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

[Christoph Hellwig <hch@infradead.org>]

On Thu, Nov 10, 2005 at 04:35:47PM -0500, John T. Kohl wrote:
> 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.

so coda is broken aswell, news at 11 ;-)  when a file operation takes
a stuct file, directly or indirecly you absolutely must pass down
a stuct file of that filesystem.  so wrap your address operations and
pass down the proper nfs file stuct.