Re: [RFC PATCH] file as directory
- From: Al Viro <viro@xxxxxxxxxxxxxxxx>
- Date: Tue, 22 May 2007 23:10:46 +0100
On Tue, May 22, 2007 at 08:48:49PM +0200, Miklos Szeredi wrote:
Why do we want this?
--------------------
That depends on who you ask. My answer is this:
'foo.tar.gz/foo/bar' or
'foo.tar.gz/contents/foo/bar'
or something similar.
Others might suggest accessing streams, resource forks or extended
attributes through such an interface. However this patch only deals
with the non-directory case, so directories would be excluded from
that interface.
But otherwise this patch doesn't limit the uses of the "file as
directory" concept in any way. It just adds the infrastructure to
support these whacky beasts.
How is it done?
---------------
(See this [1] thread for more discussion on the subject)
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? 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?
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.
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