linux-kernel - Re: [PATCH 17/38] union-mount: Union mounts documentation

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Message-Id: <E1OQgpf-0001DJ-3w@pomaz-ex.szeredi.hu>
Date:	Mon, 21 Jun 2010 15:14:35 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	raa.lkml@...il.com, viro@...iv.linux.org.uk, miklos@...redi.hu,
	jblunck@...e.de, hch@...radead.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 17/38] union-mount: Union mounts documentation

On Thu, 17 Jun 2010, Valerie Aurora wrote:
> On Thu, Jun 17, 2010 at 10:01:30AM +0200, Alex Riesen wrote:
> > On Tue, Jun 15, 2010 at 20:39, Valerie Aurora <vaurora@...hat.com> wrote:
> > > +git://git.kernel.org/pub/scm/fs/ext2/val/e2fsprogs.git
> > > +
> > > +Currently, whiteout directory entries are not returned to userland.
> > > +While the directory type for whiteouts, DT_WHT, has been defined for
> > > +many years, very little userland code handles them. ??Userland will
> > > +never see fallthru directory entries.
> > 
> > This may be a dumb question (I must admit I did only very little research),
> > but how does one cleanup the topmost layer of whiteouts and fallthroughs,
> > so that the entries of lower layer(s) can be made visible again?
> 
> I'm not sure how best to do this.  We don't want to add more system
> calls.  One thought of mine has been to do this offline, when the file
> system is unmounted.  For example, e2fsck could add a feature to
> delete whiteouts and fallthrus.  Another option is to add a flag to an
> existing system call.

One more advantage of doing whiteouts, etc. with hard links and
extended attributes instead of as special filesystem objects.  That
way they are visible (unless part of a union) and can be treated as
normal filesystem objects.

Miklos
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