diff --git a/include/asm-um/processor-generic.h b/include/asm-um/processor-generic.h
index 4d94049..b953b1a 100644
--- a/include/asm-um/processor-generic.h
+++ b/include/asm-um/processor-generic.h
@@ -17,6 +17,10 @@
 struct mm_struct;
 
 struct thread_struct {
+	/* This flag is set to 1 before calling do_fork (and analyzed in
+	 * copy_thread) to mark that we are begin called from userspace (fork /
+	 * vfork / clone), and reset to 0 after. It is left to 0 when called
+	 * from kernelspace (i.e. kernel_thread() or fork_idle(), as of 2.6.11). */
 	int forking;
 	int nsyscalls;
 	struct pt_regs regs;