diff --git a/include/linux/idr.h b/include/linux/idr.h
index e968db71e33a94548160cb38c216289ab1577c30..88607523e2df9dd61c97c2e3802177a8750372d5 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -117,10 +117,13 @@ void idr_init(struct idr *idp);
 /*
  * IDA - IDR based id allocator, use when translation from id to
  * pointer isn't necessary.
+ *
+ * IDA_BITMAP_LONGS is calculated to be one less to accommodate
+ * ida_bitmap->nr_busy so that the whole struct fits in 128 bytes.
  */
 #define IDA_CHUNK_SIZE		128	/* 128 bytes per chunk */
-#define IDA_BITMAP_LONGS	(128 / sizeof(long) - 1)
-#define IDA_BITMAP_BITS		(IDA_BITMAP_LONGS * sizeof(long) * 8)
+#define IDA_BITMAP_LONGS	(IDA_CHUNK_SIZE / sizeof(long) - 1)
+#define IDA_BITMAP_BITS 	(IDA_BITMAP_LONGS * sizeof(long) * 8)
 
 struct ida_bitmap {
 	long			nr_busy;