linux-kernel - Re: [RFC PATCH] file as directory

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Message-Id: <E1Hql7t-0001EQ-00@dorka.pomaz.szeredi.hu>
Date:	Wed, 23 May 2007 09:19:17 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	viro@....linux.org.uk
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH] file as directory

> > > Interesting...  How do you deal with mount propagation and things like
> > > mount --move?
> > 
> > Moving (or doing other mount operations on) an ancestor shouldn't be a
> > problem.  Moving this mount itself is not allowed, and neither is
> > doing bind or pivot_root.  Maybe bind could be allowed...
> 
> Eh...  Arbitrary limitations are fun, aren't they?

But these mounts _are_ special.  There is really no point in moving or
pivoting them.

> > When doing recursive bind on ancestor, these mounts are skipped.
> 
> What about clone copying your namespace?

In that case they are cloned, but only those survive which have refs
in the new namespace.

> What about MNT_SLAVE stuff being set up prior to that lookup?

These mounts are not propagated.  Or at least I hope so.  Propagation
stuff is a bit too complicated for my poor little brain.

> More interesting question: should independent lookups of that sucker
> on different paths end up with the same superblock (and vfsmount for
> each) or should we get fully independent mount on each?  The latter
> would be interesting wrt cache coherency...

I think they should be the same superblock, same dentry.  What would
be the advantage of doing otherwise?

> > > As for unlink...  How do you deal with having that thing
> > > mounted, mounting something _under_ it (so that vfsmount would be kept
> > > busy) and then unlinking that sucker?
> > 
> > Yeah, that's a good point.  Current patch doesn't deal with that.
> > Simplest solution could be to disallow submounting these.  Don't think
> > it makes much sense anyway.
> 
> Arbitrary limitations... (and that's where revalidate horrors come in, BTW).
> BTW^2: what if fs mounted that way will happen to have such node itself?

I think doing this recursively should be allowed.  "Releasing last ref
cleans up the mess" should work in that case.

> I'm not saying that it's unfeasible or won't lead to interesting things,
> but it really needs semantics done right...

Agreed :)

Miklos
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