diff --git a/include/linux/gcd.h b/include/linux/gcd.h
new file mode 100644
index 0000000..0ac2621
--- /dev/null
+++ b/include/linux/gcd.h
@@ -0,0 +1,8 @@
+#ifndef _GCD_H
+#define _GCD_H
+
+#include <linux/kernel.h>
+
+unsigned long gcd(unsigned long a, unsigned long b) __attribute_const__;
+
+#endif /* _GCD_H */
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3f2644c..5b6b448 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -257,5 +257,10 @@
 	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
 	(type *)( (char *)__mptr - offsetof(type,member) );})
 
+/*
+ * swap - swap value of @a and @b
+ */
+#define swap(a, b) \
+	do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
 
 #endif /* _LINUX_KERNEL_H */
diff --git a/lib/Makefile b/lib/Makefile
index 226570a..1684649 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -51,3 +51,4 @@
 obj-y			+= wchar.o
 obj-y			+= libfile.o
 obj-y			+= bitmap.o
+obj-y			+= gcd.o
diff --git a/lib/gcd.c b/lib/gcd.c
new file mode 100644
index 0000000..86bdba9
--- /dev/null
+++ b/lib/gcd.c
@@ -0,0 +1,18 @@
+#include <linux/gcd.h>
+
+/* Greatest common divisor */
+unsigned long gcd(unsigned long a, unsigned long b)
+{
+	unsigned long r;
+
+	if (a < b)
+		swap(a, b);
+
+	if (!b)
+		return a;
+	while ((r = a % b) != 0) {
+		a = b;
+		b = r;
+	}
+	return b;
+}