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

["John T. Kohl" <jtk@us.ibm.com>]

>>>>> "Shaya" == Shaya Potter <spotter@cs.columbia.edu> writes:

>>>>>> > On Thu, Nov 10, 2005 at 11:32:22AM -0600, Dave Kleikamp wrote:
Dave>> The following patch allows stackable file systems, such as ClearCase's
Dave>> mvfs, to run atop nfs.  mvfs has it's own file and inode structures, but
Dave>> points its inode->i_mapping to the lower file system's mapping.  This
Dave>> causes problems when nfs's address space operations try to extract the
Dave>> open context from file->private_data.
Dave>> 
Dave>> The patch adds a small overhead of checking the file structure to see if
Dave>> it contains an inode that is not the mapping's host.
Dave>> 
Dave>> I am curious if there are any other stackable file systems that could
Dave>> benefit from this.

John>> Let me explain a bit more what's going on here.  MVFS would like to do
John>> the same thing that CODA does.  In the file->mmap() operation, CODA and
John>> MVFS want to set up paging operations to be handled by the backing store
John>> inode.  See for example fs/coda/file.c:coda_file_mmap(), it sets
John>> coda_inode-> i_mapping = host_inode->i_mapping.
John>> 
John>> But this fails when host_inode is an NFS inode.  NFS assumes
John>> that when it gets paging operations, it can look at the file pointer
John>> passed to the address_space_operations' readpage function, and that file
John>> pointer will be for an open NFS file.  If NFS is a backing store inode,
John>> the file pointer is for the stacked file system's open file.
John>> 
John>> CODA certainly won't work today with NFS host inodes and mapped files.
John>> I'm not surprised nobody noticed, since that seems like a poor way to
John>> use CODA.  Using NFS backing store is a primary use case for ClearCase
John>> MVFS, so we noticed.

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

Hmm, so thinking about this more, I think the minimal patch would only
change how the file structure is found for the address space operations,
and leave the other ones alone.  (And FYI, yes MVFS does have a (struct
file *) open on the backing store inode, and uses it for
non-mapping-related calls.)

-- 
John Kohl
Senior Software Engineer - Rational Software - IBM Software Group
Lexington, Massachusetts, USA
jtk@us.ibm.com
<http://www.ibm.com/software/rational/>