diff --git a/man2/fallocate.2 b/man2/fallocate.2
index 54d634096..3ed14601d 100644
--- a/man2/fallocate.2
+++ b/man2/fallocate.2
@@ -79,6 +79,16 @@ but the file size will not be changed even if
 is greater than the file size.
 Preallocating zeroed blocks beyond the end of the file in this manner
 is useful for optimizing append workloads.
+
+If the
+.B FALLOC_FL_UNSHARE
+flag is specified in
+.IR mode ,
+shared file data extents will be made private to the file to guarantee
+that a subsequent write will not fail due to lack of space.
+Typically, this will be done by performing a copy-on-write operation on
+all shared data in the file.
+This flag may not be supported by all filesystems.
 .PP
 Because allocation is done in block size chunks,
 .BR fallocate ()