Task Control Groups: basic task cgroup framework

Generic Process Control Groups
--------------------------

There have recently been various proposals floating around for
resource management/accounting and other task grouping subsystems in
the kernel, including ResGroups, User BeanCounters, NSProxy
cgroups, and others.  These all need the basic abstraction of being
able to group together multiple processes in an aggregate, in order to
track/limit the resources permitted to those processes, or control
other behaviour of the processes, and all implement this grouping in
different ways.

This patchset provides a framework for tracking and grouping processes
into arbitrary "cgroups" and assigning arbitrary state to those
groupings, in order to control the behaviour of the cgroup as an
aggregate.

The intention is that the various resource management and
virtualization/cgroup efforts can also become task cgroup
clients, with the result that:

- the userspace APIs are (somewhat) normalised

- it's easier to test e.g. the ResGroups CPU controller in
 conjunction with the BeanCounters memory controller, or use either of
them as the resource-control portion of a virtual server system.

- the additional kernel footprint of any of the competing resource
 management systems is substantially reduced, since it doesn't need
 to provide process grouping/containment, hence improving their
 chances of getting into the kernel

This patch:

Add the main task cgroups framework - the cgroup filesystem, and the
basic structures for tracking membership and associating subsystem state
objects to tasks.

Signed-off-by: Paul Menage <menage@google.com>
Cc: Serge E. Hallyn <serue@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Kirill Korotaev <dev@openvz.org>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 1199 insertions, 0 deletions

diff --git a/kernel/Makefile b/kernel/Makefile
index 001bd3b65dd1..ea8c8a12e19a 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -36,6 +36,7 @@ obj-$(CONFIG_PM) += power/
 obj-$(CONFIG_BSD_PROCESS_ACCT) += acct.o
 obj-$(CONFIG_KEXEC) += kexec.o
 obj-$(CONFIG_COMPAT) += compat.o
+obj-$(CONFIG_CGROUPS) += cgroup.o
 obj-$(CONFIG_CPUSETS) += cpuset.o
 obj-$(CONFIG_IKCONFIG) += configs.o
 obj-$(CONFIG_STOP_MACHINE) += stop_machine.o
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
new file mode 100644
index 000000000000..6ba857bec71b
--- /dev/null
+++ b/kernel/cgroup.c
@@ -0,0 +1,1198 @@
+/*
+ *  kernel/cgroup.c
+ *
+ *  Generic process-grouping system.
+ *
+ *  Based originally on the cpuset system, extracted by Paul Menage
+ *  Copyright (C) 2006 Google, Inc
+ *
+ *  Copyright notices from the original cpuset code:
+ *  --------------------------------------------------
+ *  Copyright (C) 2003 BULL SA.
+ *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
+ *
+ *  Portions derived from Patrick Mochel's sysfs code.
+ *  sysfs is Copyright (c) 2001-3 Patrick Mochel
+ *
+ *  2003-10-10 Written by Simon Derr.
+ *  2003-10-22 Updates by Stephen Hemminger.
+ *  2004 May-July Rework by Paul Jackson.
+ *  ---------------------------------------------------
+ *
+ *  This file is subject to the terms and conditions of the GNU General Public
+ *  License.  See the file COPYING in the main directory of the Linux
+ *  distribution for more details.
+ */
+
+#include <linux/cgroup.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/kernel.h>
+#include <linux/list.h>
+#include <linux/mm.h>
+#include <linux/mutex.h>
+#include <linux/mount.h>
+#include <linux/pagemap.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/seq_file.h>
+#include <linux/slab.h>
+#include <linux/magic.h>
+#include <linux/spinlock.h>
+#include <linux/string.h>
+
+#include <asm/atomic.h>
+
+/* Generate an array of cgroup subsystem pointers */
+#define SUBSYS(_x) &_x ## _subsys,
+
+static struct cgroup_subsys *subsys[] = {
+#include <linux/cgroup_subsys.h>
+};
+
+/*
+ * A cgroupfs_root represents the root of a cgroup hierarchy,
+ * and may be associated with a superblock to form an active
+ * hierarchy
+ */
+struct cgroupfs_root {
+        struct super_block *sb;
+
+        /*
+         * The bitmask of subsystems intended to be attached to this
+         * hierarchy
+         */
+        unsigned long subsys_bits;
+
+        /* The bitmask of subsystems currently attached to this hierarchy */
+        unsigned long actual_subsys_bits;
+
+        /* A list running through the attached subsystems */
+        struct list_head subsys_list;
+
+        /* The root cgroup for this hierarchy */
+        struct cgroup top_cgroup;
+
+        /* Tracks how many cgroups are currently defined in hierarchy.*/
+        int number_of_cgroups;
+
+        /* A list running through the mounted hierarchies */
+        struct list_head root_list;
+
+        /* Hierarchy-specific flags */
+        unsigned long flags;
+};
+
+
+/*
+ * The "rootnode" hierarchy is the "dummy hierarchy", reserved for the
+ * subsystems that are otherwise unattached - it never has more than a
+ * single cgroup, and all tasks are part of that cgroup.
+ */
+static struct cgroupfs_root rootnode;
+
+/* The list of hierarchy roots */
+
+static LIST_HEAD(roots);
+
+/* dummytop is a shorthand for the dummy hierarchy's top cgroup */
+#define dummytop (&rootnode.top_cgroup)
+
+/* This flag indicates whether tasks in the fork and exit paths should
+ * take callback_mutex and check for fork/exit handlers to call. This
+ * avoids us having to do extra work in the fork/exit path if none of the
+ * subsystems need to be called.
+ */
+static int need_forkexit_callback;
+
+/* bits in struct cgroup flags field */
+enum {
+        CONT_REMOVED,
+};
+
+/* convenient tests for these bits */
+inline int cgroup_is_removed(const struct cgroup *cont)
+{
+        return test_bit(CONT_REMOVED, &cont->flags);
+}
+
+/* bits in struct cgroupfs_root flags field */
+enum {
+        ROOT_NOPREFIX, /* mounted subsystems have no named prefix */
+};
+
+/*
+ * for_each_subsys() allows you to iterate on each subsystem attached to
+ * an active hierarchy
+ */
+#define for_each_subsys(_root, _ss) \
+list_for_each_entry(_ss, &_root->subsys_list, sibling)
+
+/* for_each_root() allows you to iterate across the active hierarchies */
+#define for_each_root(_root) \
+list_for_each_entry(_root, &roots, root_list)
+
+/*
+ * There is one global cgroup mutex. We also require taking
+ * task_lock() when dereferencing a task's cgroup subsys pointers.
+ * See "The task_lock() exception", at the end of this comment.
+ *
+ * A task must hold cgroup_mutex to modify cgroups.
+ *
+ * Any task can increment and decrement the count field without lock.
+ * So in general, code holding cgroup_mutex can't rely on the count
+ * field not changing.  However, if the count goes to zero, then only
+ * attach_task() can increment it again.  Because a count of zero
+ * means that no tasks are currently attached, therefore there is no
+ * way a task attached to that cgroup can fork (the other way to
+ * increment the count).  So code holding cgroup_mutex can safely
+ * assume that if the count is zero, it will stay zero. Similarly, if
+ * a task holds cgroup_mutex on a cgroup with zero count, it
+ * knows that the cgroup won't be removed, as cgroup_rmdir()
+ * needs that mutex.
+ *
+ * The cgroup_common_file_write handler for operations that modify
+ * the cgroup hierarchy holds cgroup_mutex across the entire operation,
+ * single threading all such cgroup modifications across the system.
+ *
+ * The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
+ * (usually) take cgroup_mutex.  These are the two most performance
+ * critical pieces of code here.  The exception occurs on cgroup_exit(),
+ * when a task in a notify_on_release cgroup exits.  Then cgroup_mutex
+ * is taken, and if the cgroup count is zero, a usermode call made
+ * to /sbin/cgroup_release_agent with the name of the cgroup (path
+ * relative to the root of cgroup file system) as the argument.
+ *
+ * A cgroup can only be deleted if both its 'count' of using tasks
+ * is zero, and its list of 'children' cgroups is empty.  Since all
+ * tasks in the system use _some_ cgroup, and since there is always at
+ * least one task in the system (init, pid == 1), therefore, top_cgroup
+ * always has either children cgroups and/or using tasks.  So we don't
+ * need a special hack to ensure that top_cgroup cannot be deleted.
+ *
+ *      The task_lock() exception
+ *
+ * The need for this exception arises from the action of
+ * attach_task(), which overwrites one tasks cgroup pointer with
+ * another.  It does so using cgroup_mutexe, however there are
+ * several performance critical places that need to reference
+ * task->cgroup without the expense of grabbing a system global
+ * mutex.  Therefore except as noted below, when dereferencing or, as
+ * in attach_task(), modifying a task'ss cgroup pointer we use
+ * task_lock(), which acts on a spinlock (task->alloc_lock) already in
+ * the task_struct routinely used for such matters.
+ *
+ * P.S.  One more locking exception.  RCU is used to guard the
+ * update of a tasks cgroup pointer by attach_task()
+ */
+
+static DEFINE_MUTEX(cgroup_mutex);
+
+/**
+ * cgroup_lock - lock out any changes to cgroup structures
+ *
+ */
+
+void cgroup_lock(void)
+{
+        mutex_lock(&cgroup_mutex);
+}
+
+/**
+ * cgroup_unlock - release lock on cgroup changes
+ *
+ * Undo the lock taken in a previous cgroup_lock() call.
+ */
+
+void cgroup_unlock(void)
+{
+        mutex_unlock(&cgroup_mutex);
+}
+
+/*
+ * A couple of forward declarations required, due to cyclic reference loop:
+ * cgroup_mkdir -> cgroup_create -> cgroup_populate_dir ->
+ * cgroup_add_file -> cgroup_create_file -> cgroup_dir_inode_operations
+ * -> cgroup_mkdir.
+ */
+
+static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, int mode);
+static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);
+static int cgroup_populate_dir(struct cgroup *cont);
+static struct inode_operations cgroup_dir_inode_operations;
+
+static struct inode *cgroup_new_inode(mode_t mode, struct super_block *sb)
+{
+        struct inode *inode = new_inode(sb);
+        static struct backing_dev_info cgroup_backing_dev_info = {
+                .capabilities   = BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK,
+        };
+
+        if (inode) {
+                inode->i_mode = mode;
+                inode->i_uid = current->fsuid;
+                inode->i_gid = current->fsgid;
+                inode->i_blocks = 0;
+                inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
+                inode->i_mapping->backing_dev_info = &cgroup_backing_dev_info;
+        }
+        return inode;
+}
+
+static void cgroup_diput(struct dentry *dentry, struct inode *inode)
+{
+        /* is dentry a directory ? if so, kfree() associated cgroup */
+        if (S_ISDIR(inode->i_mode)) {
+                struct cgroup *cont = dentry->d_fsdata;
+                BUG_ON(!(cgroup_is_removed(cont)));
+                kfree(cont);
+        }
+        iput(inode);
+}
+
+static void remove_dir(struct dentry *d)
+{
+        struct dentry *parent = dget(d->d_parent);
+
+        d_delete(d);
+        simple_rmdir(parent->d_inode, d);
+        dput(parent);
+}
+
+static void cgroup_clear_directory(struct dentry *dentry)
+{
+        struct list_head *node;
+
+        BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
+        spin_lock(&dcache_lock);
+        node = dentry->d_subdirs.next;
+        while (node != &dentry->d_subdirs) {
+                struct dentry *d = list_entry(node, struct dentry, d_u.d_child);
+                list_del_init(node);
+                if (d->d_inode) {
+                        /* This should never be called on a cgroup
+                         * directory with child cgroups */
+                        BUG_ON(d->d_inode->i_mode & S_IFDIR);
+                        d = dget_locked(d);
+                        spin_unlock(&dcache_lock);
+                        d_delete(d);
+                        simple_unlink(dentry->d_inode, d);
+                        dput(d);
+                        spin_lock(&dcache_lock);
+                }
+                node = dentry->d_subdirs.next;
+        }
+        spin_unlock(&dcache_lock);
+}
+
+/*
+ * NOTE : the dentry must have been dget()'ed
+ */
+static void cgroup_d_remove_dir(struct dentry *dentry)
+{
+        cgroup_clear_directory(dentry);
+
+        spin_lock(&dcache_lock);
+        list_del_init(&dentry->d_u.d_child);
+        spin_unlock(&dcache_lock);
+        remove_dir(dentry);
+}
+
+static int rebind_subsystems(struct cgroupfs_root *root,
+                              unsigned long final_bits)
+{
+        unsigned long added_bits, removed_bits;
+        struct cgroup *cont = &root->top_cgroup;
+        int i;
+
+        removed_bits = root->actual_subsys_bits & ~final_bits;
+        added_bits = final_bits & ~root->actual_subsys_bits;
+        /* Check that any added subsystems are currently free */
+        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                unsigned long long bit = 1ull << i;
+                struct cgroup_subsys *ss = subsys[i];
+                if (!(bit & added_bits))
+                        continue;
+                if (ss->root != &rootnode) {
+                        /* Subsystem isn't free */
+                        return -EBUSY;
+                }
+        }
+
+        /* Currently we don't handle adding/removing subsystems when
+         * any child cgroups exist. This is theoretically supportable
+         * but involves complex error handling, so it's being left until
+         * later */
+        if (!list_empty(&cont->children))
+                return -EBUSY;
+
+        /* Process each subsystem */
+        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                struct cgroup_subsys *ss = subsys[i];
+                unsigned long bit = 1UL << i;
+                if (bit & added_bits) {
+                        /* We're binding this subsystem to this hierarchy */
+                        BUG_ON(cont->subsys[i]);
+                        BUG_ON(!dummytop->subsys[i]);
+                        BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
+                        cont->subsys[i] = dummytop->subsys[i];
+                        cont->subsys[i]->cgroup = cont;
+                        list_add(&ss->sibling, &root->subsys_list);
+                        rcu_assign_pointer(ss->root, root);
+                        if (ss->bind)
+                                ss->bind(ss, cont);
+
+                } else if (bit & removed_bits) {
+                        /* We're removing this subsystem */
+                        BUG_ON(cont->subsys[i] != dummytop->subsys[i]);
+                        BUG_ON(cont->subsys[i]->cgroup != cont);
+                        if (ss->bind)
+                                ss->bind(ss, dummytop);
+                        dummytop->subsys[i]->cgroup = dummytop;
+                        cont->subsys[i] = NULL;
+                        rcu_assign_pointer(subsys[i]->root, &rootnode);
+                        list_del(&ss->sibling);
+                } else if (bit & final_bits) {
+                        /* Subsystem state should already exist */
+                        BUG_ON(!cont->subsys[i]);
+                } else {
+                        /* Subsystem state shouldn't exist */
+                        BUG_ON(cont->subsys[i]);
+                }
+        }
+        root->subsys_bits = root->actual_subsys_bits = final_bits;
+        synchronize_rcu();
+
+        return 0;
+}
+
+static int cgroup_show_options(struct seq_file *seq, struct vfsmount *vfs)
+{
+        struct cgroupfs_root *root = vfs->mnt_sb->s_fs_info;
+        struct cgroup_subsys *ss;
+
+        mutex_lock(&cgroup_mutex);
+        for_each_subsys(root, ss)
+                seq_printf(seq, ",%s", ss->name);
+        if (test_bit(ROOT_NOPREFIX, &root->flags))
+                seq_puts(seq, ",noprefix");
+        mutex_unlock(&cgroup_mutex);
+        return 0;
+}
+
+struct cgroup_sb_opts {
+        unsigned long subsys_bits;
+        unsigned long flags;
+};
+
+/* Convert a hierarchy specifier into a bitmask of subsystems and
+ * flags. */
+static int parse_cgroupfs_options(char *data,
+                                     struct cgroup_sb_opts *opts)
+{
+        char *token, *o = data ?: "all";
+
+        opts->subsys_bits = 0;
+        opts->flags = 0;
+
+        while ((token = strsep(&o, ",")) != NULL) {
+                if (!*token)
+                        return -EINVAL;
+                if (!strcmp(token, "all")) {
+                        opts->subsys_bits = (1 << CGROUP_SUBSYS_COUNT) - 1;
+                } else if (!strcmp(token, "noprefix")) {
+                        set_bit(ROOT_NOPREFIX, &opts->flags);
+                } else {
+                        struct cgroup_subsys *ss;
+                        int i;
+                        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                                ss = subsys[i];
+                                if (!strcmp(token, ss->name)) {
+                                        set_bit(i, &opts->subsys_bits);
+                                        break;
+                                }
+                        }
+                        if (i == CGROUP_SUBSYS_COUNT)
+                                return -ENOENT;
+                }
+        }
+
+        /* We can't have an empty hierarchy */
+        if (!opts->subsys_bits)
+                return -EINVAL;
+
+        return 0;
+}
+
+static int cgroup_remount(struct super_block *sb, int *flags, char *data)
+{
+        int ret = 0;
+        struct cgroupfs_root *root = sb->s_fs_info;
+        struct cgroup *cont = &root->top_cgroup;
+        struct cgroup_sb_opts opts;
+
+        mutex_lock(&cont->dentry->d_inode->i_mutex);
+        mutex_lock(&cgroup_mutex);
+
+        /* See what subsystems are wanted */
+        ret = parse_cgroupfs_options(data, &opts);
+        if (ret)
+                goto out_unlock;
+
+        /* Don't allow flags to change at remount */
+        if (opts.flags != root->flags) {
+                ret = -EINVAL;
+                goto out_unlock;
+        }
+
+        ret = rebind_subsystems(root, opts.subsys_bits);
+
+        /* (re)populate subsystem files */
+        if (!ret)
+                cgroup_populate_dir(cont);
+
+ out_unlock:
+        mutex_unlock(&cgroup_mutex);
+        mutex_unlock(&cont->dentry->d_inode->i_mutex);
+        return ret;
+}
+
+static struct super_operations cgroup_ops = {
+        .statfs = simple_statfs,
+        .drop_inode = generic_delete_inode,
+        .show_options = cgroup_show_options,
+        .remount_fs = cgroup_remount,
+};
+
+static void init_cgroup_root(struct cgroupfs_root *root)
+{
+        struct cgroup *cont = &root->top_cgroup;
+        INIT_LIST_HEAD(&root->subsys_list);
+        INIT_LIST_HEAD(&root->root_list);
+        root->number_of_cgroups = 1;
+        cont->root = root;
+        cont->top_cgroup = cont;
+        INIT_LIST_HEAD(&cont->sibling);
+        INIT_LIST_HEAD(&cont->children);
+}
+
+static int cgroup_test_super(struct super_block *sb, void *data)
+{
+        struct cgroupfs_root *new = data;
+        struct cgroupfs_root *root = sb->s_fs_info;
+
+        /* First check subsystems */
+        if (new->subsys_bits != root->subsys_bits)
+            return 0;
+
+        /* Next check flags */
+        if (new->flags != root->flags)
+                return 0;
+
+        return 1;
+}
+
+static int cgroup_set_super(struct super_block *sb, void *data)
+{
+        int ret;
+        struct cgroupfs_root *root = data;
+
+        ret = set_anon_super(sb, NULL);
+        if (ret)
+                return ret;
+
+        sb->s_fs_info = root;
+        root->sb = sb;
+
+        sb->s_blocksize = PAGE_CACHE_SIZE;
+        sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
+        sb->s_magic = CGROUP_SUPER_MAGIC;
+        sb->s_op = &cgroup_ops;
+
+        return 0;
+}
+
+static int cgroup_get_rootdir(struct super_block *sb)
+{
+        struct inode *inode =
+                cgroup_new_inode(S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR, sb);
+        struct dentry *dentry;
+
+        if (!inode)
+                return -ENOMEM;
+
+        inode->i_op = &simple_dir_inode_operations;
+        inode->i_fop = &simple_dir_operations;
+        inode->i_op = &cgroup_dir_inode_operations;
+        /* directories start off with i_nlink == 2 (for "." entry) */
+        inc_nlink(inode);
+        dentry = d_alloc_root(inode);
+        if (!dentry) {
+                iput(inode);
+                return -ENOMEM;
+        }
+        sb->s_root = dentry;
+        return 0;
+}
+
+static int cgroup_get_sb(struct file_system_type *fs_type,
+                         int flags, const char *unused_dev_name,
+                         void *data, struct vfsmount *mnt)
+{
+        struct cgroup_sb_opts opts;
+        int ret = 0;
+        struct super_block *sb;
+        struct cgroupfs_root *root;
+
+        /* First find the desired set of subsystems */
+        ret = parse_cgroupfs_options(data, &opts);
+        if (ret)
+                return ret;
+
+        root = kzalloc(sizeof(*root), GFP_KERNEL);
+        if (!root)
+                return -ENOMEM;
+
+        init_cgroup_root(root);
+        root->subsys_bits = opts.subsys_bits;
+        root->flags = opts.flags;
+
+        sb = sget(fs_type, cgroup_test_super, cgroup_set_super, root);
+
+        if (IS_ERR(sb)) {
+                kfree(root);
+                return PTR_ERR(sb);
+        }
+
+        if (sb->s_fs_info != root) {
+                /* Reusing an existing superblock */
+                BUG_ON(sb->s_root == NULL);
+                kfree(root);
+                root = NULL;
+        } else {
+                /* New superblock */
+                struct cgroup *cont = &root->top_cgroup;
+
+                BUG_ON(sb->s_root != NULL);
+
+                ret = cgroup_get_rootdir(sb);
+                if (ret)
+                        goto drop_new_super;
+
+                mutex_lock(&cgroup_mutex);
+
+                ret = rebind_subsystems(root, root->subsys_bits);
+                if (ret == -EBUSY) {
+                        mutex_unlock(&cgroup_mutex);
+                        goto drop_new_super;
+                }
+
+                /* EBUSY should be the only error here */
+                BUG_ON(ret);
+
+                list_add(&root->root_list, &roots);
+
+                sb->s_root->d_fsdata = &root->top_cgroup;
+                root->top_cgroup.dentry = sb->s_root;
+
+                BUG_ON(!list_empty(&cont->sibling));
+                BUG_ON(!list_empty(&cont->children));
+                BUG_ON(root->number_of_cgroups != 1);
+
+                /*
+                 * I believe that it's safe to nest i_mutex inside
+                 * cgroup_mutex in this case, since no-one else can
+                 * be accessing this directory yet. But we still need
+                 * to teach lockdep that this is the case - currently
+                 * a cgroupfs remount triggers a lockdep warning
+                 */
+                mutex_lock(&cont->dentry->d_inode->i_mutex);
+                cgroup_populate_dir(cont);
+                mutex_unlock(&cont->dentry->d_inode->i_mutex);
+                mutex_unlock(&cgroup_mutex);
+        }
+
+        return simple_set_mnt(mnt, sb);
+
+ drop_new_super:
+        up_write(&sb->s_umount);
+        deactivate_super(sb);
+        return ret;
+}
+
+static void cgroup_kill_sb(struct super_block *sb) {
+        struct cgroupfs_root *root = sb->s_fs_info;
+        struct cgroup *cont = &root->top_cgroup;
+        int ret;
+
+        BUG_ON(!root);
+
+        BUG_ON(root->number_of_cgroups != 1);
+        BUG_ON(!list_empty(&cont->children));
+        BUG_ON(!list_empty(&cont->sibling));
+
+        mutex_lock(&cgroup_mutex);
+
+        /* Rebind all subsystems back to the default hierarchy */
+        ret = rebind_subsystems(root, 0);
+        /* Shouldn't be able to fail ... */
+        BUG_ON(ret);
+
+        if (!list_empty(&root->root_list))
+                list_del(&root->root_list);
+        mutex_unlock(&cgroup_mutex);
+
+        kfree(root);
+        kill_litter_super(sb);
+}
+
+static struct file_system_type cgroup_fs_type = {
+        .name = "cgroup",
+        .get_sb = cgroup_get_sb,
+        .kill_sb = cgroup_kill_sb,
+};
+
+static inline struct cgroup *__d_cont(struct dentry *dentry)
+{
+        return dentry->d_fsdata;
+}
+
+static inline struct cftype *__d_cft(struct dentry *dentry)
+{
+        return dentry->d_fsdata;
+}
+
+/*
+ * Called with cgroup_mutex held.  Writes path of cgroup into buf.
+ * Returns 0 on success, -errno on error.
+ */
+int cgroup_path(const struct cgroup *cont, char *buf, int buflen)
+{
+        char *start;
+
+        if (cont == dummytop) {
+                /*
+                 * Inactive subsystems have no dentry for their root
+                 * cgroup
+                 */
+                strcpy(buf, "/");
+                return 0;
+        }
+
+        start = buf + buflen;
+
+        *--start = '\0';
+        for (;;) {
+                int len = cont->dentry->d_name.len;
+                if ((start -= len) < buf)
+                        return -ENAMETOOLONG;
+                memcpy(start, cont->dentry->d_name.name, len);
+                cont = cont->parent;
+                if (!cont)
+                        break;
+                if (!cont->parent)
+                        continue;
+                if (--start < buf)
+                        return -ENAMETOOLONG;
+                *start = '/';
+        }
+        memmove(buf, start, buf + buflen - start);
+        return 0;
+}
+
+/* The various types of files and directories in a cgroup file system */
+
+enum cgroup_filetype {
+        FILE_ROOT,
+        FILE_DIR,
+        FILE_TASKLIST,
+};
+
+static ssize_t cgroup_file_write(struct file *file, const char __user *buf,
+                                                size_t nbytes, loff_t *ppos)
+{
+        struct cftype *cft = __d_cft(file->f_dentry);
+        struct cgroup *cont = __d_cont(file->f_dentry->d_parent);
+
+        if (!cft)
+                return -ENODEV;
+        if (!cft->write)
+                return -EINVAL;
+
+        return cft->write(cont, cft, file, buf, nbytes, ppos);
+}
+
+static ssize_t cgroup_read_uint(struct cgroup *cont, struct cftype *cft,
+                                   struct file *file,
+                                   char __user *buf, size_t nbytes,
+                                   loff_t *ppos)
+{
+        char tmp[64];
+        u64 val = cft->read_uint(cont, cft);
+        int len = sprintf(tmp, "%llu\n", (unsigned long long) val);
+
+        return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
+}
+
+static ssize_t cgroup_file_read(struct file *file, char __user *buf,
+                                   size_t nbytes, loff_t *ppos)
+{
+        struct cftype *cft = __d_cft(file->f_dentry);
+        struct cgroup *cont = __d_cont(file->f_dentry->d_parent);
+
+        if (!cft)
+                return -ENODEV;
+
+        if (cft->read)
+                return cft->read(cont, cft, file, buf, nbytes, ppos);
+        if (cft->read_uint)
+                return cgroup_read_uint(cont, cft, file, buf, nbytes, ppos);
+        return -EINVAL;
+}
+
+static int cgroup_file_open(struct inode *inode, struct file *file)
+{
+        int err;
+        struct cftype *cft;
+
+        err = generic_file_open(inode, file);
+        if (err)
+                return err;
+
+        cft = __d_cft(file->f_dentry);
+        if (!cft)
+                return -ENODEV;
+        if (cft->open)
+                err = cft->open(inode, file);
+        else
+                err = 0;
+
+        return err;
+}
+
+static int cgroup_file_release(struct inode *inode, struct file *file)
+{
+        struct cftype *cft = __d_cft(file->f_dentry);
+        if (cft->release)
+                return cft->release(inode, file);
+        return 0;
+}
+
+/*
+ * cgroup_rename - Only allow simple rename of directories in place.
+ */
+static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
+                            struct inode *new_dir, struct dentry *new_dentry)
+{
+        if (!S_ISDIR(old_dentry->d_inode->i_mode))
+                return -ENOTDIR;
+        if (new_dentry->d_inode)
+                return -EEXIST;
+        if (old_dir != new_dir)
+                return -EIO;
+        return simple_rename(old_dir, old_dentry, new_dir, new_dentry);
+}
+
+static struct file_operations cgroup_file_operations = {
+        .read = cgroup_file_read,
+        .write = cgroup_file_write,
+        .llseek = generic_file_llseek,
+        .open = cgroup_file_open,
+        .release = cgroup_file_release,
+};
+
+static struct inode_operations cgroup_dir_inode_operations = {
+        .lookup = simple_lookup,
+        .mkdir = cgroup_mkdir,
+        .rmdir = cgroup_rmdir,
+        .rename = cgroup_rename,
+};
+
+static int cgroup_create_file(struct dentry *dentry, int mode,
+                                struct super_block *sb)
+{
+        static struct dentry_operations cgroup_dops = {
+                .d_iput = cgroup_diput,
+        };
+
+        struct inode *inode;
+
+        if (!dentry)
+                return -ENOENT;
+        if (dentry->d_inode)
+                return -EEXIST;
+
+        inode = cgroup_new_inode(mode, sb);
+        if (!inode)
+                return -ENOMEM;
+
+        if (S_ISDIR(mode)) {
+                inode->i_op = &cgroup_dir_inode_operations;
+                inode->i_fop = &simple_dir_operations;
+
+                /* start off with i_nlink == 2 (for "." entry) */
+                inc_nlink(inode);
+
+                /* start with the directory inode held, so that we can
+                 * populate it without racing with another mkdir */
+                mutex_lock(&inode->i_mutex);
+        } else if (S_ISREG(mode)) {
+                inode->i_size = 0;
+                inode->i_fop = &cgroup_file_operations;
+        }
+        dentry->d_op = &cgroup_dops;
+        d_instantiate(dentry, inode);
+        dget(dentry);   /* Extra count - pin the dentry in core */
+        return 0;
+}
+
+/*
+ *      cgroup_create_dir - create a directory for an object.
+ *      cont:   the cgroup we create the directory for.
+ *              It must have a valid ->parent field
+ *              And we are going to fill its ->dentry field.
+ *      dentry: dentry of the new container
+ *      mode:   mode to set on new directory.
+ */
+static int cgroup_create_dir(struct cgroup *cont, struct dentry *dentry,
+                                int mode)
+{
+        struct dentry *parent;
+        int error = 0;
+
+        parent = cont->parent->dentry;
+        error = cgroup_create_file(dentry, S_IFDIR | mode, cont->root->sb);
+        if (!error) {
+                dentry->d_fsdata = cont;
+                inc_nlink(parent->d_inode);
+                cont->dentry = dentry;
+                dget(dentry);
+        }
+        dput(dentry);
+
+        return error;
+}
+
+int cgroup_add_file(struct cgroup *cont,
+                       struct cgroup_subsys *subsys,
+                       const struct cftype *cft)
+{
+        struct dentry *dir = cont->dentry;
+        struct dentry *dentry;
+        int error;
+
+        char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
+        if (subsys && !test_bit(ROOT_NOPREFIX, &cont->root->flags)) {
+                strcpy(name, subsys->name);
+                strcat(name, ".");
+        }
+        strcat(name, cft->name);
+        BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));
+        dentry = lookup_one_len(name, dir, strlen(name));
+        if (!IS_ERR(dentry)) {
+                error = cgroup_create_file(dentry, 0644 | S_IFREG,
+                                                cont->root->sb);
+                if (!error)
+                        dentry->d_fsdata = (void *)cft;
+                dput(dentry);
+        } else
+                error = PTR_ERR(dentry);
+        return error;
+}
+
+int cgroup_add_files(struct cgroup *cont,
+                        struct cgroup_subsys *subsys,
+                        const struct cftype cft[],
+                        int count)
+{
+        int i, err;
+        for (i = 0; i < count; i++) {
+                err = cgroup_add_file(cont, subsys, &cft[i]);
+                if (err)
+                        return err;
+        }
+        return 0;
+}
+
+static int cgroup_populate_dir(struct cgroup *cont)
+{
+        int err;
+        struct cgroup_subsys *ss;
+
+        /* First clear out any existing files */
+        cgroup_clear_directory(cont->dentry);
+
+        for_each_subsys(cont->root, ss) {
+                if (ss->populate && (err = ss->populate(ss, cont)) < 0)
+                        return err;
+        }
+
+        return 0;
+}
+
+static void init_cgroup_css(struct cgroup_subsys_state *css,
+                               struct cgroup_subsys *ss,
+                               struct cgroup *cont)
+{
+        css->cgroup = cont;
+        atomic_set(&css->refcnt, 0);
+        css->flags = 0;
+        if (cont == dummytop)
+                set_bit(CSS_ROOT, &css->flags);
+        BUG_ON(cont->subsys[ss->subsys_id]);
+        cont->subsys[ss->subsys_id] = css;
+}
+
+/*
+ *      cgroup_create - create a cgroup
+ *      parent: cgroup that will be parent of the new cgroup.
+ *      name:           name of the new cgroup. Will be strcpy'ed.
+ *      mode:           mode to set on new inode
+ *
+ *      Must be called with the mutex on the parent inode held
+ */
+
+static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
+                             int mode)
+{
+        struct cgroup *cont;
+        struct cgroupfs_root *root = parent->root;
+        int err = 0;
+        struct cgroup_subsys *ss;
+        struct super_block *sb = root->sb;
+
+        cont = kzalloc(sizeof(*cont), GFP_KERNEL);
+        if (!cont)
+                return -ENOMEM;
+
+        /* Grab a reference on the superblock so the hierarchy doesn't
+         * get deleted on unmount if there are child cgroups.  This
+         * can be done outside cgroup_mutex, since the sb can't
+         * disappear while someone has an open control file on the
+         * fs */
+        atomic_inc(&sb->s_active);
+
+        mutex_lock(&cgroup_mutex);
+
+        cont->flags = 0;
+        INIT_LIST_HEAD(&cont->sibling);
+        INIT_LIST_HEAD(&cont->children);
+
+        cont->parent = parent;
+        cont->root = parent->root;
+        cont->top_cgroup = parent->top_cgroup;
+
+        for_each_subsys(root, ss) {
+                struct cgroup_subsys_state *css = ss->create(ss, cont);
+                if (IS_ERR(css)) {
+                        err = PTR_ERR(css);
+                        goto err_destroy;
+                }
+                init_cgroup_css(css, ss, cont);
+        }
+
+        list_add(&cont->sibling, &cont->parent->children);
+        root->number_of_cgroups++;
+
+        err = cgroup_create_dir(cont, dentry, mode);
+        if (err < 0)
+                goto err_remove;
+
+        /* The cgroup directory was pre-locked for us */
+        BUG_ON(!mutex_is_locked(&cont->dentry->d_inode->i_mutex));
+
+        err = cgroup_populate_dir(cont);
+        /* If err < 0, we have a half-filled directory - oh well ;) */
+
+        mutex_unlock(&cgroup_mutex);
+        mutex_unlock(&cont->dentry->d_inode->i_mutex);
+
+        return 0;
+
+ err_remove:
+
+        list_del(&cont->sibling);
+        root->number_of_cgroups--;
+
+ err_destroy:
+
+        for_each_subsys(root, ss) {
+                if (cont->subsys[ss->subsys_id])
+                        ss->destroy(ss, cont);
+        }
+
+        mutex_unlock(&cgroup_mutex);
+
+        /* Release the reference count that we took on the superblock */
+        deactivate_super(sb);
+
+        kfree(cont);
+        return err;
+}
+
+static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, int mode)
+{
+        struct cgroup *c_parent = dentry->d_parent->d_fsdata;
+
+        /* the vfs holds inode->i_mutex already */
+        return cgroup_create(c_parent, dentry, mode | S_IFDIR);
+}
+
+static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
+{
+        struct cgroup *cont = dentry->d_fsdata;
+        struct dentry *d;
+        struct cgroup *parent;
+        struct cgroup_subsys *ss;
+        struct super_block *sb;
+        struct cgroupfs_root *root;
+        int css_busy = 0;
+
+        /* the vfs holds both inode->i_mutex already */
+
+        mutex_lock(&cgroup_mutex);
+        if (atomic_read(&cont->count) != 0) {
+                mutex_unlock(&cgroup_mutex);
+                return -EBUSY;
+        }
+        if (!list_empty(&cont->children)) {
+                mutex_unlock(&cgroup_mutex);
+                return -EBUSY;
+        }
+
+        parent = cont->parent;
+        root = cont->root;
+        sb = root->sb;
+
+        /* Check the reference count on each subsystem. Since we
+         * already established that there are no tasks in the
+         * cgroup, if the css refcount is also 0, then there should
+         * be no outstanding references, so the subsystem is safe to
+         * destroy */
+        for_each_subsys(root, ss) {
+                struct cgroup_subsys_state *css;
+                css = cont->subsys[ss->subsys_id];
+                if (atomic_read(&css->refcnt)) {
+                        css_busy = 1;
+                        break;
+                }
+        }
+        if (css_busy) {
+                mutex_unlock(&cgroup_mutex);
+                return -EBUSY;
+        }
+
+        for_each_subsys(root, ss) {
+                if (cont->subsys[ss->subsys_id])
+                        ss->destroy(ss, cont);
+        }
+
+        set_bit(CONT_REMOVED, &cont->flags);
+        /* delete my sibling from parent->children */
+        list_del(&cont->sibling);
+        spin_lock(&cont->dentry->d_lock);
+        d = dget(cont->dentry);
+        cont->dentry = NULL;
+        spin_unlock(&d->d_lock);
+
+        cgroup_d_remove_dir(d);
+        dput(d);
+        root->number_of_cgroups--;
+
+        mutex_unlock(&cgroup_mutex);
+        /* Drop the active superblock reference that we took when we
+         * created the cgroup */
+        deactivate_super(sb);
+        return 0;
+}
+
+static void cgroup_init_subsys(struct cgroup_subsys *ss)
+{
+        struct task_struct *g, *p;
+        struct cgroup_subsys_state *css;
+        printk(KERN_ERR "Initializing cgroup subsys %s\n", ss->name);
+
+        /* Create the top cgroup state for this subsystem */
+        ss->root = &rootnode;
+        css = ss->create(ss, dummytop);
+        /* We don't handle early failures gracefully */
+        BUG_ON(IS_ERR(css));
+        init_cgroup_css(css, ss, dummytop);
+
+        /* Update all tasks to contain a subsys pointer to this state
+         * - since the subsystem is newly registered, all tasks are in
+         * the subsystem's top cgroup. */
+
+        /* If this subsystem requested that it be notified with fork
+         * events, we should send it one now for every process in the
+         * system */
+
+        read_lock(&tasklist_lock);
+        init_task.cgroups.subsys[ss->subsys_id] = css;
+        if (ss->fork)
+                ss->fork(ss, &init_task);
+
+        do_each_thread(g, p) {
+                printk(KERN_INFO "Setting task %p css to %p (%d)\n", css, p, p->pid);
+                p->cgroups.subsys[ss->subsys_id] = css;
+                if (ss->fork)
+                        ss->fork(ss, p);
+        } while_each_thread(g, p);
+        read_unlock(&tasklist_lock);
+
+        need_forkexit_callback |= ss->fork || ss->exit;
+
+        ss->active = 1;
+}
+
+/**
+ * cgroup_init_early - initialize cgroups at system boot, and
+ * initialize any subsystems that request early init.
+ */
+int __init cgroup_init_early(void)
+{
+        int i;
+        init_cgroup_root(&rootnode);
+        list_add(&rootnode.root_list, &roots);
+
+        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                struct cgroup_subsys *ss = subsys[i];
+
+                BUG_ON(!ss->name);
+                BUG_ON(strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN);
+                BUG_ON(!ss->create);
+                BUG_ON(!ss->destroy);
+                if (ss->subsys_id != i) {
+                        printk(KERN_ERR "Subsys %s id == %d\n",
+                               ss->name, ss->subsys_id);
+                        BUG();
+                }
+
+                if (ss->early_init)
+                        cgroup_init_subsys(ss);
+        }
+        return 0;
+}
+
+/**
+ * cgroup_init - register cgroup filesystem and /proc file, and
+ * initialize any subsystems that didn't request early init.
+ */
+int __init cgroup_init(void)
+{
+        int err;
+        int i;
+
+        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                struct cgroup_subsys *ss = subsys[i];
+                if (!ss->early_init)
+                        cgroup_init_subsys(ss);
+        }
+
+        err = register_filesystem(&cgroup_fs_type);
+        if (err < 0)
+                goto out;
+
+out:
+        return err;
+}