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    Subject[PATCH 19/38] union-mount: Introduce union_dir structure and basic operations
    Date
    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@redhat.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)"
    + 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@tu-harburg.de)
    + * Valerie Aurora <vaurora@redhat.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@tu-harburg.de)
    + * Valerie Aurora <vaurora@redhat.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;
    +
    };

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


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