diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index baf41118660d6019b475eb27d58abe83e1aea0d4..7af2851d667c7ab0733ff177c7c1ca91d33bf85e 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -18,8 +18,10 @@ Usage
 -----
 
 If you have a block device which supports DAX, you can make a filesystem
-on it as usual.  When mounting it, use the -o dax option manually
-or add 'dax' to the options in /etc/fstab.
+on it as usual.  The DAX code currently only supports files with a block
+size equal to your kernel's PAGE_SIZE, so you may need to specify a block
+size when creating the filesystem.  When mounting it, use the "-o dax"
+option on the command line or add 'dax' to the options in /etc/fstab.
 
 
 Implementation Tips for Block Driver Writers