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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