linux-kernel - Re: [PATCH 21/32] union-mount: Make lookup work for union-mounted file systems

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Date:	Tue, 19 May 2009 13:30:38 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	jblunck@...e.de, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	bharata@...ibm.com, dwmw2@...radead.org, mszeredi@...e.cz
Subject: Re: [PATCH 21/32] union-mount: Make lookup work for union-mounted file systems

On Tue, May 19, 2009 at 06:15:52PM +0200, Miklos Szeredi wrote:
> On Mon, 18 May 2009, Jan Blunck wrote:
> > On union-mounted file systems the lookup function must also visit lower layers
> > of the union-stack when doing a lookup. This patches add support for
> > union-mounts to cached lookups and real lookups.
> > 
> > We have 3 different styles of lookup functions now:
> > - multiple pathname components, follow mounts, follow union, follow symlinks
> > - single pathname component, doesn't follow mounts, follow union, doesn't
> >   follow symlinks
> > - single pathname component doesn't follow mounts, doesn't follow unions,
> >   doesn't follow symlinks
> 
> Ugh...  I do wonder if this could be done in a less complicated way,
> there does seem to be a fair amount of duplication between these
> functions.

Yeah, I agree.  My best idea so far is not very good - have one
skeleton function and pass in function pointers for the
lookup_topmost() and build_union() functions.  Do you have any ideas?

> Worse, it looks like there are still i_mutex lock ordering issues
> (__hash_lookup_topmost()/__hash_lookup_build_union()).  What happens
> if two separate unions of two filesystems are built where the order of
> branches is reversed?

We have a similar problem in union_copyup_dir().  Hm, thinking about
this, only one of the file systems can actually change while we are
doing work.  That might help us get out of the lock ordering problems.
Thoughts?

-VAL
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