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

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Message-Id: <E1HqkS4-00017K-00@dorka.pomaz.szeredi.hu>
Date:	Wed, 23 May 2007 08:36:04 +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

> > When a non-directory object is accessed without a trailing slash, then
> > path resolution returns the object itself as usual.
> > 
> > If a non-directory object is accessed with a trailing slash, then the
> > filesystem may opt to let the file be accessed as a directory.  In
> > this case "something" (as supplied by the filesystem) is mounted on
> > top of the non-directory object.
> > 
> > This mount will have special properties:
> > 
> >  - If there's no trailing slash is after the file name, the mount
> >    won't be followed, even if the path resolution would otherwise
> >    follow mounts.
> > 
> >  - The mount only stays there while it is referenced by some external
> >    object, like a pwd or an open file.  When it is no longer
> >    referenced, it is automatically unmounted.
> > 
> >  - Unlike "real" mounts, this won't block unlink(2) or rename(2) on
> >    the underlying object.
> 
> 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...

When doing recursive bind on ancestor, these mounts are skipped.

> 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.

> I'll look through the patch tonight; it sounds interesting, assuming that
> we don't run into serious crap with locking and <shudder> revalidation
> logics.

Revalidation shouln't be a problem.  We'll just end up with an
unhashed dentry with a mount over it, which will be detached when the
vfsmount ref is dropped.

Miklos
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