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

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Message-ID: <20070522221045.GH4095@ftp.linux.org.uk>
Date:	Tue, 22 May 2007 23:10:46 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
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

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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