Re: [PATCH] ARM: memblock limit must be pmd-aligned

[Doug Berger]

On 06/26/2017 04:43 PM, Laura Abbott wrote:
> On 06/26/2017 10:23 AM, Doug Berger wrote:
>> There is a path through the adjust_lowmem_bounds() routine where if all
>> memory regions start and end on pmd-aligned addresses the memblock_limit
>> will be set to arm_lowmem_limit.
>>
>> However, since arm_lowmem_limit can be affected by the vmalloc early
>> parameter, the value of arm_lowmem_limit may not be pmd-aligned. This
>> commit corrects this oversight such that memblock_limit is always rounded
>> down to pmd-alignment.
>>
>> The pmd containing arm_lowmem_limit is cleared by prepare_page_table()
>> and without this commit it is possible for early_alloc() to allocate
>> unmapped memory in that range when mapping the lowmem.
>>
> 
> Do you have an example system or configuration where you see this
> crash?
I have observed this crash occur on systems like the bcm7445 when a
customer uses the vmalloc boot parameter to specify an odd number of
Megabytes of VMALLOC space (e.g. vmalloc=751m).  This seems to be a
popular way for them to set the low memory boundary.

As long as vmalloc is a multiple of the pmd (e.g. 2MB) there isn't a
problem, so documenting this constraint is another possible solution.
However, educating the user is more difficult in this case than working
around a questionable value to allow the boot to succeed.

-Doug
> 
> Thanks,
> Laura
> 
>> Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
>> ---
>>  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 31af3cb59a60..2ae4f9c9d757 100644
>> --- a/arch/arm/mm/mmu.c
>> +++ b/arch/arm/mm/mmu.c
>> @@ -1226,7 +1226,7 @@ void __init adjust_lowmem_bounds(void)
>>  	if (memblock_limit)
>>  		memblock_limit = round_down(memblock_limit, PMD_SIZE);
>>  	if (!memblock_limit)
>> -		memblock_limit = arm_lowmem_limit;
>> +		memblock_limit = round_down(arm_lowmem_limit, PMD_SIZE);
>>  
>>  	if (!IS_ENABLED(CONFIG_HIGHMEM) || cache_is_vipt_aliasing()) {
>>  		if (memblock_end_of_DRAM() > arm_lowmem_limit) {
>>
>