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

From: Andrew Morton <[email-masked]>
Date: Wed, 26 Feb 2020 20:03:53 -0800

On Thu, 27 Feb 2020 08:04:05 +0530 Anshuman Khandual <[email-masked]> 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 <[email-masked]>
> 
> 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??

From: Andrew Morton <[email-masked]>
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 <[email-masked]>
Cc: Anshuman Khandual <[email-masked]>
Cc: James Morse <[email-masked]>
Cc: Christophe Leroy <[email-masked]>
Signed-off-by: Andrew Morton <[email-masked]>
---

 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;
_