From mboxrd@z Thu Jan  1 00:00:00 1970
From: hooanon05@yahoo.co.jp
Subject: Re: Fallthrus as full-length symlinks?
Date: Wed, 18 Nov 2009 14:47:15 +0900
Message-ID: <30797.1258523235@jrobl>
References: <20091113174631.GD19656@shell>
Cc: linux-fsdevel@vger.kernel.org,
	David Woodhouse <dwmw2@infradead.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jan Blunck <jblunck@suse.de>,
	Christoph Hellwig <hch@infradead.org>,
	Andy Whitcroft <apw@canonical.com>,
	Scott James Remnant <scott@canonical.com>,
	Sandu Popa Marius <sandupopamarius@gmail.com>,
	Jan Rekorajski <baggins@sith.mimuw.edu.pl>,
	Arnd Bergmann <arnd@arndb.de>,
	Vladimir Dronnikov <dronnikov@gmail.com>,
	Felix Fietkau <nbd@openwrt.org>
To: Valerie Aurora <vaurora@redhat.com>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from mtoichi12.ns.itscom.net ([219.110.2.182]:57878 "EHLO
	mtoichi12.ns.itscom.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751122AbZKRFsM (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Wed, 18 Nov 2009 00:48:12 -0500
In-Reply-To: <20091113174631.GD19656@shell>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>


Valerie Aurora:
> Fallthrus were invented as a placeholders for readdir() on a
> union-mounted directory - basically, to use the top-level file
> system's readdir() cookie mechanism.  Fallthrus are persistent
> directory entries and are implemented by the underlying file system -
> such as ext2 or tmpfs - in whatever way it sees fit.  We've
> implemented them for ext2 in two ways: as a regular directory entry
> with a magic inode number, and as a regular directory entry with a
> special file type.
> 
> Recently, David Woodhouse suggested implementing fallthrus as
> full-length symlinks with a special flag.  The interesting thing about
> this idea is that it could theoretically let us rename a file from the
> low level file system to another place in the low-level file system
> without copying the contents of the file up.  Basically, we can
> arbitrarily swizzle the namespace of the low-level by maintaining a
> set of symlinks above.
> 
> Is this useful?  Is it implementable?

I think the idea of fallthru entry is good, even if it is implemented as
a special symlink.
How do you think about the file paths in /proc/pid/maps and
/proc/pid/fd?
They refer the file paths, and some apps depend upon these path. I
remember that the package manager in debian didn't work when the path is
wrong. (But I don't know whether it is true still).

Will FS have to support such case?


J. R. Okajima