Re: [RFC] Support for stackable file systems on top of nfs

[Shaya Potter <spotter@cs.columbia.edu>]

On Thu, 2005-11-10 at 16:35 -0500, John T. Kohl wrote:
> > On Thu, Nov 10, 2005 at 11:32:22AM -0600, Dave Kleikamp wrote:
> >> The following patch allows stackable file systems, such as ClearCase's
> >> mvfs, to run atop nfs.  mvfs has it's own file and inode structures, but
> >> points its inode->i_mapping to the lower file system's mapping.  This
> >> causes problems when nfs's address space operations try to extract the
> >> open context from file->private_data.
> >> 
> >> The patch adds a small overhead of checking the file structure to see if
> >> it contains an inode that is not the mapping's host.
> >> 
> >> I am curious if there are any other stackable file systems that could
> >> benefit from this.
> 
> Let me explain a bit more what's going on here.  MVFS would like to do
> the same thing that CODA does.  In the file->mmap() operation, CODA and
> MVFS want to set up paging operations to be handled by the backing store
> inode.  See for example fs/coda/file.c:coda_file_mmap(), it sets
> coda_inode->i_mapping = host_inode->i_mapping.
> 
> But this fails when host_inode is an NFS inode.  NFS assumes
> that when it gets paging operations, it can look at the file pointer
> passed to the address_space_operations' readpage function, and that file
> pointer will be for an open NFS file.  If NFS is a backing store inode,
> the file pointer is for the stacked file system's open file.
> 
> CODA certainly won't work today with NFS host inodes and mapped files.
> I'm not surprised nobody noticed, since that seems like a poor way to
> use CODA.  Using NFS backing store is a primary use case for ClearCase
> MVFS, so we noticed.

I think you'd notice it on other file systems as well.  For instance, my
experience is that GFS doesn't play nice w/ stackable file systems that
try to stack on the a_ops.  On the other hand, it's ok if it just passes
all page cache operations directly down to the lower file system.
OCFS2, on the other hand, seems to play better w/ stacking on the a_ops.