symbol() for Andrew to pick up as this patch is already
>> part of linux-next (next-20200226). But can definitely respin if that will be
>> preferred.
> 
> I didn't see this fix?  I assume it's this?  If so, are we sure it's OK to be
> added to -next without testing??

My patch just missed your response here within a minute :) Please consider this.
It has been tested on x86 and arm64.

https://patchwork.kernel.org/patch/11407715/

> 
> 
> 
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm-debug-add-tests-validating-architecture-page-table-helpers-fix
> 
> A warning gets exposed with DEBUG_VIRTUAL due to __pa() on a kernel symbol
> i.e 'start_kernel' which might be outside the linear map.  This happens
> due to kernel mapping position randomization with KASLR.
> 
> __pa_symbol() should have been used instead, for accessing the physical
> address here.  On arm64 __pa() does check for linear address with
> __is_lm_address() and switch accordingly if it is a kernel text symbol. 
> Nevertheless, its much better to use __pa_symbol() here rather than
> __pa().
> 
> Reported-by: Qian Cai <cai@lca.pw>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: James Morse <james.morse@arm.com>
> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/debug_vm_pgtable.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/mm/debug_vm_pgtable.c~mm-debug-add-tests-validating-architecture-page-table-helpers-fix
> +++ a/mm/debug_vm_pgtable.c
> @@ -331,7 +331,7 @@ void __init debug_vm_pgtable(void)
>  	 * helps avoid large memory block allocations to be used for mapping
>  	 * at higher page table levels.
>  	 */
> -	paddr = __pa(&start_kernel);
> +	paddr = __pa_symbol(&start_kernel);
>  
>  	pte_aligned = (paddr & PAGE_MASK) >> PAGE_SHIFT;
>  	pmd_aligned = (paddr & PMD_MASK) >> PAGE_SHIFT;
> _
> 
>