Implementation Proposal for Subfile GSoC

[Adam Burkepile <sadpoti%gmail.com@localhost>]

I'll be implementing subfile support and wanted to get some feedback  
on what the community thinks of the additions/design decisions my  
mentor and I have been thinking of making. Any questions and/or  
comments are welcome.


Changes that need to be made to the file system
          o Add a flag to the file system superblock to show if  
subfiles are supported on the filesystem
          o Add two new inode types ( IFSFDIR and IFSFREG )

An IFSFDIR:
* is referenced by exactly one base inode, add a back pointer to the  
inode struct
* contents are same as normal directory, struct direct.
* contains a “.” entry that references the IFSFDIR.
* contains a “..” entry that references the base inode
* entries may only reference IFSFREG there may be no subdirectories or  
device nodes
* is instantiated on demand
* is removed when the base inode is removed
* does not have independent access control (uid, gid, mode), refer the  
the base inode

An IFSFREG:
* is just like an IFREG
* could (should?) contain a back pointer to the IFSFDIR
* can only have a link count of 1
* can be deleted explicitly (see subfile_remove(), below)
* is deleted when the base inode is deleted
* does not have independent access control (uid, gid, mode), refer the  
the base inode

When a base inode is deleted, the subfile tree has to be deleted also.  
Changes to remove() would be needed.


OS Level - System Calls
    * o int subfile_fdopen (int basefilefd, char * subfilename, int  
open_flags);
                + subfile_fdopen acts much like regular fdopen(2)  
except it is passed a basefile fd to open then opens the subfile of  
the basefile.
       o int subfile_open (char * basefilename, char * subfilename,  
int open_flags);
                + subfile_fdopen is the same as subfile_open except  
it uses a file descriptor to reference the basefile.

Both of these open functions would return a normal file descriptor to  
the subfile upon a successful open, a fd that can be used with normal  
read, write, close, etc.

   *     o opendir -modified to open the sub_file folder of a file if  
the file’s inode contains a sub_file folder reference
          o int subfile_stat (char * basefilename, char *  
subfilename, struct stat * sb);
          o int subfile_fdstat (int basefilefd, char * subfilename,  
struct stat * sb);
          o int subfile_remove (char * basefilename, char *  
subfilename);
          o int subfile_fdremove (int basefilefd, char * subfilename);


Adam Burkepile
sadpoti%gmail.com@localhost