Subject: [PATCH] arm: mm: fix lowmem virtual address range check

From: [email-masked] (Wang Nan)
Date: Wed, 14 May 2014 18:24:12 +0800

On 2014/5/14 18:11, Will Deacon wrote:
> On Wed, May 14, 2014 at 07:03:59AM +0100, Wang Nan wrote:
>> This patch makes sure the argument of __phys_to_virt is a valid physical
>> address when clear lowmem memory maps.
>>
>> The last few lines prepare_page_table() clear page mapping in the gap
>> between largest low physical memory and the upper bound of lowmem. It
>> uses __phys_to_virt(end) to calculate virtual address from where the
>> clearing start.
>>
>> However, if the platform uses private nonliner __phys_to_virt(), 'end'
>> may goes into another mapping region.
>>
>> This patch uses __phys_to_virt(end - 1) + 1 for insurance purposes.
>>
>> Signed-off-by: Wang Nan <[email-masked]>
>> Cc: Geng Hui <[email-masked]>
>> Cc: Will Deacon <[email-masked]>
>> ---
>>  arch/arm/mm/mmu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
>> index b68c6b2..87340ee 100644
>> --- a/arch/arm/mm/mmu.c
>> +++ b/arch/arm/mm/mmu.c
>> @@ -1217,7 +1217,7 @@ static inline void prepare_page_table(void)
>>  	 * Clear out all the kernel space mappings, except for the first
>>  	 * memory bank, up to the vmalloc region.
>>  	 */
>> -	for (addr = __phys_to_virt(end);
>> +	for (addr = __phys_to_virt(end - 1) + 1;
>>  	     addr < VMALLOC_START; addr += PMD_SIZE)
>>  		pmd_clear(pmd_off_k(addr));
> 
> This looks correct to me, but I'd be interested to know which platform this
> is falling over on. Only realview seems to override __phys_to_virt and I
> don't think we want to add more of that if we can help it.
> 
> Will
> 

I'm working on realview code now, trying to make it support CONFIG_ARM_PATCH_PHYS_VIRT,
and found this problem when working on its private __phys_to_virt. Realview is the only
in-kernel arm board which uses sparse memory. I think it is a good example when testing
sparse memory support of tools such as kexec-tools, kdump and crash.