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

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Message-ID: <20100419141248.GK10776@bolzano.suse.de>
Date:	Mon, 19 Apr 2010 16:12:48 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	Jamie Lokier <jamie@...reable.org>
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

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 ...

Regards,
	Jan

-- 
Jan Blunck <jblunck@...e.de>
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