1 files changed, 17 insertions, 20 deletions

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index a97ee0801475..08a7d313538a 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -438,20 +438,20 @@ static unsigned long __init init_range_memory_mapping(
 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).
+         * 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.
+         * to map 1G range with PTE. Hence we use one less than the
+         * difference of page table level shifts.
          *
-         * 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.
+         * Don't need to worry about overflow in the top-down case, on 32bit,
+         * when step_size is 0, round_down() returns 0 for start, and that
+         * turns it into 0x100000000ULL.
+         * In the bottom-up case, round_up(x, 0) returns 0 though too, which
+         * needs to be taken into consideration by the code below.
          */
-        return step_size << 5;
+        return step_size << (PMD_SHIFT - PAGE_SHIFT - 1);
 }
 
 /**
@@ -471,7 +471,6 @@ static void __init memory_map_top_down(unsigned long map_start,
         unsigned long step_size;
         unsigned long addr;
         unsigned long mapped_ram_size = 0;
-        unsigned long new_mapped_ram_size;
 
         /* xen has big range in reserved near end of ram, skip it at first.*/
         addr = memblock_find_in_range(map_start, map_end, PMD_SIZE, PMD_SIZE);
@@ -496,14 +495,12 @@ static void __init memory_map_top_down(unsigned long map_start,
                                 start = map_start;
                 } else
                         start = map_start;
-                new_mapped_ram_size = init_range_memory_mapping(start,
+                mapped_ram_size += init_range_memory_mapping(start,
                                                         last_start);
                 last_start = start;
                 min_pfn_mapped = last_start >> PAGE_SHIFT;
-                /* only increase step_size after big range get mapped */
-                if (new_mapped_ram_size > mapped_ram_size)
+                if (mapped_ram_size >= step_size)
                         step_size = get_new_step_size(step_size);
-                mapped_ram_size += new_mapped_ram_size;
         }
 
         if (real_end < map_end)
@@ -524,7 +521,7 @@ static void __init memory_map_top_down(unsigned long map_start,
 static void __init memory_map_bottom_up(unsigned long map_start,
                                         unsigned long map_end)
 {
-        unsigned long next, new_mapped_ram_size, start;
+        unsigned long next, start;
         unsigned long mapped_ram_size = 0;
         /* step_size need to be small so pgt_buf from BRK could cover it */
         unsigned long step_size = PMD_SIZE;
@@ -539,19 +536,19 @@ static void __init memory_map_bottom_up(unsigned long map_start,
          * for page table.
          */
         while (start < map_end) {
-                if (map_end - start > step_size) {
+                if (step_size && map_end - start > step_size) {
                         next = round_up(start + 1, step_size);
                         if (next > map_end)
                                 next = map_end;
-                } else
+                } else {
                         next = map_end;
+                }
 
-                new_mapped_ram_size = init_range_memory_mapping(start, next);
+                mapped_ram_size += init_range_memory_mapping(start, next);
                 start = next;
 
-                if (new_mapped_ram_size > mapped_ram_size)
+                if (mapped_ram_size >= step_size)
                         step_size = get_new_step_size(step_size);
-                mapped_ram_size += new_mapped_ram_size;
         }
 }