linux-kernel - Re: [PATCH 19/38] union-mount: Introduce union

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Message-Id: <E1OgfJb-0000n9-KZ@pomaz-ex.szeredi.hu>
Date:	Wed, 04 Aug 2010 16:51:31 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	viro@...iv.linux.org.uk, miklos@...redi.hu, jblunck@...e.de,
	hch@...radead.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, vaurora@...hat.com
Subject: Re: [PATCH 19/38] union-mount: Introduce union_dir structure and basic operations

On Tue, 15 Jun 2010, Valerie Aurora wrote:
> This patch adds the basic structures and operations of VFS-based union
> mounts (but not the ability to mount or lookup unioned file systems).
> Each directory in a unioned file system has an associated union stack
> created when the directory is first looked up.  The union stack is a
> union_dir structure kept in a hash table indexed by mount and dentry
> of the directory; thus, specific paths are unioned, not dentries
> alone.  The union_dir keeps a pointer to the upper path and the lower
> path and can be looked up by either path.  Currently only two layers
> are supported, but the union_dir struct is flexible enough to allow
> more than two layers.
> 
> This particular version of union mounts is based on ideas by Jan
> Blunck, Bharata Rao, and many others.
> 
> Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> ---


> --- a/include/linux/dcache.h
> +++ b/include/linux/dcache.h
> @@ -100,7 +100,9 @@ struct dentry {
>  	struct hlist_node d_hash;	/* lookup hash list */
>  	struct dentry *d_parent;	/* parent directory */
>  	struct qstr d_name;
> -
> +#ifdef CONFIG_UNION_MOUNT
> +	struct union_dir *d_union_dir;	/* head of union stack */
> +#endif

This botches the carefully tuned length of struct dentry.  At least a
FIXME comment needs to be added that this is something to be
addressed.

Why was the hash table concept dropped?  The header comment still
talks about that?

Miklos
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