cgroup.c

/*
 *  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/proc_fs.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/backing-dev.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/magic.h>
#include <linux/spinlock.h>
#include <linux/string.h>
#include <linux/sort.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);
static int root_count;

/* 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)

/* Link structure for associating css_set objects with cgroups */
struct cg_cgroup_link {
	/*
	 * List running through cg_cgroup_links associated with a
	 * cgroup, anchored on cgroup->css_sets
	 */
	struct list_head cont_link_list;
	/*
	 * List running through cg_cgroup_links pointing at a
	 * single css_set object, anchored on css_set->cg_links
	 */
	struct list_head cg_link_list;
	struct css_set *cg;
};

/* The default css_set - used by init and its children prior to any
 * hierarchies being mounted. It contains a pointer to the root state
 * for each subsystem. Also used to anchor the list of css_sets. Not
 * reference-counted, to improve performance when child cgroups
 * haven't been created.
 */

static struct css_set init_css_set;
static struct cg_cgroup_link init_css_set_link;

/* css_set_lock protects the list of css_set objects, and the
 * chain of tasks off each css_set.  Nests outside task->alloc_lock
 * due to cgroup_iter_start() */
static DEFINE_RWLOCK(css_set_lock);
static int css_set_count;

/* We don't maintain the lists running through each css_set to its
 * task until after the first call to cgroup_iter_start(). This
 * reduces the fork()/exit() overhead for people who have cgroups
 * compiled into their kernel but not actually in use */
static int use_task_css_set_links;

/* When we create or destroy a css_set, the operation simply
 * takes/releases a reference count on all the cgroups referenced
 * by subsystems in this css_set. This can end up multiple-counting
 * some cgroups, but that's OK - the ref-count is just a
 * busy/not-busy indicator; ensuring that we only count each cgroup
 * once would require taking a global lock to ensure that no
 * subsystems moved between hierarchies while we were doing so.
 *
 * Possible TODO: decide at boot time based on the number of
 * registered subsystems and the number of CPUs or NUMA nodes whether
 * it's better for performance to ref-count every subsystem, or to
 * take a global lock and only add one ref count to each hierarchy.
 */

/*
 * unlink a css_set from the list and free it
 */
static void release_css_set(struct kref *k)
{
	struct css_set *cg = container_of(k, struct css_set, ref);
	int i;

	write_lock(&css_set_lock);
	list_del(&cg->list);
	css_set_count--;
	while (!list_empty(&cg->cg_links)) {
		struct cg_cgroup_link *link;
		link = list_entry(cg->cg_links.next,
				  struct cg_cgroup_link, cg_link_list);
		list_del(&link->cg_link_list);
		list_del(&link->cont_link_list);
		kfree(link);
	}
	write_unlock(&css_set_lock);
	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
		atomic_dec(&cg->subsys[i]->cgroup->count);
	kfree(cg);
}

/*
 * refcounted get/put for css_set objects
 */
static inline void get_css_set(struct css_set *cg)
{
	kref_get(&cg->ref);
}

static inline void put_css_set(struct css_set *cg)
{
	kref_put(&cg->ref, release_css_set);
}

/*
 * find_existing_css_set() is a helper for
 * find_css_set(), and checks to see whether an existing
 * css_set is suitable. This currently walks a linked-list for
 * simplicity; a later patch will use a hash table for better
 * performance
 *
 * oldcg: the cgroup group that we're using before the cgroup
 * transition
 *
 * cont: the cgroup that we're moving into
 *
 * template: location in which to build the desired set of subsystem
 * state objects for the new cgroup group
 */

static struct css_set *find_existing_css_set(
	struct css_set *oldcg,
	struct cgroup *cont,
	struct cgroup_subsys_state *template[])
{
	int i;
	struct cgroupfs_root *root = cont->root;
	struct list_head *l = &init_css_set.list;

	/* Built the set of subsystem state objects that we want to
	 * see in the new css_set */
	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
		if (root->subsys_bits & (1ull << i)) {
			/* Subsystem is in this hierarchy. So we want
			 * the subsystem state from the new
			 * cgroup */
			template[i] = cont->subsys[i];
		} else {
			/* Subsystem is not in this hierarchy, so we
			 * don't want to change the subsystem state */
			template[i] = oldcg->subsys[i];
		}
	}

	/* Look through existing cgroup groups to find one to reuse */
	do {
		struct css_set *cg =
			list_entry(l, struct css_set, list);

		if (!memcmp(template, cg->subsys, sizeof(cg->subsys))) {
			/* All subsystems matched */
			return cg;
		}
		/* Try the next cgroup group */
		l = l->next;
	} while (l != &init_css_set.list);

	/* No existing cgroup group matched */
	return NULL;
}

/*
 * allocate_cg_links() allocates "count" cg_cgroup_link structures
 * and chains them on tmp through their cont_link_list fields. Returns 0 on
 * success or a negative error
 */

static int allocate_cg_links(int count, struct list_head *tmp)
{
	struct cg_cgroup_link *link;
	int i;
	INIT_LIST_HEAD(tmp);
	for (i = 0; i < count; i++) {
		link = kmalloc(sizeof(*link), GFP_KERNEL);
		if (!link) {
			while (!list_empty(tmp)) {
				link = list_entry(tmp->next,
						  struct cg_cgroup_link,
						  cont_link_list);
				list_del(&link->cont_link_list);
				kfree(link);
			}
			return -ENOMEM;
		}
		list_add(&link->cont_link_list, tmp);
	}
	return 0;
}

static void free_cg_links(struct list_head *tmp)
{
	while (!list_empty(tmp)) {
		struct cg_cgroup_link *link;
		link = list_entry(tmp->next,
				  struct cg_cgroup_link,
				  cont_link_list);
		list_del(&link->cont_link_list);
		kfree(link);
	}
}

/*
 * find_css_set() takes an existing cgroup group and a
 * cgroup object, and returns a css_set object that's
 * equivalent to the old group, but with the given cgroup
 * substituted into the appropriate hierarchy. Must be called with
 * cgroup_mutex held
 */

static struct css_set *find_css_set(
	struct css_set *oldcg, struct cgroup *cont)
{
	struct css_set *res;
	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
	int i;

	struct list_head tmp_cg_links;
	struct cg_cgroup_link *link;

	/* First see if we already have a cgroup group that matches
	 * the desired set */
	write_lock(&css_set_lock);
	res = find_existing_css_set(oldcg, cont, template);
	if (res)
		get_css_set(res);
	write_unlock(&css_set_lock);

	if (res)
		return res;

	res = kmalloc(sizeof(*res), GFP_KERNEL);
	if (!res)
		return NULL;

	/* Allocate all the cg_cgroup_link objects that we'll need */
	if (allocate_cg_links(root_count, &tmp_cg_links) < 0) {
		kfree(res);
		return NULL;
	}

	kref_init(&res->ref);
	INIT_LIST_HEAD(&res->cg_links);
	INIT_LIST_HEAD(&res->tasks);

	/* Copy the set of subsystem state objects generated in
	 * find_existing_css_set() */
	memcpy(res->subsys, template, sizeof(res->subsys));

	write_lock(&css_set_lock);
	/* Add reference counts and links from the new css_set. */
	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
		struct cgroup *cont = res->subsys[i]->cgroup;
		struct cgroup_subsys *ss = subsys[i];
		atomic_inc(&cont->count);
		/*
		 * We want to add a link once per cgroup, so we
		 * only do it for the first subsystem in each
		 * hierarchy
		 */
		if (ss->root->subsys_list.next == &ss->sibling) {
			BUG_ON(list_empty(&tmp_cg_links));
			link = list_entry(tmp_cg_links.next,
					  struct cg_cgroup_link,
					  cont_link_list);
			list_del(&link->cont_link_list);
			list_add(&link->cont_link_list, &cont->css_sets);
			link->cg = res;
			list_add(&link->cg_link_list, &res->cg_links);
		}
	}
	if (list_empty(&rootnode.subsys_list)) {
		link = list_entry(tmp_cg_links.next,
				  struct cg_cgroup_link,
				  cont_link_list);
		list_del(&link->cont_link_list);
		list_add(&link->cont_link_list, &dummytop->css_sets);
		link->cg = res;
		list_add(&link->cg_link_list, &res->cg_links);
	}

	BUG_ON(!list_empty(&tmp_cg_links));

	/* Link this cgroup group into the list */
	list_add(&res->list, &init_css_set.list);
	css_set_count++;
	INIT_LIST_HEAD(&res->tasks);
	write_unlock(&css_set_lock);

	return res;
}

/*
 * 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 file_operations proc_cgroupstats_operations;

static struct backing_dev_info cgroup_backing_dev_info = {
	.capabilities	= BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK,
};

static struct inode *cgroup_new_inode(mode_t mode, struct super_block *sb)
{
	struct inode *inode = new_inode(sb);

	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);
	INIT_LIST_HEAD(&cont->css_sets);
}

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;
	struct list_head tmp_cg_links, *l;
	INIT_LIST_HEAD(&tmp_cg_links);

	/* 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;
		struct inode *inode;

		BUG_ON(sb->s_root != NULL);

		ret = cgroup_get_rootdir(sb);
		if (ret)
			goto drop_new_super;
		inode = sb->s_root->d_inode;

		mutex_lock(&inode->i_mutex);
		mutex_lock(&cgroup_mutex);

		/*
		 * We're accessing css_set_count without locking
		 * css_set_lock here, but that's OK - it can only be
		 * increased by someone holding cgroup_lock, and
		 * that's us. The worst that can happen is that we
		 * have some link structures left over
		 */
		ret = allocate_cg_links(css_set_count, &tmp_cg_links);
		if (ret) {
			mutex_unlock(&cgroup_mutex);
			mutex_unlock(&inode->i_mutex);
			goto drop_new_super;
		}

		ret = rebind_subsystems(root, root->subsys_bits);
		if (ret == -EBUSY) {
			mutex_unlock(&cgroup_mutex);
			mutex_unlock(&inode->i_mutex);
			goto drop_new_super;
		}

		/* EBUSY should be the only error here */
		BUG_ON(ret);

		list_add(&root->root_list, &roots);
		root_count++;

		sb->s_root->d_fsdata = &root->top_cgroup;
		root->top_cgroup.dentry = sb->s_root;

		/* Link the top cgroup in this hierarchy into all
		 * the css_set objects */
		write_lock(&css_set_lock);
		l = &init_css_set.list;
		do {
			struct css_set *cg;
			struct cg_cgroup_link *link;
			cg = list_entry(l, struct css_set, list);
			BUG_ON(list_empty(&tmp_cg_links));
			link = list_entry(tmp_cg_links.next,
					  struct cg_cgroup_link,
					  cont_link_list);
			list_del(&link->cont_link_list);
			link->cg = cg;
			list_add(&link->cont_link_list,
				 &root->top_cgroup.css_sets);
			list_add(&link->cg_link_list, &cg->cg_links);
			l = l->next;
		} while (l != &init_css_set.list);
		write_unlock(&css_set_lock);

		free_cg_links(&tmp_cg_links);

		BUG_ON(!list_empty(&cont->sibling));
		BUG_ON(!list_empty(&cont->children));
		BUG_ON(root->number_of_cgroups != 1);

		cgroup_populate_dir(cont);
		mutex_unlock(&inode->i_mutex);
		mutex_unlock(&cgroup_mutex);
	}

	return simple_set_mnt(mnt, sb);

 drop_new_super:
	up_write(&sb->s_umount);
	deactivate_super(sb);
	free_cg_links(&tmp_cg_links);
	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);

	/*
	 * Release all the links from css_sets to this hierarchy's
	 * root cgroup
	 */
	write_lock(&css_set_lock);
	while (!list_empty(&cont->css_sets)) {
		struct cg_cgroup_link *link;
		link = list_entry(cont->css_sets.next,
				  struct cg_cgroup_link, cont_link_list);
		list_del(&link->cg_link_list);
		list_del(&link->cont_link_list);
		kfree(link);
	}
	write_unlock(&css_set_lock);

	if (!list_empty(&root->root_list)) {
		list_del(&root->root_list);
		root_count--;
	}
	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;
}