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

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Message-ID: <20130315121220.GY21522@ZenIV.linux.org.uk>
Date:	Fri, 15 Mar 2013 12:12:20 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
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, Mar 15, 2013 at 08:15:18AM +0000, James Bottomley wrote:

> 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.

The trouble with such mechanisms is that they tend to end up depending on
fairly non-trivial properties of underlying fs.  Try aufs one on btrfs,
see how soon you spot the problem.  It's nice when a method turns out
to be really redundant and implementable in uniform way via other methods
present; see fh_to_dentry history for example of situation where it hadn't...
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