linux-kernel - Re: [PATCH 0/9] overlay filesystem: request for inclusion (v17)

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Message-ID: <1363335318.2459.4.camel@dabdike>
Date:	Fri, 15 Mar 2013 08:15:18 +0000
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Miklos Szeredi <miklos@...redi.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org, apw@...onical.com,
	nbd@...nwrt.org, neilb@...e.de, jordipujolp@...il.com,
	ezk@....cs.sunysb.edu, dhowells@...hat.com,
	sedat.dilek@...glemail.com, mszeredi@...e.cz
Subject: Re: [PATCH 0/9] overlay filesystem: request for inclusion (v17)

On Fri, 2013-03-15 at 05:13 +0000, Al Viro wrote:
> On Fri, Mar 15, 2013 at 02:09:14PM +0900, J. R. Okajima wrote:
> 
> > If so, it has a big disadvantage for the layer-fs (or branch-fs) to have
> > to implement a new method for whiteout.
> > 
> > Overlayfs implements whiteout as symlink+xattr which consumes an
> > inode. And you don't like it, right?
> > What I showed is another generic approach without xattr where the new
> > method to whiteout is unnecessary.
> 
> I'm yet to see the reason that would make implementing that method a big
> disadvantage, TBH...

It's the fact that a directory entry based whiteout limits the amount of
change to the VFS, but has to be supported by underlying filesystems.
The generic_dirent_fallthrough() mechanism is a nice way of hiding it,
but there are still quite a few fs specific mods in the union mount tree
because of this.  Having to modify filesystems to me indicates the
mechanism is a bit fragile.  If we could do whiteouts purely in the VFS,
so it would work for any filesystem (without needing filesystem
modifications) that would seem to be a more robust approach.  I'm not
saying we can definitely do this in an elegant way ... I'm just saying
that if someone comes up with it, it's obviously preferable.

James

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