diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index a97ee0801475a2e25df60f336c552408fe1d2b63..08a7d313538a72dfc51227b01546bab34a8f7bee 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;
 	}
 }