x86/mm: Add 'step_size' comments to init_mem_mapping()

Current code uses macro to shift by 5, but there is no explanation
why there's no worry about an overflow there.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Jacob Shin <jacob.shin@amd.com>
Link: http://lkml.kernel.org/r/1378519629-10433-1-git-send-email-yinghai@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 20 insertions, 3 deletions

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 04664cdb7fda..ce32017c5e38 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -399,8 +399,25 @@ static unsigned long __init init_range_memory_mapping(
         return mapped_ram_size;
 }
 
-/* (PUD_SHIFT-PMD_SHIFT)/2 */
-#define STEP_SIZE_SHIFT 5
+static unsigned long __init get_new_step_size(unsigned long step_size)
+{
+        /*
+         * Explain why we shift by 5 and why we don't have to worry about
+         * 'step_size << 5' overflowing:
+         *
+         * initial mapped size is PMD_SIZE (2M).
+         * We can not set step_size to be PUD_SIZE (1G) yet.
+         * In worse case, when we cross the 1G boundary, and
+         * PG_LEVEL_2M is not set, we will need 1+1+512 pages (2M + 8k)
+         * to map 1G range with PTE. Use 5 as shift for now.
+         *
+         * Don't need to worry about overflow, on 32bit, when step_size
+         * is 0, round_down() returns 0 for start, and that turns it
+         * into 0x100000000ULL.
+         */
+        return step_size << 5;
+}
+
 void __init init_mem_mapping(void)
 {
         unsigned long end, real_end, start, last_start;
@@ -449,7 +466,7 @@ void __init init_mem_mapping(void)
                 min_pfn_mapped = last_start >> PAGE_SHIFT;
                 /* only increase step_size after big range get mapped */
                 if (new_mapped_ram_size > mapped_ram_size)
-                        step_size <<= STEP_SIZE_SHIFT;
+                        step_size = get_new_step_size(step_size);
                 mapped_ram_size += new_mapped_ram_size;
         }