diff --git a/autoconf/spl-build.m4 b/autoconf/spl-build.m4
index 2f09fa87e5..b311902e93 100644
--- a/autoconf/spl-build.m4
+++ b/autoconf/spl-build.m4
@@ -439,3 +439,21 @@ AC_DEFUN([SPL_AC_CTL_UNNUMBERED],
 	        AC_MSG_RESULT(no)
 	])
 ])
+
+dnl #
+dnl # 2.6.16 API change.
+dnl # Check if 'fls64()' is available
+dnl #
+AC_DEFUN([SPL_AC_FLS64],
+       [AC_MSG_CHECKING([whether fls64() is available])
+       SPL_LINUX_TRY_COMPILE([
+               #include <linux/bitops.h>
+       ],[
+               return fls64(0);
+       ],[
+               AC_MSG_RESULT(yes)
+               AC_DEFINE(HAVE_FLS64, 1, [fls64() is available])
+       ],[
+               AC_MSG_RESULT(no)
+       ])
+])
diff --git a/configure.ac b/configure.ac
index c4d424ea02..99d341a661 100644
--- a/configure.ac
+++ b/configure.ac
@@ -51,6 +51,7 @@ SPL_AC_SET_SHRINKER
 SPL_AC_PATH_IN_NAMEIDATA
 SPL_AC_TASK_CURR
 SPL_AC_CTL_UNNUMBERED
+SPL_AC_FLS64
 
 TOPDIR=`/bin/pwd`
 
diff --git a/include/linux/bitops_compat.c b/include/linux/bitops_compat.c
new file mode 100644
index 0000000000..8e1e25809a
--- /dev/null
+++ b/include/linux/bitops_compat.c
@@ -0,0 +1,19 @@
+#ifndef _SPL_BITOPS_COMPAT_H
+#define _SPL_BITOPS_COMPAT_H
+
+#include <linux/bitops.h>
+
+#ifndef HAVE_FLS64
+
+static inline int fls64(__u64 x)
+{
+       __u32 h = x >> 32;
+       if (h)
+               return fls(h) + 32;
+       return fls(x);
+}
+
+#endif /* HAVE_FLS64 */
+
+#endif /* _SPL_BITOPS_COMPAT_H */
+