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

Mark Rutland mark.rutland at arm.com
Mon May 11 03:31:30 PDT 2015

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.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Reported-by: Stefan Agner <stefan at agner.ch>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Hans de Goede <hdegoede at redhat.com>
Cc: Laura Abbott <labbott at redhat.com>
Cc: Russell King <rmk+kernel at arm.linux.org.uk>
Cc: Steve Capper <steve.capper at linaro.org>
 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;

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