linux-ext4 - Re: [PATCH 13/35] fallthru: ext2 fallthru support

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Message-ID: <20100419133028.GA3631@shareable.org>
Date:	Mon, 19 Apr 2010 14:30:28 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Jan Blunck <jblunck@...e.de>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	Valerie Aurora <vaurora@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 13/35] fallthru: ext2 fallthru support

Jan Blunck wrote:
> On Mon, Apr 19, David Woodhouse wrote:
> 
> > On Mon, 2010-04-19 at 14:40 +0200, Jan Blunck wrote:
> > > On Thu, Apr 15, Valerie Aurora wrote:
> > > 
> > > > Add support for fallthru directory entries to ext2.
> > > > 
> > > > XXX - Makes up inode number for fallthru entry
> > > > XXX - Might be better implemented as special symlinks
> > > 
> > > Better not. David Woodhouse actually convinced me of moving away from the
> > > special symlink approach. The whiteouts have been implemented as special
> > > symlinks before.
> > 
> > I certainly asked whether you really need a real 'struct inode' for
> > whiteouts, and suggested that they should be represented _purely_ as a
> > dentry with type DT_WHT.
> > 
> > I don't much like the manifestation of that in this patch though,
> > especially with the made-up inode number. (ISTR I had other
> > jffs2-specific objections too, which I'll dig out and forward).
> 
> Yes, this patches still have issues that Val and me are aware off. I can't
> remember anything jffs2-specific though.
> 
> We return that inode number because we don't want to lookup the name on the
> other filesystem during readdir. Therefore returning DT_UNKNOWN to let the
> userspace decide if it needs to stat the file was the easiest workaround. I
> know that POSIX requires d_ino and d_name but on the other hand it does not
> require anything more on how long d_ino is valid.

Although the lifetime of d_ino might very, I know some programs (not
public) that will break if they see a d_ino which is wrongly matching
the st_ino of another file somewhere on the same st_dev.  They will
assume the name is a hard link to the other file, without calling
stat(), which I think is a reasonable assumption and a useful optimisation.

So the made-up d_ino should at least be careful to not match an inode
number of another file which has a stable st_ino.

Why not zero for d_ino?

-- Jamie

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