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    SubjectRe: [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@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)"

    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@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);

    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@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;
    > +

    Extra blank line, intended?

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