Re: Fallthrus as full-length symlinks?

[Valerie Aurora <vaurora@redhat.com>]

On Wed, Nov 18, 2009 at 02:47:15PM +0900, hooanon05@yahoo.co.jp wrote:
> 
> Valerie Aurora:
> > Fallthrus were invented as a placeholders for readdir() on a
> > union-mounted directory - basically, to use the top-level file
> > system's readdir() cookie mechanism.  Fallthrus are persistent
> > directory entries and are implemented by the underlying file system -
> > such as ext2 or tmpfs - in whatever way it sees fit.  We've
> > implemented them for ext2 in two ways: as a regular directory entry
> > with a magic inode number, and as a regular directory entry with a
> > special file type.
> > 
> > Recently, David Woodhouse suggested implementing fallthrus as
> > full-length symlinks with a special flag.  The interesting thing about
> > this idea is that it could theoretically let us rename a file from the
> > low level file system to another place in the low-level file system
> > without copying the contents of the file up.  Basically, we can
> > arbitrarily swizzle the namespace of the low-level by maintaining a
> > set of symlinks above.
> > 
> > Is this useful?  Is it implementable?
> 
> I think the idea of fallthru entry is good, even if it is implemented as
> a special symlink.
> How do you think about the file paths in /proc/pid/maps and
> /proc/pid/fd?
> They refer the file paths, and some apps depend upon these path. I
> remember that the package manager in debian didn't work when the path is
> wrong. (But I don't know whether it is true still).
> 
> Will FS have to support such case?

My current thinking is that an overlay shouldn't be mounted over /proc
or other special file systems.  I think that a writable layer should
only be mounted over exactly one file system, no submounts allowed.

-VAL