Re: [PATCH] VFS: new fgetattr() file operation

[Miklos Szeredi <miklos@szeredi.hu>]

> Miklos Szeredi wrote:
> > I don't think Christoph will like the patch better, regardless of how
> > I change the description.
> >
> > Of course, I'm open to suggestion on how to improve the interface, but
> > I think fundamentally this is the only way to correctly deal with the
> > below problem.
> >
> > Anyway, here's the patch another time, please consider adding it to
> > -mm.  For 2.6.25 obviously.
> >
> > Thanks,
> > Miklos
> > ----
> >
> > From: Miklos Szeredi <mszeredi@suse.cz>
> >
> > Add a new file operation: f_op->fgetattr(), that is invoked by
> > fstat().  Fall back to i_op->getattr() if it is not defined.
> >
> > We need this because fstat() semantics can in some cases be better
> > implemented if the filesystem has the open file available.
> >
> > Let's take the following example: we have a network filesystem, with
> > the server implemented as an unprivileged userspace process running on
> > a UNIX system (this is basically what sshfs does).
> >
> > We want the filesystem to follow the familiar UNIX file semantics as
> > closely as possible.  If for example we have this sequence of events,
> > we still would like fstat to work correctly:
> >
> >  1) file X is opened on client
> >  2) file X is renamed to Y on server
> >  3) fstat() is performed on open file descriptor on client
> >
> > This is only possible if the filesystem server acutally uses fstat()
> > on a file descriptor obtained when the file was opened.  Which means,
> > the filesystem client needs a way to get this information from the
> > VFS.
> >
> >   
> 
> This true iff the protocol that this mythical

Not mythical at all.  As noted in the description, there's sshfs, a
live and quite popular example of this sort of filesystem.

> network file system uses the name of the file on the server to
> actually identify the file on the server.

The constraint is that the server has to be an ordinary unprivileged
process.  How should it identify the file, other than by name, or by
an open file descriptor?

> Clearly, this is broken on many levels.  It can't handle
> situations as described nor can it handle different instances
> of the same filename being used.

Can you please give concrete examples what it can't handle, and how
should the implementation be improved to be able to handle it, given
the above constraints?

> This is why NFS, a network file system, does not use the filename
> as part of the file handle.

And the nfs server isn't a userspace process, or if it is, it must use
horrible hacks to convert the file handle to a name, that don't work
half the time.

> Wouldn't you be better off by attempting to implement an "open
> by ino" operation and an operation to get the generation count
> for the file and then modifying the network protocol of interest
> to use these as identifiers for the file to be manipulated?

You mean an "open by inode" on the userspace API?  My guess, it
wouldn't get very far.

Anyway, that would still not work on old servers, and servers running
other OS's.

Note, the point is _not_ to make a brand new NFS replacement
filesystem, that can use names instead of file handles.  The point is
to use existing infrastructure, to make the setup as easy as ssh'ing
to a different machine.  And sshfs does just that.

Miklos
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