Message-Id: <1281134124-17041-20-git-send-email-vaurora@redhat.com>
Date:	Fri,  6 Aug 2010 15:35:05 -0700
From:	Valerie Aurora <vaurora@...hat.com>
To:	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	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,
	Valerie Aurora <vaurora@...hat.com>
Subject: [PATCH 19/38] union-mount: Introduce union_dir structure and basic operations

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 B. 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 |    5 ++-
 6 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..47409c9 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 overlays) (EXPERIMENTAL)"
+       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 e6ec1d3..936acf0 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 2cd367a..c7b6e67 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -959,6 +959,9 @@ 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);
+	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 0904716..84657da 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -100,7 +100,10 @@ struct dentry {
 	struct hlist_node d_hash;	/* lookup hash list */
 	struct dentry *d_parent;	/* parent directory */
 	struct qstr d_name;
-
+	/* XXX Changes size of dentry, perhaps should be re-tuned. */
+#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
-- 
1.6.3.3

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