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

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

On Thu, 2005-11-10 at 17:18 -0500, Trond Myklebust wrote:
> On Thu, 2005-11-10 at 16:36 -0500, Shaya Potter wrote:
> > On Thu, 2005-11-10 at 16:24 -0500, Trond Myklebust wrote:
> > > On Thu, 2005-11-10 at 11:32 -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.
> > > 
> > > NACK
> > > 
> > > This is a fundamentally flawed approach. The nfs_find_open_context() is
> > > designed for the mmap() case were you have a valid vm_area_struct, that
> > > has a "struct file" with a valid NFS open context attached to it.
> > > Existence of the file is guaranteed, but the readpage() and writepage()
> > > interfaces don't actually pass the struct file down to the filesystem.
> > > 
> > > This is clearly not the case here.
> > > 
> > > If you want to make mvfs work correctly with NFS, then have it set up a
> > > valid NFS struct file, and use that file with the NFS functions.
> > > Anything else is borken.
> > 
> > from my experiences with stackable file systems, I'm not sure the above
> > is totally correct.  i.e. vm_area_structs only around while a process is
> > in use.  Once a process exits, the file and vm_area_struct go away.
> 
> ...calling file->f_ops->release() in the process. Guess what happens
> there?

from my experience, writepage is called signficantly after a process is
gone.  I've seen this via instrumenting writepage() in using stackable
file systems and a simple program that maps a file, modifies it, and
then exits.  the kernel hit the writepage() function 30 seconds or so
after the process existed.

ext2_release_file just does a ext2_discard_prealloc, which has nothing
to do with written data.

In looking at nfs, nfs_file_release does write out the data by calling
filemap_fdatawrite(), but it seems somewhat unique in that regard, but I
guess we are talking about NFS here. :)