From WhitixDoc

The Whitix virtual filesystem abstracts the implementation of each filesystem (including FAT, ext3, reiserfs and isofs) and presents a consistent view of the filesystem to userspace and the rest of the kernel. Each filesystem driver must expose a particular interface to the virtual filesystem through a structure of function pointers. ...

Where to find the code

All of the virtual filesystem code can be found in fs/vfs, with the implementation of each filesystem type in fs/{type}.