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

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Message-ID: <20100419142315.GA2688@shell>
Date:	Mon, 19 Apr 2010 10:23:15 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Jan Blunck <jblunck@...e.de>
Cc:	Jamie Lokier <jamie@...reable.org>,
	David Woodhouse <dwmw2@...radead.org>,
	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

On Mon, Apr 19, 2010 at 04:12:48PM +0200, Jan Blunck wrote:
> On Mon, Apr 19, Jamie Lokier wrote:
> 
> > 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?
> > 
> 
> Hmm, why not. Or even the ino of the directory we are reading from ...

I don't recall there being any technical reason not to look up the
real inode number.  I just wrote it that we because I was lazy.  So I
like returning the directory's d_ino better than a single magic
number, but I'd at least like to try returning the real inode number
too.

-VAL
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