Subject: [PATCH] ARM: mm: handle non-pmd-aligned end of RAM

From: [email-masked] (Hans de Goede)
Date: Wed, 13 May 2015 15:40:16 +0200

Hi,

On 11-05-15 13:43, Stefan Agner wrote:
> On 2015-05-11 12:31, Mark Rutland wrote:
>> At boot time we round the memblock limit down to section size in an
>> attempt to ensure that we will have mapped this RAM with section
>> mappings prior to allocating from it. When mapping RAM we iterate over
>> PMD-sized chunks, creating these section mappings.
>>
>> Section mappings are only created when the end of a chunk is aligned to
>> section size. Unfortunately, with classic page tables (where PMD_SIZE is
>> 2 * SECTION_SIZE) this means that if a chunk is between 1M and 2M in
>> size the first 1M will not be mapped despite having been accounted for
>> in the memblock limit. This has been observed to result in page tables
>> being allocated from unmapped memory, causing boot-time hangs.
>>
>> This patch modifies the memblock limit rounding to always round down to
>> PMD_SIZE instead of SECTION_SIZE. For classic MMU this means that we
>> will round the memblock limit down to a 2M boundary, matching the limits
>> on section mappings, and preventing allocations from unmapped memory.
>> For LPAE there should be no change as PMD_SIZE == SECTION_SIZE.
>
> Thanks Mark, just tested that patch on the hardware I had the issue,
> looks good.
>
> Tested-by: Stefan Agner <[email-masked]>

Same for me, this also fixes the issue I was seeing no an Allwinner A33
tablet with 1024z600 lcd screen.

Tested-by: Hans de Goede <[email-masked]>

Can we get this Cc-ed to stable at vger.kernel.org please? At least for 4.0 ?

Regards,

Hans

>
>>
>> Signed-off-by: Mark Rutland <[email-masked]>
>> Reported-by: Stefan Agner <[email-masked]>
>> Cc: Catalin Marinas <[email-masked]>
>> Cc: Hans de Goede <[email-masked]>
>> Cc: Laura Abbott <[email-masked]>
>> Cc: Russell King <[email-masked]>
>> Cc: Steve Capper <[email-masked]>
>> ---
>>   arch/arm/mm/mmu.c | 20 ++++++++++----------
>>   1 file changed, 10 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
>> index 4e6ef89..7186382 100644
>> --- a/arch/arm/mm/mmu.c
>> +++ b/arch/arm/mm/mmu.c
>> @@ -1112,22 +1112,22 @@ void __init sanity_check_meminfo(void)
>>   			}
>>
>>   			/*
>> -			 * Find the first non-section-aligned page, and point
>> +			 * Find the first non-pmd-aligned page, and point
>>   			 * memblock_limit at it. This relies on rounding the
>> -			 * limit down to be section-aligned, which happens at
>> -			 * the end of this function.
>> +			 * limit down to be pmd-aligned, which happens at the
>> +			 * end of this function.
>>   			 *
>>   			 * With this algorithm, the start or end of almost any
>> -			 * bank can be non-section-aligned. The only exception
>> -			 * is that the start of the bank 0 must be section-
>> +			 * bank can be non-pmd-aligned. The only exception is
>> +			 * that the start of the bank 0 must be section-
>>   			 * aligned, since otherwise memory would need to be
>>   			 * allocated when mapping the start of bank 0, which
>>   			 * occurs before any free memory is mapped.
>>   			 */
>>   			if (!memblock_limit) {
>> -				if (!IS_ALIGNED(block_start, SECTION_SIZE))
>> +				if (!IS_ALIGNED(block_start, PMD_SIZE))
>>   					memblock_limit = block_start;
>> -				else if (!IS_ALIGNED(block_end, SECTION_SIZE))
>> +				else if (!IS_ALIGNED(block_end, PMD_SIZE))
>>   					memblock_limit = arm_lowmem_limit;
>>   			}
>>
>> @@ -1137,12 +1137,12 @@ void __init sanity_check_meminfo(void)
>>   	high_memory = __va(arm_lowmem_limit - 1) + 1;
>>
>>   	/*
>> -	 * Round the memblock limit down to a section size.  This
>> +	 * Round the memblock limit down to a pmd size.  This
>>   	 * helps to ensure that we will allocate memory from the
>> -	 * last full section, which should be mapped.
>> +	 * last full pmd, which should be mapped.
>>   	 */
>>   	if (memblock_limit)
>> -		memblock_limit = round_down(memblock_limit, SECTION_SIZE);
>> +		memblock_limit = round_down(memblock_limit, PMD_SIZE);
>>   	if (!memblock_limit)
>>   		memblock_limit = arm_lowmem_limit;
>