Porting XFS to Linux (4) - Refinements

Direct use of Linux data structures
Passing down dentry
struct statfs vs statvfs

Duplicate code removal
Linux does _much_ more work in common code
About 2000 LOC gone

Use the generic I/O code
Early versions uses pagebuf-based I/O path
Now uses generic Linux routines
Delalloc was hard to fit into this model