Re: Fallthrus as full-length symlinks?

[Valerie Aurora <vaurora@redhat.com>]

On Fri, Nov 13, 2009 at 01:46:15PM -0500, Erez Zadok wrote:
> In message <20091113174631.GD19656@shell>, Valerie Aurora writes:
> > 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.
> 
> Other than a possible improvement to ->rename, what's wrong with the idea of
> a special dirent flag?  I kinda liked that idea: it's simple and requires
> only a small amount of change to lower file systems.  Any idea in which you
> have to record the whiteouts using an actual file or inode is more
> cumbersome.

You have it right - the major advantage is a possible simplification
to rename().

> > Recently, David Woodhouse suggested implementing fallthrus as
> > full-length symlinks with a special flag.
> 
> Where does this "special flag" go?  Is it persistent?  Is it new?  Would
> that mean having to change lower file systems to teach them about this flag?
> 
> Is there a way of doing it w/o having to change lower f/s code at all?
> That'll be a major advantage if possible.

I can't think of a way to do it without using up namespace - but
perhaps there is some part of the symlink target namespace that has no
valid meaning that we could use instead, like Arnd's self symlink.

-VAL