[PATCH] arm: improve non-section-aligned low memory mapping

[Min-Hua Chen]

>From d8dbec3573b02afd8a23fe10f92bc0d324b0c951 Mon Sep 17 00:00:00 2001
From: Min-Hua Chen <orca.chen at gmail.com>
Date: Sun, 26 Apr 2015 15:07:44 +0800
Subject: [PATCH] arm: improve non-section-aligned low memory mapping

In current design, the memblock.current_limit is set to
a section-aligned value in sanity_check_meminfo().

However, the section-aligned memblock may become non-section-aligned
after arm_memblock_init(). For example, the first section-aligned
memblock is 0x00000000-0x01000000 and sanity_check_meminfo sets
current_limit to 0x01000000. After arm_memblock_init, two memory blocks
[0x00c00000 - 0x00d00000] and [0x00ff0000 - 0x01000000] are reserved
by memblock_reserve() and make the original memory block
[0x00000000-0x01000000] becomes:

[0x00000000-0x00c00000]
[0x00d00000-0x00ff0000]

When creating the low memory mapping for [0x00d00000-0x00ff0000],
since the memory block is non-section-aligned, it will need to create
a second level page table. But the current_limit is set to 0x01000000,
and it's possible to allocate a unmapped memory block.

call flow:

setup_arch
 + sanity_check_meminfo
 + arm_memblock_init
 + paging_init
    + map_lowmem
    + bootmem_init

Move the memblock_set_current_limit logic to map_lowmem(), we point
the memblock current_limit to the first section-aligned memblock block.
Since map_lowmem() is called after arm_memblock_init(), there is no way to
change memblock layout. So we can say that the first section-aligned limit
is valid during map_lowmem(). Hence fix the problem described above.

Another change is the implementation of find_limits().
In commit: 1c2f87c22566cd057bc8cde10c37ae9da1a1bb76, the max_low is
set by memblock_get_current_limit(). However memblock.current_limit
can be changed by memblock_set_current_limit() anypoint before
find_limits().

It's better to use arm_lowmem_limit to be max_lowmem in two ways:
First, arm_lowmem_limit cannot be changed by a public API. Second, the
high_memory is set by arm_lowmem_limit and is a natural limit of
low memory area in bootmem_init().

Signed-off-by: Min-Hua Chen <orca.chen at gmail.com>
---
 arch/arm/mm/init.c |    2 +-
 arch/arm/mm/mmu.c  |   44 ++++++++++----------------------------------
 2 files changed, 11 insertions(+), 35 deletions(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 2495c8c..6a618f9 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -138,7 +138,7 @@ void show_mem(unsigned int filter)
 static void __init find_limits(unsigned long *min, unsigned long *max_low,
        unsigned long *max_high)
 {
- *max_low = PFN_DOWN(memblock_get_current_limit());
+ *max_low = PFN_DOWN(arm_lowmem_limit);
  *min = PFN_UP(memblock_start_of_DRAM());
  *max_high = PFN_DOWN(memblock_end_of_DRAM());
 }
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 4e6ef89..dbc484d 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -1068,7 +1068,6 @@ phys_addr_t arm_lowmem_limit __initdata = 0;

 void __init sanity_check_meminfo(void)
 {
- phys_addr_t memblock_limit = 0;
  int highmem = 0;
  phys_addr_t vmalloc_limit = __pa(vmalloc_min - 1) + 1;
  struct memblock_region *reg;
@@ -1110,43 +1109,10 @@ void __init sanity_check_meminfo(void)
  else
  arm_lowmem_limit = block_end;
  }
-
- /*
- * Find the first non-section-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.
- *
- * 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-
- * 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))
- memblock_limit = block_start;
- else if (!IS_ALIGNED(block_end, SECTION_SIZE))
- memblock_limit = arm_lowmem_limit;
- }
-
  }
  }

  high_memory = __va(arm_lowmem_limit - 1) + 1;
-
- /*
- * Round the memblock limit down to a section size.  This
- * helps to ensure that we will allocate memory from the
- * last full section, which should be mapped.
- */
- if (memblock_limit)
- memblock_limit = round_down(memblock_limit, SECTION_SIZE);
- if (!memblock_limit)
- memblock_limit = arm_lowmem_limit;
-
- memblock_set_current_limit(memblock_limit);
 }

 static inline void prepare_page_table(void)
@@ -1331,6 +1297,7 @@ static void __init map_lowmem(void)
  struct memblock_region *reg;
  phys_addr_t kernel_x_start = round_down(__pa(_stext), SECTION_SIZE);
  phys_addr_t kernel_x_end = round_up(__pa(__init_end), SECTION_SIZE);
+ phys_addr_t section_block_limit = 0;

  /* Map all the lowmem memory banks. */
  for_each_memblock(memory, reg) {
@@ -1384,6 +1351,15 @@ static void __init map_lowmem(void)
  create_mapping(&map);
  }
  }
+
+ /*
+ * Find the first section-aligned memblock and set
+ * memblock_limit at it.
+ */
+ if (!section_memblock_limit && !(end & ~SECTION_MASK)) {
+ section_memblock_limit = end;
+ memblock_set_current_limit(section_memblock_limit);
+ }
  }
 }

-- 
1.7.10.4