kernel/pid.c - kernel/msm - Gitiles

 /*
  * Generic pidhash and scalable, time-bounded PID allocator
  *
  * (C) 2002-2003 William Irwin, IBM
  * (C) 2004 William Irwin, Oracle
  * (C) 2002-2004 Ingo Molnar, Red Hat
  *
  * pid-structures are backing objects for tasks sharing a given ID to chain
  * against. There is very little to them aside from hashing them and
  * parking tasks using given ID's on a list.
  *
  * The hash is always changed with the tasklist_lock write-acquired,
  * and the hash is only accessed with the tasklist_lock at least
  * read-acquired, so there's no additional SMP locking needed here.
  *
  * We have a list of bitmap pages, which bitmaps represent the PID space.
  * Allocating and freeing PIDs is completely lockless. The worst-case
  * allocation scenario when all but one out of 1 million PIDs possible are
  * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
  * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
  *
  * Pid namespaces:
  *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
  *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
  *     Many thanks to Oleg Nesterov for comments and help
  *
  */

 #include <linux/mm.h>
 #include <linux/export.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/rculist.h>
 #include <linux/bootmem.h>
 #include <linux/hash.h>
 #include <linux/pid_namespace.h>
 #include <linux/init_task.h>
 #include <linux/syscalls.h>

 #define pid_hashfn(nr, ns)	\
 	hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
 static struct hlist_head *pid_hash;
 static unsigned int pidhash_shift = 4;
 struct pid init_struct_pid = INIT_STRUCT_PID;

 int pid_max = PID_MAX_DEFAULT;

 #define RESERVED_PIDS		300

 int pid_max_min = RESERVED_PIDS + 1;
 int pid_max_max = PID_MAX_LIMIT;

 #define BITS_PER_PAGE		(PAGE_SIZE*8)
 #define BITS_PER_PAGE_MASK	(BITS_PER_PAGE-1)

 static inline int mk_pid(struct pid_namespace *pid_ns,
 		struct pidmap *map, int off)
 {
 	return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
 }

 #define find_next_offset(map, off)					\
 		find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

 /*
  * PID-map pages start out as NULL, they get allocated upon
  * first use and are never deallocated. This way a low pid_max
  * value does not cause lots of bitmaps to be allocated, but
  * the scheme scales to up to 4 million PIDs, runtime.
  */
 struct pid_namespace init_pid_ns = {
 	.kref = {
 		.refcount       = ATOMIC_INIT(2),
 	},
 	.pidmap = {
 		[ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
 	},
 	.last_pid = 0,
 	.level = 0,
 	.child_reaper = &init_task,
 };
 EXPORT_SYMBOL_GPL(init_pid_ns);

 int is_container_init(struct task_struct *tsk)
 {
 	int ret = 0;
 	struct pid *pid;

 	rcu_read_lock();
 	pid = task_pid(tsk);
 	if (pid != NULL && pid->numbers[pid->level].nr == 1)
 		ret = 1;
 	rcu_read_unlock();

 	return ret;
 }
 EXPORT_SYMBOL(is_container_init);

 /*
  * Note: disable interrupts while the pidmap_lock is held as an
  * interrupt might come in and do read_lock(&tasklist_lock).
  *
  * If we don't disable interrupts there is a nasty deadlock between
  * detach_pid()->free_pid() and another cpu that does
  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  * read_lock(&tasklist_lock);
  *
  * After we clean up the tasklist_lock and know there are no
  * irq handlers that take it we can leave the interrupts enabled.
  * For now it is easier to be safe than to prove it can't happen.
  */

 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

 static void free_pidmap(struct upid *upid)
 {
 	int nr = upid->nr;
 	struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 	int offset = nr & BITS_PER_PAGE_MASK;

 	clear_bit(offset, map->page);
 	atomic_inc(&map->nr_free);
 }

 /*
  * If we started walking pids at 'base', is 'a' seen before 'b'?
  */
 static int pid_before(int base, int a, int b)
 {
 	/*
 	 * This is the same as saying
 	 *
 	 * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 	 * and that mapping orders 'a' and 'b' with respect to 'base'.
 	 */
 	return (unsigned)(a - base) < (unsigned)(b - base);
 }

 /*
  * We might be racing with someone else trying to set pid_ns->last_pid
  * at the pid allocation time (there's also a sysctl for this, but racing
  * with this one is OK, see comment in kernel/pid_namespace.c about it).
  * We want the winner to have the "later" value, because if the
  * "earlier" value prevails, then a pid may get reused immediately.
  *
  * Since pids rollover, it is not sufficient to just pick the bigger
  * value.  We have to consider where we started counting from.
  *
  * 'base' is the value of pid_ns->last_pid that we observed when
  * we started looking for a pid.
  *
  * 'pid' is the pid that we eventually found.
  */
 static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 {
 	int prev;
 	int last_write = base;
 	do {
 		prev = last_write;
 		last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 	} while ((prev != last_write) && (pid_before(base, last_write, pid)));
 }

 static int alloc_pidmap(struct pid_namespace *pid_ns)
 {
 	int i, offset, max_scan, pid, last = pid_ns->last_pid;
 	struct pidmap *map;

 	pid = last + 1;
 	if (pid >= pid_max)
 		pid = RESERVED_PIDS;
 	offset = pid & BITS_PER_PAGE_MASK;
 	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 	/*
 	 * If last_pid points into the middle of the map->page we
 	 * want to scan this bitmap block twice, the second time
 	 * we start with offset == 0 (or RESERVED_PIDS).
 	 */
 	max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 	for (i = 0; i <= max_scan; ++i) {
 		if (unlikely(!map->page)) {
 			void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 			/*
 			 * Free the page if someone raced with us
 			 * installing it:
 			 */
 			spin_lock_irq(&pidmap_lock);
 			if (!map->page) {
 				map->page = page;
 				page = NULL;
 			}
 			spin_unlock_irq(&pidmap_lock);
 			kfree(page);
 			if (unlikely(!map->page))
 				break;
 		}
 		if (likely(atomic_read(&map->nr_free))) {
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
 					set_last_pid(pid_ns, last, pid);
 					return pid;
 				}
 				offset = find_next_offset(map, offset);
 				pid = mk_pid(pid_ns, map, offset);
 			} while (offset < BITS_PER_PAGE && pid < pid_max);
 		}
 		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 			++map;
 			offset = 0;
 		} else {
 			map = &pid_ns->pidmap[0];
 			offset = RESERVED_PIDS;
 			if (unlikely(last == offset))
 				break;
 		}
 		pid = mk_pid(pid_ns, map, offset);
 	}
 	return -1;
 }

 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 {
 	int offset;
 	struct pidmap *map, *end;

 	if (last >= PID_MAX_LIMIT)
 		return -1;

 	offset = (last + 1) & BITS_PER_PAGE_MASK;
 	map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 	end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 	for (; map < end; map++, offset = 0) {
 		if (unlikely(!map->page))
 			continue;
 		offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 		if (offset < BITS_PER_PAGE)
 			return mk_pid(pid_ns, map, offset);
 	}
 	return -1;
 }

 void put_pid(struct pid *pid)
 {
 	struct pid_namespace *ns;

 	if (!pid)
 		return;

 	ns = pid->numbers[pid->level].ns;
 	if ((atomic_read(&pid->count) == 1) ||
 	     atomic_dec_and_test(&pid->count)) {
 		kmem_cache_free(ns->pid_cachep, pid);
 		put_pid_ns(ns);
 	}
 }
 EXPORT_SYMBOL_GPL(put_pid);

 static void delayed_put_pid(struct rcu_head *rhp)
 {
 	struct pid *pid = container_of(rhp, struct pid, rcu);
 	put_pid(pid);
 }

 void free_pid(struct pid *pid)
 {
 	/* We can be called with write_lock_irq(&tasklist_lock) held */
 	int i;
 	unsigned long flags;

 	spin_lock_irqsave(&pidmap_lock, flags);
 	for (i = 0; i <= pid->level; i++)
 		hlist_del_rcu(&pid->numbers[i].pid_chain);
 	spin_unlock_irqrestore(&pidmap_lock, flags);

 	for (i = 0; i <= pid->level; i++)
 		free_pidmap(pid->numbers + i);

 	call_rcu(&pid->rcu, delayed_put_pid);
 }

 struct pid *alloc_pid(struct pid_namespace *ns)
 {
 	struct pid *pid;
 	enum pid_type type;
 	int i, nr;
 	struct pid_namespace *tmp;
 	struct upid *upid;

 	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 	if (!pid)
 		goto out;

 	tmp = ns;
 	for (i = ns->level; i >= 0; i--) {
 		nr = alloc_pidmap(tmp);
 		if (nr < 0)
 			goto out_free;

 		pid->numbers[i].nr = nr;
 		pid->numbers[i].ns = tmp;
 		tmp = tmp->parent;
 	}

 	get_pid_ns(ns);
 	pid->level = ns->level;
 	atomic_set(&pid->count, 1);
 	for (type = 0; type < PIDTYPE_MAX; ++type)
 		INIT_HLIST_HEAD(&pid->tasks[type]);

 	upid = pid->numbers + ns->level;
 	spin_lock_irq(&pidmap_lock);
 	for ( ; upid >= pid->numbers; --upid)
 		hlist_add_head_rcu(&upid->pid_chain,
 				&pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 	spin_unlock_irq(&pidmap_lock);

 out:
 	return pid;

 out_free:
 	while (++i <= ns->level)
 		free_pidmap(pid->numbers + i);

 	kmem_cache_free(ns->pid_cachep, pid);
 	pid = NULL;
 	goto out;
 }

 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 {
 	struct hlist_node *elem;
 	struct upid *pnr;

 	hlist_for_each_entry_rcu(pnr, elem,
 			&pid_hash[pid_hashfn(nr, ns)], pid_chain)
 		if (pnr->nr == nr && pnr->ns == ns)
 			return container_of(pnr, struct pid,
 					numbers[ns->level]);

 	return NULL;
 }
 EXPORT_SYMBOL_GPL(find_pid_ns);

 struct pid *find_vpid(int nr)
 {
 	return find_pid_ns(nr, current->nsproxy->pid_ns);
 }
 EXPORT_SYMBOL_GPL(find_vpid);

 /*
  * attach_pid() must be called with the tasklist_lock write-held.
  */
 void attach_pid(struct task_struct *task, enum pid_type type,
 		struct pid *pid)
 {
 	struct pid_link *link;

 	link = &task->pids[type];
 	link->pid = pid;
 	hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 }

 static void __change_pid(struct task_struct *task, enum pid_type type,
 			struct pid *new)
 {
 	struct pid_link *link;
 	struct pid *pid;
 	int tmp;

 	link = &task->pids[type];
 	pid = link->pid;

 	hlist_del_rcu(&link->node);
 	link->pid = new;

 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 		if (!hlist_empty(&pid->tasks[tmp]))
 			return;

 	free_pid(pid);
 }

 void detach_pid(struct task_struct *task, enum pid_type type)
 {
 	__change_pid(task, type, NULL);
 }

 void change_pid(struct task_struct *task, enum pid_type type,
 		struct pid *pid)
 {
 	__change_pid(task, type, pid);
 	attach_pid(task, type, pid);
 }

 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 void transfer_pid(struct task_struct *old, struct task_struct *new,
 			   enum pid_type type)
 {
 	new->pids[type].pid = old->pids[type].pid;
 	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 }

 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 {
 	struct task_struct *result = NULL;
 	if (pid) {
 		struct hlist_node *first;
 		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 					      lockdep_tasklist_lock_is_held());
 		if (first)
 			result = hlist_entry(first, struct task_struct, pids[(type)].node);
 	}
 	return result;
 }
 EXPORT_SYMBOL(pid_task);

 /*
  * Must be called under rcu_read_lock().
  */
 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 {
 	rcu_lockdep_assert(rcu_read_lock_held(),
 			   "find_task_by_pid_ns() needs rcu_read_lock()"
 			   " protection");
 	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 }

 struct task_struct *find_task_by_vpid(pid_t vnr)
 {
 	return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
 }
 EXPORT_SYMBOL_GPL(find_task_by_vpid);

 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 {
 	struct pid *pid;
 	rcu_read_lock();
 	if (type != PIDTYPE_PID)
 		task = task->group_leader;
 	pid = get_pid(task->pids[type].pid);
 	rcu_read_unlock();
 	return pid;
 }
 EXPORT_SYMBOL_GPL(get_task_pid);

 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 {
 	struct task_struct *result;
 	rcu_read_lock();
 	result = pid_task(pid, type);
 	if (result)
 		get_task_struct(result);
 	rcu_read_unlock();
 	return result;
 }
 EXPORT_SYMBOL_GPL(get_pid_task);

 struct pid *find_get_pid(pid_t nr)
 {
 	struct pid *pid;

 	rcu_read_lock();
 	pid = get_pid(find_vpid(nr));
 	rcu_read_unlock();

 	return pid;
 }
 EXPORT_SYMBOL_GPL(find_get_pid);

 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 {
 	struct upid *upid;
 	pid_t nr = 0;

 	if (pid && ns->level <= pid->level) {
 		upid = &pid->numbers[ns->level];
 		if (upid->ns == ns)
 			nr = upid->nr;
 	}
 	return nr;
 }

 pid_t pid_vnr(struct pid *pid)
 {
 	return pid_nr_ns(pid, current->nsproxy->pid_ns);
 }
 EXPORT_SYMBOL_GPL(pid_vnr);

 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 			struct pid_namespace *ns)
 {
 	pid_t nr = 0;

 	rcu_read_lock();
 	if (!ns)
 		ns = current->nsproxy->pid_ns;
 	if (likely(pid_alive(task))) {
 		if (type != PIDTYPE_PID)
 			task = task->group_leader;
 		nr = pid_nr_ns(task->pids[type].pid, ns);
 	}
 	rcu_read_unlock();

 	return nr;
 }
 EXPORT_SYMBOL(__task_pid_nr_ns);

 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 {
 	return pid_nr_ns(task_tgid(tsk), ns);
 }
 EXPORT_SYMBOL(task_tgid_nr_ns);

 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 {
 	return ns_of_pid(task_pid(tsk));
 }
 EXPORT_SYMBOL_GPL(task_active_pid_ns);

 /*
  * Used by proc to find the first pid that is greater than or equal to nr.
  *
  * If there is a pid at nr this function is exactly the same as find_pid_ns.
  */
 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 {
 	struct pid *pid;

 	do {
 		pid = find_pid_ns(nr, ns);
 		if (pid)
 			break;
 		nr = next_pidmap(ns, nr);
 	} while (nr > 0);

 	return pid;
 }

 /*
  * The pid hash table is scaled according to the amount of memory in the
  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
  * more.
  */
 void __init pidhash_init(void)
 {
 	unsigned int i, pidhash_size;

 	pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 					   HASH_EARLY | HASH_SMALL,
 					   &pidhash_shift, NULL, 4096);
 	pidhash_size = 1U << pidhash_shift;

 	for (i = 0; i < pidhash_size; i++)
 		INIT_HLIST_HEAD(&pid_hash[i]);
 }

 void __init pidmap_init(void)
 {
 	/* bump default and minimum pid_max based on number of cpus */
 	pid_max = min(pid_max_max, max_t(int, pid_max,
 				PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 	pid_max_min = max_t(int, pid_max_min,
 				PIDS_PER_CPU_MIN * num_possible_cpus());
 	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);

 	init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 	/* Reserve PID 0. We never call free_pidmap(0) */
 	set_bit(0, init_pid_ns.pidmap[0].page);
 	atomic_dec(&init_pid_ns.pidmap[0].nr_free);

 	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 			SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 }
	/*
	* Generic pidhash and scalable, time-bounded PID allocator
	*
	* (C) 2002-2003 William Irwin, IBM
	* (C) 2004 William Irwin, Oracle
	* (C) 2002-2004 Ingo Molnar, Red Hat
	*
	* pid-structures are backing objects for tasks sharing a given ID to chain
	* against. There is very little to them aside from hashing them and
	* parking tasks using given ID's on a list.
	*
	* The hash is always changed with the tasklist_lock write-acquired,
	* and the hash is only accessed with the tasklist_lock at least
	* read-acquired, so there's no additional SMP locking needed here.
	*
	* We have a list of bitmap pages, which bitmaps represent the PID space.
	* Allocating and freeing PIDs is completely lockless. The worst-case
	* allocation scenario when all but one out of 1 million PIDs possible are
	* allocated already: the scanning of 32 list entries and at most PAGE_SIZE
	* bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
	*
	* Pid namespaces:
	* (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
	* (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
	* Many thanks to Oleg Nesterov for comments and help
	*
	*/

	#include <linux/mm.h>
	#include <linux/export.h>
	#include <linux/slab.h>
	#include <linux/init.h>
	#include <linux/rculist.h>
	#include <linux/bootmem.h>
	#include <linux/hash.h>
	#include <linux/pid_namespace.h>
	#include <linux/init_task.h>
	#include <linux/syscalls.h>

	#define pid_hashfn(nr, ns) \
	hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
	static struct hlist_head *pid_hash;
	static unsigned int pidhash_shift = 4;
	struct pid init_struct_pid = INIT_STRUCT_PID;

	int pid_max = PID_MAX_DEFAULT;

	#define RESERVED_PIDS 300

	int pid_max_min = RESERVED_PIDS + 1;
	int pid_max_max = PID_MAX_LIMIT;

	#define BITS_PER_PAGE (PAGE_SIZE*8)
	#define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)

	static inline int mk_pid(struct pid_namespace *pid_ns,
	struct pidmap *map, int off)
	{
	return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
	}

	#define find_next_offset(map, off) \
	find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

	/*
	* PID-map pages start out as NULL, they get allocated upon
	* first use and are never deallocated. This way a low pid_max
	* value does not cause lots of bitmaps to be allocated, but
	* the scheme scales to up to 4 million PIDs, runtime.
	*/
	struct pid_namespace init_pid_ns = {
	.kref = {
	.refcount = ATOMIC_INIT(2),
	},
	.pidmap = {
	[ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
	},
	.last_pid = 0,
	.level = 0,
	.child_reaper = &init_task,
	};
	EXPORT_SYMBOL_GPL(init_pid_ns);

	int is_container_init(struct task_struct *tsk)
	{
	int ret = 0;
	struct pid *pid;

	rcu_read_lock();
	pid = task_pid(tsk);
	if (pid != NULL && pid->numbers[pid->level].nr == 1)
	ret = 1;
	rcu_read_unlock();

	return ret;
	}
	EXPORT_SYMBOL(is_container_init);

	/*
	* Note: disable interrupts while the pidmap_lock is held as an
	* interrupt might come in and do read_lock(&tasklist_lock).
	*
	* If we don't disable interrupts there is a nasty deadlock between
	* detach_pid()->free_pid() and another cpu that does
	* spin_lock(&pidmap_lock) followed by an interrupt routine that does
	* read_lock(&tasklist_lock);
	*
	* After we clean up the tasklist_lock and know there are no
	* irq handlers that take it we can leave the interrupts enabled.
	* For now it is easier to be safe than to prove it can't happen.
	*/

	static __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

	static void free_pidmap(struct upid *upid)
	{
	int nr = upid->nr;
	struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
	int offset = nr & BITS_PER_PAGE_MASK;

	clear_bit(offset, map->page);
	atomic_inc(&map->nr_free);
	}

	/*
	* If we started walking pids at 'base', is 'a' seen before 'b'?
	*/
	static int pid_before(int base, int a, int b)
	{
	/*
	* This is the same as saying
	*
	* (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
	* and that mapping orders 'a' and 'b' with respect to 'base'.
	*/
	return (unsigned)(a - base) < (unsigned)(b - base);
	}

	/*
	* We might be racing with someone else trying to set pid_ns->last_pid
	* at the pid allocation time (there's also a sysctl for this, but racing
	* with this one is OK, see comment in kernel/pid_namespace.c about it).
	* We want the winner to have the "later" value, because if the
	* "earlier" value prevails, then a pid may get reused immediately.
	*
	* Since pids rollover, it is not sufficient to just pick the bigger
	* value. We have to consider where we started counting from.
	*
	* 'base' is the value of pid_ns->last_pid that we observed when
	* we started looking for a pid.
	*
	* 'pid' is the pid that we eventually found.
	*/
	static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
	{
	int prev;
	int last_write = base;
	do {
	prev = last_write;
	last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
	} while ((prev != last_write) && (pid_before(base, last_write, pid)));
	}

	static int alloc_pidmap(struct pid_namespace *pid_ns)
	{
	int i, offset, max_scan, pid, last = pid_ns->last_pid;
	struct pidmap *map;

	pid = last + 1;
	if (pid >= pid_max)
	pid = RESERVED_PIDS;
	offset = pid & BITS_PER_PAGE_MASK;
	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
	/*
	* If last_pid points into the middle of the map->page we
	* want to scan this bitmap block twice, the second time
	* we start with offset == 0 (or RESERVED_PIDS).
	*/
	max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
	for (i = 0; i <= max_scan; ++i) {
	if (unlikely(!map->page)) {
	void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
	/*
	* Free the page if someone raced with us
	* installing it:
	*/
	spin_lock_irq(&pidmap_lock);
	if (!map->page) {
	map->page = page;
	page = NULL;
	}
	spin_unlock_irq(&pidmap_lock);
	kfree(page);
	if (unlikely(!map->page))
	break;
	}
	if (likely(atomic_read(&map->nr_free))) {
	do {
	if (!test_and_set_bit(offset, map->page)) {
	atomic_dec(&map->nr_free);
	set_last_pid(pid_ns, last, pid);
	return pid;
	}
	offset = find_next_offset(map, offset);
	pid = mk_pid(pid_ns, map, offset);
	} while (offset < BITS_PER_PAGE && pid < pid_max);
	}
	if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
	++map;
	offset = 0;
	} else {
	map = &pid_ns->pidmap[0];
	offset = RESERVED_PIDS;
	if (unlikely(last == offset))
	break;
	}
	pid = mk_pid(pid_ns, map, offset);
	}
	return -1;
	}

	int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
	{
	int offset;
	struct pidmap map, end;

	if (last >= PID_MAX_LIMIT)
	return -1;

	offset = (last + 1) & BITS_PER_PAGE_MASK;
	map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
	end = &pid_ns->pidmap[PIDMAP_ENTRIES];
	for (; map < end; map++, offset = 0) {
	if (unlikely(!map->page))
	continue;
	offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
	if (offset < BITS_PER_PAGE)
	return mk_pid(pid_ns, map, offset);
	}
	return -1;
	}

	void put_pid(struct pid *pid)
	{
	struct pid_namespace *ns;

	if (!pid)
	return;

	ns = pid->numbers[pid->level].ns;
	if ((atomic_read(&pid->count) == 1) \|\|
	atomic_dec_and_test(&pid->count)) {
	kmem_cache_free(ns->pid_cachep, pid);
	put_pid_ns(ns);
	}
	}
	EXPORT_SYMBOL_GPL(put_pid);

	static void delayed_put_pid(struct rcu_head *rhp)
	{
	struct pid *pid = container_of(rhp, struct pid, rcu);
	put_pid(pid);
	}

	void free_pid(struct pid *pid)
	{
	/* We can be called with write_lock_irq(&tasklist_lock) held */
	int i;
	unsigned long flags;

	spin_lock_irqsave(&pidmap_lock, flags);
	for (i = 0; i <= pid->level; i++)
	hlist_del_rcu(&pid->numbers[i].pid_chain);
	spin_unlock_irqrestore(&pidmap_lock, flags);

	for (i = 0; i <= pid->level; i++)
	free_pidmap(pid->numbers + i);

	call_rcu(&pid->rcu, delayed_put_pid);
	}

	struct pid alloc_pid(struct pid_namespace ns)
	{
	struct pid *pid;
	enum pid_type type;
	int i, nr;
	struct pid_namespace *tmp;
	struct upid *upid;

	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
	if (!pid)
	goto out;

	tmp = ns;
	for (i = ns->level; i >= 0; i--) {
	nr = alloc_pidmap(tmp);
	if (nr < 0)
	goto out_free;

	pid->numbers[i].nr = nr;
	pid->numbers[i].ns = tmp;
	tmp = tmp->parent;
	}

	get_pid_ns(ns);
	pid->level = ns->level;
	atomic_set(&pid->count, 1);
	for (type = 0; type < PIDTYPE_MAX; ++type)
	INIT_HLIST_HEAD(&pid->tasks[type]);

	upid = pid->numbers + ns->level;
	spin_lock_irq(&pidmap_lock);
	for ( ; upid >= pid->numbers; --upid)
	hlist_add_head_rcu(&upid->pid_chain,
	&pid_hash[pid_hashfn(upid->nr, upid->ns)]);
	spin_unlock_irq(&pidmap_lock);

	out:
	return pid;

	out_free:
	while (++i <= ns->level)
	free_pidmap(pid->numbers + i);

	kmem_cache_free(ns->pid_cachep, pid);
	pid = NULL;
	goto out;
	}

	struct pid find_pid_ns(int nr, struct pid_namespace ns)
	{
	struct hlist_node *elem;
	struct upid *pnr;

	hlist_for_each_entry_rcu(pnr, elem,
	&pid_hash[pid_hashfn(nr, ns)], pid_chain)
	if (pnr->nr == nr && pnr->ns == ns)
	return container_of(pnr, struct pid,
	numbers[ns->level]);

	return NULL;
	}
	EXPORT_SYMBOL_GPL(find_pid_ns);

	struct pid *find_vpid(int nr)
	{
	return find_pid_ns(nr, current->nsproxy->pid_ns);
	}
	EXPORT_SYMBOL_GPL(find_vpid);

	/*
	* attach_pid() must be called with the tasklist_lock write-held.
	*/
	void attach_pid(struct task_struct *task, enum pid_type type,
	struct pid *pid)
	{
	struct pid_link *link;

	link = &task->pids[type];
	link->pid = pid;
	hlist_add_head_rcu(&link->node, &pid->tasks[type]);
	}

	static void __change_pid(struct task_struct *task, enum pid_type type,
	struct pid *new)
	{
	struct pid_link *link;
	struct pid *pid;
	int tmp;

	link = &task->pids[type];
	pid = link->pid;

	hlist_del_rcu(&link->node);
	link->pid = new;

	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
	if (!hlist_empty(&pid->tasks[tmp]))
	return;

	free_pid(pid);
	}

	void detach_pid(struct task_struct *task, enum pid_type type)
	{
	__change_pid(task, type, NULL);
	}

	void change_pid(struct task_struct *task, enum pid_type type,
	struct pid *pid)
	{
	__change_pid(task, type, pid);
	attach_pid(task, type, pid);
	}

	/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
	void transfer_pid(struct task_struct old, struct task_struct new,
	enum pid_type type)
	{
	new->pids[type].pid = old->pids[type].pid;
	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
	}

	struct task_struct pid_task(struct pid pid, enum pid_type type)
	{
	struct task_struct *result = NULL;
	if (pid) {
	struct hlist_node *first;
	first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
	lockdep_tasklist_lock_is_held());
	if (first)
	result = hlist_entry(first, struct task_struct, pids[(type)].node);
	}
	return result;
	}
	EXPORT_SYMBOL(pid_task);

	/*
	* Must be called under rcu_read_lock().
	*/
	struct task_struct find_task_by_pid_ns(pid_t nr, struct pid_namespace ns)
	{
	rcu_lockdep_assert(rcu_read_lock_held(),
	"find_task_by_pid_ns() needs rcu_read_lock()"
	" protection");
	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
	}

	struct task_struct *find_task_by_vpid(pid_t vnr)
	{
	return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
	}
	EXPORT_SYMBOL_GPL(find_task_by_vpid);

	struct pid get_task_pid(struct task_struct task, enum pid_type type)
	{
	struct pid *pid;
	rcu_read_lock();
	if (type != PIDTYPE_PID)
	task = task->group_leader;
	pid = get_pid(task->pids[type].pid);
	rcu_read_unlock();
	return pid;
	}
	EXPORT_SYMBOL_GPL(get_task_pid);

	struct task_struct get_pid_task(struct pid pid, enum pid_type type)
	{
	struct task_struct *result;
	rcu_read_lock();
	result = pid_task(pid, type);
	if (result)
	get_task_struct(result);
	rcu_read_unlock();
	return result;
	}
	EXPORT_SYMBOL_GPL(get_pid_task);

	struct pid *find_get_pid(pid_t nr)
	{
	struct pid *pid;

	rcu_read_lock();
	pid = get_pid(find_vpid(nr));
	rcu_read_unlock();

	return pid;
	}
	EXPORT_SYMBOL_GPL(find_get_pid);

	pid_t pid_nr_ns(struct pid pid, struct pid_namespace ns)
	{
	struct upid *upid;
	pid_t nr = 0;

	if (pid && ns->level <= pid->level) {
	upid = &pid->numbers[ns->level];
	if (upid->ns == ns)
	nr = upid->nr;
	}
	return nr;
	}

	pid_t pid_vnr(struct pid *pid)
	{
	return pid_nr_ns(pid, current->nsproxy->pid_ns);
	}
	EXPORT_SYMBOL_GPL(pid_vnr);

	pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
	struct pid_namespace *ns)
	{
	pid_t nr = 0;

	rcu_read_lock();
	if (!ns)
	ns = current->nsproxy->pid_ns;
	if (likely(pid_alive(task))) {
	if (type != PIDTYPE_PID)
	task = task->group_leader;
	nr = pid_nr_ns(task->pids[type].pid, ns);
	}
	rcu_read_unlock();

	return nr;
	}
	EXPORT_SYMBOL(__task_pid_nr_ns);

	pid_t task_tgid_nr_ns(struct task_struct tsk, struct pid_namespace ns)
	{
	return pid_nr_ns(task_tgid(tsk), ns);
	}
	EXPORT_SYMBOL(task_tgid_nr_ns);

	struct pid_namespace task_active_pid_ns(struct task_struct tsk)
	{
	return ns_of_pid(task_pid(tsk));
	}
	EXPORT_SYMBOL_GPL(task_active_pid_ns);

	/*
	* Used by proc to find the first pid that is greater than or equal to nr.
	*
	* If there is a pid at nr this function is exactly the same as find_pid_ns.
	*/
	struct pid find_ge_pid(int nr, struct pid_namespace ns)
	{
	struct pid *pid;

	do {
	pid = find_pid_ns(nr, ns);
	if (pid)
	break;
	nr = next_pidmap(ns, nr);
	} while (nr > 0);

	return pid;
	}

	/*
	* The pid hash table is scaled according to the amount of memory in the
	* machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or
	* more.
	*/
	void __init pidhash_init(void)
	{
	unsigned int i, pidhash_size;

	pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
	HASH_EARLY \| HASH_SMALL,
	&pidhash_shift, NULL, 4096);
	pidhash_size = 1U << pidhash_shift;

	for (i = 0; i < pidhash_size; i++)
	INIT_HLIST_HEAD(&pid_hash[i]);
	}

	void __init pidmap_init(void)
	{
	/* bump default and minimum pid_max based on number of cpus */
	pid_max = min(pid_max_max, max_t(int, pid_max,
	PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
	pid_max_min = max_t(int, pid_max_min,
	PIDS_PER_CPU_MIN * num_possible_cpus());
	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);

	init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
	/* Reserve PID 0. We never call free_pidmap(0) */
	set_bit(0, init_pid_ns.pidmap[0].page);
	atomic_dec(&init_pid_ns.pidmap[0].nr_free);

	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
	SLAB_HWCACHE_ALIGN \| SLAB_PANIC);
	}