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

From: [email-masked] (Mark Rutland)
Date: Wed, 13 May 2015 15:11:52 +0100

On Wed, May 13, 2015 at 02:40:16PM +0100, Hans de Goede wrote:
> 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]>

Great. Thanks for testing!

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

Sure, done.

Russell, on the assumption you're ok with the patch I've submitted it to
the patch system as 8356/1.

Thanks,
Mark.

> 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;
> >
>