+/*
+ * 1<<6=64 pages -> 256K chunk when page size is 4K. This gives us
+ * the benefit that all the chunks are 64 pages aligned then the
+ * bitmaps are always aligned to LONG.
+ */
+#define CLEAR_BITMAP_SHIFT_MIN 6
+/*
+ * 1<<18=256K pages -> 1G chunk when page size is 4K. This is the
+ * default value to use if no one specified.
+ */
+#define CLEAR_BITMAP_SHIFT_DEFAULT 18
+/*
+ * 1<<31=2G pages -> 8T chunk when page size is 4K. This should be
+ * big enough and make sure we won't overflow easily.
+ */
+#define CLEAR_BITMAP_SHIFT_MAX 31
+