Date:	Tue, 13 Jul 2010 12:39:46 +0800
From:	Ian Kent <raven@...maw.net>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	Jan Blunck <jblunck@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 19/38] union-mount: Introduce union_dir structure and
 basic operations

On Tue, Jun 15, 2010 at 11:39:49AM -0700, 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>
> ---
>  fs/Kconfig             |   13 +++++
>  fs/Makefile            |    1 +
>  fs/dcache.c            |    3 +
>  fs/union.c             |  119 ++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/union.h             |   66 ++++++++++++++++++++++++++
>  include/linux/dcache.h |    4 +-
>  include/linux/fs.h     |    1 +
>  7 files changed, 206 insertions(+), 1 deletions(-)
>  create mode 100644 fs/union.c
>  create mode 100644 fs/union.h
> 
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 5f85b59..f99c3a9 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -59,6 +59,19 @@ source "fs/notify/Kconfig"
>  
>  source "fs/quota/Kconfig"
>  
> +config UNION_MOUNT
> +       bool "Union mounts (writable overlasy) (EXPERIMENTAL)"

Spelling of overlay

> +       depends on EXPERIMENTAL
> +       help
> +         Union mounts allow you to mount a transparent writable
> +	 layer over a read-only file system, for example, an ext3
> +	 partition on a hard drive over a CD-ROM root file system
> +	 image.
> +
> +	 See <file:Documentation/filesystems/union-mounts.txt> for details.
> +
> +	 If unsure, say N.
> +
>  source "fs/autofs/Kconfig"
>  source "fs/autofs4/Kconfig"
>  source "fs/fuse/Kconfig"
> diff --git a/fs/Makefile b/fs/Makefile
> index 97f340f..1949af2 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -52,6 +52,7 @@ obj-$(CONFIG_NFS_COMMON)	+= nfs_common/
>  obj-$(CONFIG_GENERIC_ACL)	+= generic_acl.o
>  
>  obj-y				+= quota/
> +obj-$(CONFIG_UNION_MOUNT)	+= union.o
>  
>  obj-$(CONFIG_PROC_FS)		+= proc/
>  obj-y				+= partitions/
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 1575af4..54ff5a3 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -960,6 +960,9 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
>  	INIT_LIST_HEAD(&dentry->d_lru);
>  	INIT_LIST_HEAD(&dentry->d_subdirs);
>  	INIT_LIST_HEAD(&dentry->d_alias);
> +#ifdef CONFIG_UNION_MOUNT
> +	dentry->d_union_dir = NULL;
> +#endif
>  
>  	if (parent) {
>  		dentry->d_parent = dget(parent);
> diff --git a/fs/union.c b/fs/union.c
> new file mode 100644
> index 0000000..02abb7c
> --- /dev/null
> +++ b/fs/union.c
> @@ -0,0 +1,119 @@
> + /*
> + * VFS-based union mounts for Linux
> + *
> + * Copyright (C) 2004-2007 IBM Corporation, IBM Deutschland Entwicklung GmbH.
> + * Copyright (C) 2007-2009 Novell Inc.
> + * Copyright (C) 2009-2010 Red Hat, Inc.
> + *
> + *   Author(s): Jan Blunck (j.blunck@...harburg.de)
> + *              Valerie Aurora <vaurora@...hat.com>
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; version 2
> + * of the License.
> + */
> +
> +#include <linux/bootmem.h>
> +#include <linux/init.h>
> +#include <linux/types.h>
> +#include <linux/fs.h>
> +#include <linux/mount.h>
> +#include <linux/fs_struct.h>
> +#include <linux/slab.h>
> +
> +#include "union.h"
> +
> +static struct kmem_cache *union_cache;
> +
> +static int __init init_union(void)
> +{
> +	union_cache = KMEM_CACHE(union_dir, SLAB_PANIC | SLAB_MEM_SPREAD);

I know there are other places where we just assume KMEM_CACHE() returns
non-null but is that really the right thing to do?

> +	return 0;
> +}
> +
> +fs_initcall(init_union);
> +
> +/**
> + * union_alloc - allocate a union_dir
> + *
> + * @path: path of directory underneath another directory
> + *
> + * Allocate a union_dir for this directory.  We only allocate
> + * union_dirs for the second and lower layers - the read-only layers.
> + * Top-level dentries don't have a union_dir, just a pointer to the
> + * union_dir of the directory in the layer below it.  u_lower is
> + * initialized to NULL by default.  If there is another layer below
> + * this and a matching directory in the layer, then we allocate a
> + * union_dir for it and then set u_lower of the above union_dir to
> + * point to it.
> + */
> +
> +static struct union_dir *union_alloc(struct path *path)
> +{
> +	struct union_dir *ud;
> +
> +	BUG_ON(!S_ISDIR(path->dentry->d_inode->i_mode));
> +
> +	ud = kmem_cache_alloc(union_cache, GFP_ATOMIC);
> +	if (!ud)
> +		return NULL;
> +
> +	ud->u_this = *path;
> +	ud->u_lower = NULL;
> +
> +	return ud;
> +}
> +
> +static void union_put(struct union_dir *ud)
> +{
> +	path_put(&ud->u_this);
> +	kmem_cache_free(union_cache, ud);
> +}
> +
> +/**
> + * union_add_dir - Add another layer to a unioned directory
> + *
> + * @upper - directory in the previous layer
> + * @lower - directory in the current layer
> + * @next_ud - location of pointer to this union_dir
> + *
> + * Must have a reference (i.e., call path_get()) to @lower before
> + * passing to this function.
> + */
> +
> +int union_add_dir(struct path *upper, struct path *lower,
> +		  struct union_dir **next_ud)
> +{
> +	struct union_dir *ud;
> +
> +	BUG_ON(*next_ud != NULL);
> +
> +	ud = union_alloc(lower);
> +	if (!ud)
> +		return -ENOMEM;
> +	*next_ud = ud;
> +
> +	return 0;
> +}
> +
> +/**
> + * d_free_unions - free all unions for this dentry
> + *
> + * @dentry - topmost dentry in the union stack to remove
> + *
> + * This must be called when freeing a dentry.
> + */
> +void d_free_unions(struct dentry *dentry)
> +{
> +	struct union_dir *this, *next;
> +
> +	this = dentry->d_union_dir;
> +
> +	while (this != NULL) {
> +		next = this->u_lower;
> +		union_put(this);
> +		this = next;
> +	}
> +	dentry->d_union_dir = NULL;
> +}
> diff --git a/fs/union.h b/fs/union.h
> new file mode 100644
> index 0000000..04efc1f
> --- /dev/null
> +++ b/fs/union.h
> @@ -0,0 +1,66 @@
> + /*
> + * VFS-based union mounts for Linux
> + *
> + * Copyright (C) 2004-2007 IBM Corporation, IBM Deutschland Entwicklung GmbH.
> + * Copyright (C) 2007-2009 Novell Inc.
> + * Copyright (C) 2009-2010 Red Hat, Inc.
> + *
> + *   Author(s): Jan Blunck (j.blunck@...harburg.de)
> + *              Valerie Aurora <vaurora@...hat.com>
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; version 2
> + * of the License.
> + */
> +#ifndef __LINUX_UNION_H
> +#define __LINUX_UNION_H
> +#ifdef __KERNEL__
> +
> +#ifdef CONFIG_UNION_MOUNT
> +
> +/*
> + * WARNING! Confusing terminology alert.
> + *
> + * Note that the directions "up" and "down" in union mounts are the
> + * opposite of "up" and "down" in normal VFS operation terminology.
> + * "up" in the rest of the VFS means "towards the root of the mount
> + * tree."  If you mount B on top of A, following B "up" will get you
> + * A.  In union mounts, "up" means "towards the most recently mounted
> + * layer of the union stack."  If you union mount B on top of A,
> + * following A "up" will get you to B.  Another way to put it is that
> + * "up" in the VFS means going from this mount towards the direction
> + * of its mnt->mnt_parent pointer, but "up" in union mounts means
> + * going in the opposite direction (until you run out of union
> + * layers).
> + */
> +
> +/*
> + * The union_dir structure.  Basically just a singly-linked list with
> + * a pointer to the referenced dentry, whose head is d_union_dir in
> + * the dentry of the topmost directory.  We can't link this list
> + * purely through list elements in the dentry because lower layer
> + * dentries can be part of multiple union stacks.  However, the
> + * topmost dentry is only part of one union stack.  So we point at the
> + * lower layer dentries through a linked list rooted in the topmost
> + * dentry.
> + */
> +struct union_dir {
> +	struct path u_this;		/* this is me */
> +	struct union_dir *u_lower;	/* this is what I overlay */
> +};
> +
> +#define IS_MNT_UNION(mnt)	((mnt)->mnt_flags & MNT_UNION)
> +
> +extern int union_add_dir(struct path *, struct path *, struct union_dir **);
> +extern void d_free_unions(struct dentry *);
> +
> +#else /* CONFIG_UNION_MOUNT */
> +
> +#define IS_MNT_UNION(x)			(0)
> +#define union_add_dir(x, y, z)		({ BUG(); (NULL); })
> +#define d_free_unions(x)		do { } while (0)
> +
> +#endif	/* CONFIG_UNION_MOUNT */
> +#endif	/* __KERNEL__ */
> +#endif	/* __LINUX_UNION_H */
> diff --git a/include/linux/dcache.h b/include/linux/dcache.h
> index 01d6011..509a637 100644
> --- 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
>  	struct list_head d_lru;		/* LRU list */
>  	/*
>  	 * d_child and d_rcu can share memory
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index dbd9881..32e6988 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1395,6 +1395,7 @@ struct super_block {
>  	 * read-only.
>  	 */
>  	int s_hard_readonly_users;
> +

Extra blank line, intended?

>  };
>  
>  extern struct timespec current_fs_time(struct super_block *sb);
> -- 
> 1.6.3.3
> 
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