linux-kernel - Re: [PATCH 20/41] union-mount: Introduce union

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Date:	Tue, 26 Jan 2010 17:38:05 -0500
From:	Valerie Aurora <vaurora@...hat.com>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	Jan Blunck <jblunck@...e.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Andy Whitcroft <apw@...onical.com>,
	Scott James Remnant <scott@...onical.com>,
	Sandu Popa Marius <sandupopamarius@...il.com>,
	Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Arnd Bergmann <arnd@...db.de>,
	Vladimir Dronnikov <dronnikov@...il.com>,
	Felix Fietkau <nbd@...nwrt.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 20/41] union-mount: Introduce union_mount structure

On Mon, Nov 30, 2009 at 03:46:29AM -0500, Erez Zadok wrote:
> In message <1256152779-10054-21-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> > From: Jan Blunck <jblunck@...e.de>
> > 
> > This patch adds the basic structures of VFS based union mounts. It is a new
> > implementation based on some of my old ideas that influenced Bharata B Rao
> > <bharata@...ux.vnet.ibm.com> who came up with the proposal to let the
> > union_mount struct only point to the next layer in the union stack. I rewrote
> > nearly all of the central patches around lookup and the dcache interaction.
> > 
> > Advantages of the new implementation:
> > - the new union stack is no longer tied directly to one dentry
> > - the union stack enables dentries to be part of more than one union
> >   (bind mounts)
> > - it is unnecessary to traverse the union stack when de/referencing a dentry
> > - caching of union stack information still driven by dentry cache
> > 
> > XXX - is_unionized() is pretty heavy-weight for non-union file systems
> > on a union mount-enabled kernel.  May be simplified by assuming one or
> > more of:
> > 
> > - Two layers only
> > - One-to-one association between layers (doesn't union submounts)
> > - Writable layer mounted in only one place
> 
> Yes, is_unionized() does appear to be heavy.  Is it correct to assume that
> every such dentry will have gotten looked up or traversed as part of a
> union?  If so, can we just set a flag in the dentry to mark it as
> D_THIS_IS_PART_OF_A_UNION?  Even if you could, what happens when a union r-w
> layer is removed: could there be leftover dentries marked as part of a
> union, which are no longer really part of it?

dentries aren't themselves part of a unioned file system - they are
shared among all the mounts of a superblock/device. A dentry is
unioned or not only in the context of a particular mount.  So we can't
mark the dentry itself since it will be unioned in one mount (say,
/mnt/union) and not in another (/mnt/ro).  A dentry can be looked up
in another mount of one of the components of the union before the
union mount is created, so we can't mark a dentry at lookup anyway.

The vfsmount for the top layer <dentry,mnt> pair is marked with
MNT_UNION, but the vfsmount for the bottom layer is not.  I think we
could mark the lower vfsmount with a flag (probably not MNT_UNION but
MNT_UNION_LOWER or something like that) and check for that.  Al?  Jan?
Christoph?

-VAL
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