On Thu, 27 Feb 2020 08:04:05 +0530 Anshuman Khandual [off-list ref] wrote:

quoted

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??



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 <redacted>
Cc: Anshuman Khandual <redacted>
Cc: James Morse <james.morse@arm.com>
Cc: Christophe Leroy <redacted>
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;

_