Re: tmpfs, NFS, file handles

[Neil Brown]

On Wednesday February 20, [e-mail] wrote:
> Hi, 
> 
> > "Peter J. Braam" wrote:
> ...
> > > Is there a suggested solution for fh_to_dentry and dentry_to_fh for
> > > tmpfs?
> > > 
> > > An "iget" based solution might work but at present tmpfs inodes are
> > > not hashed.
> On Wed, Feb 20, 2002 at 11:56:40AM -0500, Jeff Garzik wrote:
> ...
> > I talked to neil brown about NFS and ramfs... he mentioned using
> > iunique()
... but Trond had a better idea....
> 
> 
> So do I understand that hashing tmpfs inodes is perhaps the way to go?
> 
> Would the following also work? 
> 
>  - have a 32 bit counter: set inode->i_ino to count++
>  - up the generation number each time the counter warps. 

You don't just need a number in inode->i_ino.  You also need to be
able to find an inode given that number.
So you need to store all the inodes in a hash table.
But you don't want to penalise non-NFS users.

I would probably:
   leave i_ino as set by new_inode
   initialise inode->i_generation to CURRENT_TIME

   in dentry_to_fh,
     check if list_empty(&inode->i_hash)
       if it is, then add the inode to some hash table indexed by the
           address of the inode
       put the address of the inode, i_ino and i_generation in the filehandle

   in fh_to_dentry,
     lookup the given address in the hash table.
     if it is found, check the i_ino and i_generation

That means you are only hashing inodes exported by NFS, and you have
a pretty good guarantee of uniqueness (providing time doesn't go
backwards).

NeilBrown