diff --git a/Documentation/Subpage.rst b/Documentation/Subpage.rst
new file mode 100644
index 00000000..0aadf3c0
--- /dev/null
+++ b/Documentation/Subpage.rst
@@ -0,0 +1,11 @@
+Subpage support
+===============
+
+Subpage block size support, or just *subpage* for short, is a feature to allow
+using a filesystem that has different size of data block size (*sectorsize*)
+and the host CPU page size. For easier implementation the support was limited
+to the exactly same size of the block and page. On x86_64 this is typically
+4KiB, but there are other architectures commonly used that make use of larger
+pages, like 64KiB on 64bit ARM or PowerPC. A filesystem created on one cannot
+be mounted on the other.  The subpage support is still work in progress in 5.18
+but the support is incrementally added with each release.
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 01e563a4..fd430805 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -35,6 +35,7 @@ Welcome to BTRFS documentation!
    Scrub
    Seeding-device
    Send-receive
+   Subpage
    Subvolumes
    Swapfile
    Tree-checker