[PATCH v8 0/4] cgroups: add pids subsystem

[PATCH v8 0/4] cgroups: add pids subsystem

[Aleksa Sarai]

This is an alternate version of the pids v7 patchset[1]. The main
difference is that this version doesn't add a new callback for
an association change during a fork -- it lets subsystems handle such
cases by themselves by allowing for a private pointer to be passed
around between the callbacks.

* Pass a cgroup_fork_state structure between the
  cgroup_{can,cancel,post}_fork hooks which stores the private opaque
  pointer for each subsystem. This allows for subsystems to decide which
  (if any) state to save in the can_fork callback to refer back to in
  the post_fork callback.

[1]: https://lkml.org/lkml/2015/3/31/1058

Aleksa Sarai (4):
  cgroups: use bitmask to filter for_each_subsys
  cgroups: replace explicit ss_mask checking with for_each_subsys_which
  cgroups: allow a cgroup subsystem to reject a fork
  cgroups: implement the PIDs subsystem

 include/linux/cgroup.h        |  35 +++-
 include/linux/cgroup_subsys.h |   4 +
 init/Kconfig                  |  16 ++
 kernel/Makefile               |   1 +
 kernel/cgroup.c               | 208 ++++++++++++++++++------
 kernel/cgroup_freezer.c       |   2 +-
 kernel/cgroup_pids.c          | 367 ++++++++++++++++++++++++++++++++++++++++++
 kernel/fork.c                 |  31 +++-
 kernel/sched/core.c           |   2 +-
 9 files changed, 608 insertions(+), 58 deletions(-)
 create mode 100644 kernel/cgroup_pids.c

-- 
2.3.4

[PATCH v8 1/4] cgroups: use bitmask to filter for_each_subsys

[Aleksa Sarai]

Add a new macro for_each_subsys_which that allows all enabled cgroup
subsystems to be filtered by a bitmask, such that mask & (1 << ssid)
determines if the subsystem is to be processed in the loop body (where
ssid is the unique id of the subsystem).

Also replace the need_forkexit_callback with two separate bitmasks for
each callback to make (ss->{fork,exit}) checks unnecessary.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 kernel/cgroup.c | 43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 29a7b2c..1d69a085 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -175,12 +175,14 @@ static DEFINE_IDR(cgroup_hierarchy_idr);
  */
 static u64 css_serial_nr_next = 1;
 
-/* This flag indicates whether tasks in the fork and exit paths should
+/*
+ * These bitmask flags indicate whether tasks in the fork and exit paths should
  * 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 __read_mostly;
+static int need_fork_callback __read_mostly;
+static int need_exit_callback __read_mostly;
 
 static struct cftype cgroup_dfl_base_files[];
 static struct cftype cgroup_legacy_base_files[];
@@ -409,6 +411,19 @@ static int notify_on_release(const struct cgroup *cgrp)
 	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT &&		\
 	     (((ss) = cgroup_subsys[ssid]) || true); (ssid)++)
 
+/**
+ * for_each_subsys_which - filter for_each_subsys with a bitmask
+ * @ss_mask: the bitmask
+ * @ss: the iteration cursor
+ * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
+ *
+ * The block will only run for cases where the ssid-th bit (1 << ssid) of
+ * mask is set to 1.
+ */
+#define for_each_subsys_which(ss_mask, ss, ssid) \
+	for_each_subsys((ss), (ssid)) \
+		if ((ss_mask) & (1 << (ssid)))
+
 /* iterate across the hierarchies */
 #define for_each_root(root)						\
 	list_for_each_entry((root), &cgroup_roots, root_list)
@@ -4932,7 +4947,8 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 	 * init_css_set is in the subsystem's root cgroup. */
 	init_css_set.subsys[ss->id] = css;
 
-	need_forkexit_callback |= ss->fork || ss->exit;
+	need_fork_callback |= (bool) ss->fork << ss->id;
+	need_exit_callback |= (bool) ss->exit << ss->id;
 
 	/* At system boot, before all subsystems have been
 	 * registered, no tasks have been forked, so we don't
@@ -5239,11 +5255,8 @@ void cgroup_post_fork(struct task_struct *child)
 	 * css_set; otherwise, @child might change state between ->fork()
 	 * and addition to css_set.
 	 */
-	if (need_forkexit_callback) {
-		for_each_subsys(ss, i)
-			if (ss->fork)
-				ss->fork(child);
-	}
+	for_each_subsys_which(need_fork_callback, ss, i)
+		ss->fork(child);
 }
 
 /**
@@ -5287,16 +5300,12 @@ void cgroup_exit(struct task_struct *tsk)
 	cset = task_css_set(tsk);
 	RCU_INIT_POINTER(tsk->cgroups, &init_css_set);
 
-	if (need_forkexit_callback) {
-		/* see cgroup_post_fork() for details */
-		for_each_subsys(ss, i) {
-			if (ss->exit) {
-				struct cgroup_subsys_state *old_css = cset->subsys[i];
-				struct cgroup_subsys_state *css = task_css(tsk, i);
+	/* see cgroup_post_fork() for details */
+	for_each_subsys_which(need_exit_callback, ss, i) {
+		struct cgroup_subsys_state *old_css = cset->subsys[i];
+		struct cgroup_subsys_state *css = task_css(tsk, i);
 
-				ss->exit(css, old_css, tsk);
-			}
-		}
+		ss->exit(css, old_css, tsk);
 	}
 
 	if (put_cset)
-- 
2.3.4

[PATCH v8 2/4] cgroups: replace explicit ss_mask checking with for_each_subsys_which

[Aleksa Sarai]

Replace the explicit checking against ss_masks inside a for_each_subsys
block with for_each_subsys_which(ss_mask, ...), to take advantage of the
more readable macro.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 kernel/cgroup.c | 42 ++++++++++++++----------------------------
 1 file changed, 14 insertions(+), 28 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1d69a085..abd491f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1099,9 +1099,8 @@ static unsigned int cgroup_calc_child_subsys_mask(struct cgroup *cgrp,
 	while (true) {
 		unsigned int new_ss_mask = cur_ss_mask;
 
-		for_each_subsys(ss, ssid)
-			if (cur_ss_mask & (1 << ssid))
-				new_ss_mask |= ss->depends_on;
+		for_each_subsys_which(cur_ss_mask, ss, ssid)
+			new_ss_mask |= ss->depends_on;
 
 		/*
 		 * Mask out subsystems which aren't available.  This can
@@ -1238,10 +1237,7 @@ static int rebind_subsystems(struct cgroup_root *dst_root, unsigned int ss_mask)
 
 	lockdep_assert_held(&cgroup_mutex);
 
-	for_each_subsys(ss, ssid) {
-		if (!(ss_mask & (1 << ssid)))
-			continue;
-
+	for_each_subsys_which(ss_mask, ss, ssid) {
 		/* if @ss has non-root csses attached to it, can't move */
 		if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)))
 			return -EBUSY;
@@ -1278,18 +1274,14 @@ static int rebind_subsystems(struct cgroup_root *dst_root, unsigned int ss_mask)
 	 * Nothing can fail from this point on.  Remove files for the
 	 * removed subsystems and rebind each subsystem.
 	 */
-	for_each_subsys(ss, ssid)
-		if (ss_mask & (1 << ssid))
-			cgroup_clear_dir(&ss->root->cgrp, 1 << ssid);
+	for_each_subsys_which(ss_mask, ss, ssid)
+		cgroup_clear_dir(&ss->root->cgrp, 1 << ssid);
 
-	for_each_subsys(ss, ssid) {
+	for_each_subsys_which(ss_mask, ss, ssid) {
 		struct cgroup_root *src_root;
 		struct cgroup_subsys_state *css;
 		struct css_set *cset;
 
-		if (!(ss_mask & (1 << ssid)))
-			continue;
-
 		src_root = ss->root;
 		css = cgroup_css(&src_root->cgrp, ss);
 
@@ -2563,13 +2555,11 @@ static void cgroup_print_ss_mask(struct seq_file *seq, unsigned int ss_mask)
 	bool printed = false;
 	int ssid;
 
-	for_each_subsys(ss, ssid) {
-		if (ss_mask & (1 << ssid)) {
-			if (printed)
-				seq_putc(seq, ' ');
-			seq_printf(seq, "%s", ss->name);
-			printed = true;
-		}
+	for_each_subsys_which(ss_mask, ss, ssid) {
+		if (printed)
+			seq_putc(seq, ' ');
+		seq_printf(seq, "%s", ss->name);
+		printed = true;
 	}
 	if (printed)
 		seq_putc(seq, '\n');
@@ -2719,9 +2709,8 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	while ((tok = strsep(&buf, " "))) {
 		if (tok[0] == '\0')
 			continue;
-		for_each_subsys(ss, ssid) {
-			if (ss->disabled || strcmp(tok + 1, ss->name) ||
-			    ((1 << ss->id) & cgrp_dfl_root_inhibit_ss_mask))
+		for_each_subsys_which(~cgrp_dfl_root_inhibit_ss_mask, ss, ssid) {
+			if (ss->disabled || strcmp(tok + 1, ss->name))
 				continue;
 
 			if (*tok == '+') {
@@ -2808,10 +2797,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	 * still around.  In such cases, wait till it's gone using
 	 * offline_waitq.
 	 */
-	for_each_subsys(ss, ssid) {
-		if (!(css_enable & (1 << ssid)))
-			continue;
-
+	for_each_subsys_which(css_enable, ss, ssid) {
 		cgroup_for_each_live_child(child, cgrp) {
 			DEFINE_WAIT(wait);
 
-- 
2.3.4

[PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

Add a new cgroup subsystem callback can_fork that conditionally
states whether or not the fork is accepted or rejected by a cgroup
policy. In addition, add a cancel_fork callback so that if an error
occurs later in the forking process, any state modified by can_fork can
be reverted.

Allow for a private opaque pointer to be passed from the cgroup_can_fork
to cgroup_post_fork, allowing for the fork state to be stored by each
subsystem separately.

In order for a subsystem to know that a task associated with a cgroup
hierarchy is being migrated to another hierarchy, add a detach callback
to the subsystem which is run after the migration has been confirmed but
before the old_cset's refcount is dropped. This is necessary in order
for a subsystem to be able to keep a proper count of how many tasks are
associated with that subsystem.

This is in preparation for implementing the pids cgroup subsystem.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/cgroup.h  |  35 +++++++++++--
 kernel/cgroup.c         | 133 ++++++++++++++++++++++++++++++++++++++++++++----
 kernel/cgroup_freezer.c |   2 +-
 kernel/fork.c           |  31 +++++++++--
 kernel/sched/core.c     |   2 +-
 5 files changed, 185 insertions(+), 18 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b9cb94c..d4cdd7a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -25,14 +25,23 @@
 
 #ifdef CONFIG_CGROUPS
 
+struct cgroup_fork_state;
 struct cgroup_root;
 struct cgroup_subsys;
 struct cgroup;
 
+extern struct cgroup_fork_state *cgroup_cfs_alloc(void);
+extern void cgroup_cfs_free(struct cgroup_fork_state *cfs);
+
 extern int cgroup_init_early(void);
 extern int cgroup_init(void);
 extern void cgroup_fork(struct task_struct *p);
-extern void cgroup_post_fork(struct task_struct *p);
+extern int cgroup_can_fork(struct task_struct *p,
+			   struct cgroup_fork_state *cfs);
+extern void cgroup_cancel_fork(struct task_struct *p,
+			       struct cgroup_fork_state *cfs);
+extern void cgroup_post_fork(struct task_struct *p,
+			     struct cgroup_fork_state *old_cfs);
 extern void cgroup_exit(struct task_struct *p);
 extern int cgroupstats_build(struct cgroupstats *stats,
 				struct dentry *dentry);
@@ -649,7 +658,11 @@ struct cgroup_subsys {
 			      struct cgroup_taskset *tset);
 	void (*attach)(struct cgroup_subsys_state *css,
 		       struct cgroup_taskset *tset);
-	void (*fork)(struct task_struct *task);
+	void (*detach)(struct cgroup_subsys_state *old_css,
+		       struct task_struct *task);
+	int (*can_fork)(struct task_struct *task, void **private);
+	void (*cancel_fork)(struct task_struct *task, void **private);
+	void (*fork)(struct task_struct *task, void **private);
 	void (*exit)(struct cgroup_subsys_state *css,
 		     struct cgroup_subsys_state *old_css,
 		     struct task_struct *task);
@@ -943,12 +956,28 @@ struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
 
 #else /* !CONFIG_CGROUPS */
 
+struct cgroup_fork_state;
 struct cgroup_subsys_state;
 
+static inline struct cgroup_fork_state *cfs cgroup_cfs_alloc(void)
+{
+	return NULL;
+}
+static inline void cgroup_cfs_free(struct cgroup_fork_state *cfs) {}
+
 static inline int cgroup_init_early(void) { return 0; }
 static inline int cgroup_init(void) { return 0; }
 static inline void cgroup_fork(struct task_struct *p) {}
-static inline void cgroup_post_fork(struct task_struct *p) {}
+static inline int cgroup_can_fork(struct task_struct *p,
+				  struct cgroup_fork_state *cfs)
+{
+	return 0;
+}
+static inline void cgroup_cancel_fork(struct task_struct *p,
+				      struct cgroup_fork_state *cfs) {}
+static inline void cgroup_post_fork(struct task_struct *p,
+				    struct cgroup_fork_state *old_cfs) {}
+
 static inline void cgroup_exit(struct task_struct *p) {}
 
 static inline int cgroupstats_build(struct cgroupstats *stats,
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index abd491f..7b0ba2f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -176,14 +176,18 @@ static DEFINE_IDR(cgroup_hierarchy_idr);
 static u64 css_serial_nr_next = 1;
 
 /*
- * These bitmask flags indicate whether tasks in the fork and exit paths should
- * 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.
+ * These bitmask flags indicate whether tasks in the fork and exit paths
+ * should check for fork/exit handlers to call. This avoids us having to do
+ * extra work in the fork/exit path if a subsystems doesn't need to be
+ * called.
  */
 static int need_fork_callback __read_mostly;
 static int need_exit_callback __read_mostly;
 
+/* Ditto for the can_fork/cancel_fork/reapply_fork callbacks. */
+static int need_canfork_callback __read_mostly;
+static int need_cancelfork_callback __read_mostly;
+
 static struct cftype cgroup_dfl_base_files[];
 static struct cftype cgroup_legacy_base_files[];
 
@@ -412,7 +416,7 @@ static int notify_on_release(const struct cgroup *cgrp)
 	     (((ss) = cgroup_subsys[ssid]) || true); (ssid)++)
 
 /**
- * for_each_subsys_which - filter for_each_subsys with a bitmask
+ * for_each_subsys_which - filter for_each_subsys with a subsys bitmask
  * @ss_mask: the bitmask
  * @ss: the iteration cursor
  * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
@@ -2054,6 +2058,8 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
 				struct css_set *new_cset)
 {
 	struct css_set *old_cset;
+	struct cgroup_subsys_state *css;
+	int i;
 
 	lockdep_assert_held(&cgroup_mutex);
 	lockdep_assert_held(&css_set_rwsem);
@@ -2078,6 +2084,18 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
 	list_move_tail(&tsk->cg_list, &new_cset->mg_tasks);
 
 	/*
+	 * We detach from the old_cset subsystems here. We must do this
+	 * before we drop the refcount for old_cset, in order to make sure
+	 * that nobody frees it underneath us.
+	 */
+	for_each_e_css(css, i, old_cgrp) {
+		struct cgroup_subsys_state *old_css = old_cset->subsys[i];
+
+		if (old_css->ss->detach)
+			old_css->ss->detach(old_css, tsk);
+	}
+
+	/*
 	 * We just gained a reference on old_cset by taking it from the
 	 * task. As trading it for new_cset is protected by cgroup_mutex,
 	 * we're safe to drop it here; it will be freed under RCU.
@@ -2321,9 +2339,10 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader,
 	 */
 	tset.csets = &tset.dst_csets;
 
-	for_each_e_css(css, i, cgrp)
+	for_each_e_css(css, i, cgrp) {
 		if (css->ss->attach)
 			css->ss->attach(css, &tset);
+	}
 
 	ret = 0;
 	goto out_release_tset;
@@ -4935,6 +4954,8 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 
 	need_fork_callback |= (bool) ss->fork << ss->id;
 	need_exit_callback |= (bool) ss->exit << ss->id;
+	need_canfork_callback |= (bool) ss->can_fork << ss->id;
+	need_cancelfork_callback |= (bool) ss->cancel_fork << ss->id;
 
 	/* At system boot, before all subsystems have been
 	 * registered, no tasks have been forked, so we don't
@@ -5173,6 +5194,33 @@ static const struct file_operations proc_cgroupstats_operations = {
 	.release = single_release,
 };
 
+struct cgroup_fork_state {
+	void *ss_state[CGROUP_SUBSYS_COUNT];
+};
+
+/**
+ * cgroup_cfs_alloc - allocates an empty cgroup_fork_state
+ */
+struct cgroup_fork_state *cgroup_cfs_alloc(void)
+{
+	struct cgroup_fork_state *cfs;
+
+	cfs = kzalloc(sizeof(struct cgroup_fork_state), GFP_KERNEL);
+	if (!cfs)
+		return ERR_PTR(-ENOMEM);
+
+	return cfs;
+}
+
+/**
+ * cgroup_cfs_free - frees a cgroup_fork_state pointer
+ * @cfs: the pointer to free
+ */
+void cgroup_cfs_free(struct cgroup_fork_state *cfs)
+{
+	kfree(cfs);
+}
+
 /**
  * cgroup_fork - initialize cgroup related fields during copy_process()
  * @child: pointer to task_struct of forking parent process.
@@ -5188,6 +5236,63 @@ void cgroup_fork(struct task_struct *child)
 }
 
 /**
+ * cgroup_can_fork - called on a new task before the process is exposed.
+ * @child: the task in question.
+ *
+ * This calls the subsystem can_fork() callbacks. If the can_fork() callback
+ * returns an error, the fork aborts with that error code. This allows for
+ * a cgroup subsystem to conditionally allow or deny new forks.
+ */
+int cgroup_can_fork(struct task_struct *child, struct cgroup_fork_state *cfs)
+{
+	struct cgroup_subsys *ss;
+	int i, j, retval;
+
+	for_each_subsys_which(need_canfork_callback, ss, i) {
+		retval = ss->can_fork(child, &cfs->ss_state[i]);
+		if (retval)
+			goto out_revert;
+	}
+
+	return 0;
+
+out_revert:
+	for_each_subsys_which(need_cancelfork_callback, ss, j) {
+		if (j >= i)
+			break;
+		ss->cancel_fork(child, &cfs->ss_state[i]);
+	}
+
+	return retval;
+}
+
+/**
+ * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
+ * @child: the task in question
+ *
+ * This calls the cancel_fork() callbacks if a fork failed *after*
+ * cgroup_can_fork() succeded.
+ */
+void cgroup_cancel_fork(struct task_struct *child, struct cgroup_fork_state *cfs)
+{
+	struct cgroup_subsys *ss;
+	int i;
+
+	for_each_subsys_which(need_cancelfork_callback, ss, i) {
+		void **state = NULL;
+
+		/*
+		 * Only if %ss has a can_fork() callback is %cfs->ss_state[i] meaningful
+		 * -- otherwise just pass a NULL.
+		 */
+		if (need_canfork_callback & (1 << i))
+			state = &cfs->ss_state[i];
+
+		ss->cancel_fork(child, &cfs->ss_state[i]);
+	}
+}
+
+/**
  * cgroup_post_fork - called on a new task after adding it to the task list
  * @child: the task in question
  *
@@ -5197,7 +5302,7 @@ void cgroup_fork(struct task_struct *child)
  * cgroup_task_iter_start() - to guarantee that the new task ends up on its
  * list.
  */
-void cgroup_post_fork(struct task_struct *child)
+void cgroup_post_fork(struct task_struct *child, struct cgroup_fork_state *old_cfs)
 {
 	struct cgroup_subsys *ss;
 	int i;
@@ -5241,8 +5346,18 @@ void cgroup_post_fork(struct task_struct *child)
 	 * css_set; otherwise, @child might change state between ->fork()
 	 * and addition to css_set.
 	 */
-	for_each_subsys_which(need_fork_callback, ss, i)
-		ss->fork(child);
+	for_each_subsys_which(need_fork_callback, ss, i) {
+		void **state = NULL;
+
+		/*
+		 * Only if %ss has a can_fork() callback is %old_cfs->ss_state[i]
+		 * meaningful -- otherwise just pass a NULL.
+		 */
+		if (need_canfork_callback & (1 << i))
+			state = &old_cfs->ss_state[i];
+
+		ss->fork(child, state);
+	}
 }
 
 /**
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 92b98cc..6c9b090 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -203,7 +203,7 @@ static void freezer_attach(struct cgroup_subsys_state *new_css,
  * to do anything as freezer_attach() will put @task into the appropriate
  * state.
  */
-static void freezer_fork(struct task_struct *task)
+static void freezer_fork(struct task_struct *task, void **private)
 {
 	struct freezer *freezer;
 
diff --git a/kernel/fork.c b/kernel/fork.c
index cf65139..9d74393 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1196,6 +1196,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 {
 	int retval;
 	struct task_struct *p;
+	struct cgroup_fork_state *cfs;
 
 	if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
 		return ERR_PTR(-EINVAL);
@@ -1322,12 +1323,17 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	if (clone_flags & CLONE_THREAD)
 		threadgroup_change_begin(current);
 	cgroup_fork(p);
+	cfs = cgroup_cfs_alloc();
+	if (IS_ERR(cfs)) {
+		retval = PTR_ERR(cfs);
+		goto bad_fork_cleanup_threadgroup_lock;
+	}
 #ifdef CONFIG_NUMA
 	p->mempolicy = mpol_dup(p->mempolicy);
 	if (IS_ERR(p->mempolicy)) {
 		retval = PTR_ERR(p->mempolicy);
 		p->mempolicy = NULL;
-		goto bad_fork_cleanup_threadgroup_lock;
+		goto bad_fork_cfs_free;
 	}
 #endif
 #ifdef CONFIG_CPUSETS
@@ -1468,6 +1474,18 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	INIT_LIST_HEAD(&p->thread_group);
 	p->task_works = NULL;
 
+
+	/*
+	 * Ensure that the cgroup subsystem policies allow the new process to be
+	 * forked. If this fork is happening in an organization operation, then
+	 * this will not charge the correct css_set. This is fixed during
+	 * cgroup_post_fork() (when the css_set has been updated) by undoing
+	 * this operation and forcefully charging the correct css_set.
+	 */
+	retval = cgroup_can_fork(p, cfs);
+	if (retval)
+		goto bad_fork_free_pid;
+
 	/*
 	 * Make it visible to the rest of the system, but dont wake it up yet.
 	 * Need tasklist lock for parent etc handling!
@@ -1504,7 +1522,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		spin_unlock(&current->sighand->siglock);
 		write_unlock_irq(&tasklist_lock);
 		retval = -ERESTARTNOINTR;
-		goto bad_fork_free_pid;
+		goto bad_fork_cancel_cgroup;
 	}
 
 	if (likely(p->pid)) {
@@ -1546,7 +1564,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	write_unlock_irq(&tasklist_lock);
 
 	proc_fork_connector(p);
-	cgroup_post_fork(p);
+	cgroup_post_fork(p, cfs);
+	cgroup_cfs_free(cfs);
 	if (clone_flags & CLONE_THREAD)
 		threadgroup_change_end(current);
 	perf_event_fork(p);
@@ -1556,6 +1575,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	return p;
 
+bad_fork_cancel_cgroup:
+	cgroup_cancel_fork(p, cfs);
 bad_fork_free_pid:
 	if (pid != &init_struct_pid)
 		free_pid(pid);
@@ -1585,8 +1606,10 @@ bad_fork_cleanup_perf:
 bad_fork_cleanup_policy:
 #ifdef CONFIG_NUMA
 	mpol_put(p->mempolicy);
-bad_fork_cleanup_threadgroup_lock:
+bad_fork_cfs_free:
 #endif
+	cgroup_cfs_free(cfs);
+bad_fork_cleanup_threadgroup_lock:
 	if (clone_flags & CLONE_THREAD)
 		threadgroup_change_end(current);
 	delayacct_tsk_free(p);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f0f831e..75337bf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7986,7 +7986,7 @@ static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
 	sched_offline_group(tg);
 }
 
-static void cpu_cgroup_fork(struct task_struct *task)
+static void cpu_cgroup_fork(struct task_struct *task, void **private)
 {
 	sched_move_task(task);
 }
-- 
2.3.4

Re: [PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

Hello, Aleksa.

On Wed, Apr 01, 2015 at 07:57:20PM +1100, Aleksa Sarai wrote:
> +struct cgroup_fork_state {
> +	void *ss_state[CGROUP_SUBSYS_COUNT];
> +};

Can we collect the subsystems which require pre/post fork callbacks to
the front in group_subsys.h and do do CGROUP_SUBSYS_FORK_COUNT (or
whatever) instead?  Then, we don't need all these subsys bitmasks
either we can just test the index against that and be done with it.

> +
> +/**
> + * cgroup_cfs_alloc - allocates an empty cgroup_fork_state
> + */
> +struct cgroup_fork_state *cgroup_cfs_alloc(void)
> +{
> +	struct cgroup_fork_state *cfs;
> +
> +	cfs = kzalloc(sizeof(struct cgroup_fork_state), GFP_KERNEL);
> +	if (!cfs)
> +		return ERR_PTR(-ENOMEM);
> +
> +	return cfs;
> +}

Just make it a void * array and put it on stack.  Abstraction at this
level doesn't serve any purpose.  No controller code is gonna see this
anyway.

> +int cgroup_can_fork(struct task_struct *child, struct cgroup_fork_state *cfs)
> +{
> +	struct cgroup_subsys *ss;
> +	int i, j, retval;
> +
> +	for_each_subsys_which(need_canfork_callback, ss, i) {
> +		retval = ss->can_fork(child, &cfs->ss_state[i]);
> +		if (retval)
> +			goto out_revert;
> +	}
> +
> +	return 0;
> +
> +out_revert:
> +	for_each_subsys_which(need_cancelfork_callback, ss, j) {
> +		if (j >= i)
> +			break;
> +		ss->cancel_fork(child, &cfs->ss_state[i]);

cancel_fork() has no reason to update the opaque pointer.  No reason
to pass pointer of it.

> +void cgroup_cancel_fork(struct task_struct *child, struct cgroup_fork_state *cfs)
> +{
> +	struct cgroup_subsys *ss;
> +	int i;
> +
> +	for_each_subsys_which(need_cancelfork_callback, ss, i) {
> +		void **state = NULL;
> +
> +		/*
> +		 * Only if %ss has a can_fork() callback is %cfs->ss_state[i] meaningful

I don't think we do %var, do we?  % is used for macros and consts.

> +		 * -- otherwise just pass a NULL.
> +		 */
> +		if (need_canfork_callback & (1 << i))
> +			state = &cfs->ss_state[i];
> +
> +		ss->cancel_fork(child, &cfs->ss_state[i]);

Ditto, just pass the pointer itself.

> @@ -5241,8 +5346,18 @@ void cgroup_post_fork(struct task_struct *child)
>  	 * css_set; otherwise, @child might change state between ->fork()
>  	 * and addition to css_set.
>  	 */
> -	for_each_subsys_which(need_fork_callback, ss, i)
> -		ss->fork(child);
> +	for_each_subsys_which(need_fork_callback, ss, i) {
> +		void **state = NULL;
> +
> +		/*
> +		 * Only if %ss has a can_fork() callback is %old_cfs->ss_state[i]
> +		 * meaningful -- otherwise just pass a NULL.
> +		 */

Again, if you just passed the pointer, you wouldn't need the above.
Just clear the array on init and pass in whatever value is in there.

> +		if (need_canfork_callback & (1 << i))
> +			state = &old_cfs->ss_state[i];
> +
> +		ss->fork(child, state);
> +	}
>  }

Thanks.

-- 
tejun

Re: [PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

Hello, Tejun.

>> +struct cgroup_fork_state {
>> +     void *ss_state[CGROUP_SUBSYS_COUNT];
>> +};
>
> Can we collect the subsystems which require pre/post fork callbacks to
> the front in group_subsys.h and do do CGROUP_SUBSYS_FORK_COUNT (or
> whatever) instead?  Then, we don't need all these subsys bitmasks
> either we can just test the index against that and be done with it.

I tried doing this and the kernel would refuse to boot. I believe it has
something to do with the ordering of early_init subsystems, but I'm not
entirely sure (this optimisation can be dealt with later [it's non-critical],
so IMO this should be done in a separate patchset [if at all]). Also, your
later comments would fix the subsys bitmask problem (we can just pass the
default %NULL), we don't even need to test the index.

--
Aleksa Sarai (cyphar)
www.cyphar.com

Re: [PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

Hello, Aleksa.

On Fri, Apr 03, 2015 at 10:42:27AM +1100, Aleksa Sarai wrote:
> I tried doing this and the kernel would refuse to boot. I believe it has
> something to do with the ordering of early_init subsystems, but I'm not

Hmmm... yeah, failure in early_init can be tricky to debug.

> entirely sure (this optimisation can be dealt with later [it's non-critical],
> so IMO this should be done in a separate patchset [if at all]). Also, your
> later comments would fix the subsys bitmask problem (we can just pass the
> default %NULL), we don't even need to test the index.

But we'd be adding quite a few loops in relatively hot paths.  I don't
think it's a good idea to send the patches as-are because we can't
debug and fix them properly, right?  If there are hard ordering
dependencies, the range of subsystems which require fork/exit doesn't
have to be at the beginning.

Thanks.

-- 
tejun

Re: [PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

Hi Tejun,

>> I tried doing this and the kernel would refuse to boot. I believe it has
>> something to do with the ordering of early_init subsystems, but I'm not
>
> Hmmm... yeah, failure in early_init can be tricky to debug.

Okay, I took another look and it isn't that there are hard ordering
dependencies, it's because of how I wrote the tagging macro for cgroup_subsys.h
that caused a NULL deref during the early_init. Whoops.

>> entirely sure (this optimisation can be dealt with later [it's non-critical],
>> so IMO this should be done in a separate patchset [if at all]). Also, your
>> later comments would fix the subsys bitmask problem (we can just pass the
>> default %NULL), we don't even need to test the index.
>
> [...] I don't
> think it's a good idea to send the patches as-are because we can't
> debug and fix them properly, right? [...]

In what way are they hard to debug?

> [...] If there are hard ordering
> dependencies, the range of subsystems which require fork/exit doesn't
> have to be at the beginning.

I just spent a few hours trying to write it and it's just too dodgy to live.
Either you have a solution that won't work with nested tags, and is generally
just kind of bad or you have a solution that requires you to keep two separate
enumerations in sync. And of course you need to offset the index you're looking
stuff up in ss_state so that also looks bad.

But that's all besides the point because, *even if* it I had a clean solution,
it still wouldn't solve the fact that we are
> [...] adding quite a few loops in relatively hot paths.
You kind of _need_ to loop over all the subsystems in either case, the only
difference between having an array of CGROUP_SUBSYS_COUNT pointers or
CGROUP_PREFORK_COUNT is the few bytes of memory you've "saved" (at the expense
of making the callback code essentially unreadable).

--
Aleksa Sarai (cyphar)
www.cyphar.com

Re: [PATCH v8 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

On Tue, Apr 07, 2015 at 11:47:02PM +1000, Aleksa Sarai wrote:
> > [...] I don't
> > think it's a good idea to send the patches as-are because we can't
> > debug and fix them properly, right? [...]
> 
> In what way are they hard to debug?

Some failures can happen before console init and figuring out what's
going on can be tricky.

...
> difference between having an array of CGROUP_SUBSYS_COUNT pointers or
> CGROUP_PREFORK_COUNT is the few bytes of memory you've "saved" (at the expense
> of making the callback code essentially unreadable).

Hmmm?  Just define a marker macro so that the index can be extracted
and loop over the indices and call the callbacks?  Why would this be
any more complex than anything else?

-- 
tejun

[PATCH v8 4/4] cgroups: implement the PIDs subsystem

[Aleksa Sarai]

Adds a new single-purpose PIDs subsystem to limit the number of
tasks that can be forked inside a cgroup. Essentially this is an
implementation of RLIMIT_NPROC that applies to a cgroup rather than a
process tree.

However, it should be noted that organisational operations (adding and
removing tasks from a PIDs hierarchy) will *not* be prevented. Rather,
the number of tasks in the hierarchy cannot exceed the limit through
forking. This is due to the fact that, in the unified hierarchy, attach
cannot fail (and it is not possible for a task to overcome its PIDs
cgroup policy limit by attaching to a child cgroup).

PIDs are fundamentally a global resource, and it is possible to reach
PID exhaustion inside a cgroup without hitting any reasonable kmemcg
policy. Once you've hit PID exhaustion, you're only in a marginally
better state than OOM. This subsystem allows PID exhaustion inside a
cgroup to be prevented.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/cgroup_subsys.h |   4 +
 init/Kconfig                  |  16 ++
 kernel/Makefile               |   1 +
 kernel/cgroup_pids.c          | 367 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 388 insertions(+)
 create mode 100644 kernel/cgroup_pids.c

diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index e4a96fb..a198822 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -47,6 +47,10 @@ SUBSYS(net_prio)
 SUBSYS(hugetlb)
 #endif
 
+#if IS_ENABLED(CONFIG_CGROUP_PIDS)
+SUBSYS(pids)
+#endif
+
 /*
  * The following subsystems are not supported on the default hierarchy.
  */
diff --git a/init/Kconfig b/init/Kconfig
index f5dbc6d..1f135b7 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -952,6 +952,22 @@ config CGROUP_FREEZER
 	  Provides a way to freeze and unfreeze all tasks in a
 	  cgroup.
 
+config CGROUP_PIDS
+	bool "PIDs cgroup subsystem"
+	help
+	  Provides enforcement of process number limits in the scope of a
+	  cgroup. Any attempt to fork more processes than is allowed in the
+	  cgroup will fail. PIDs are fundamentally a global resource because it
+	  is fairly trivial to reach PID exhaustion before you reach even a
+	  conservative kmemcg limit. As a result, it is possible to grind a
+	  system to halt without being limited by other cgroup policies. The
+	  PIDs cgroup subsystem is designed to stop this from happening.
+
+	  It should be noted that organisational operations (such as attaching
+	  to a cgroup hierarchy will *not* be blocked by the PIDs subsystem),
+	  since the PIDs limit only affects a process's ability to fork, not to
+	  attach to a cgroup.
+
 config CGROUP_DEVICE
 	bool "Device controller for cgroups"
 	help
diff --git a/kernel/Makefile b/kernel/Makefile
index 1408b33..e823592 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -53,6 +53,7 @@ obj-$(CONFIG_BACKTRACE_SELF_TEST) += backtracetest.o
 obj-$(CONFIG_COMPAT) += compat.o
 obj-$(CONFIG_CGROUPS) += cgroup.o
 obj-$(CONFIG_CGROUP_FREEZER) += cgroup_freezer.o
+obj-$(CONFIG_CGROUP_PIDS) += cgroup_pids.o
 obj-$(CONFIG_CPUSETS) += cpuset.o
 obj-$(CONFIG_UTS_NS) += utsname.o
 obj-$(CONFIG_USER_NS) += user_namespace.o
diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
new file mode 100644
index 0000000..cb52d51
--- /dev/null
+++ b/kernel/cgroup_pids.c
@@ -0,0 +1,367 @@
+/*
+ * Process number limiting controller for cgroups.
+ *
+ * Used to allow a cgroup hierarchy to stop any new processes
+ * from fork()ing after a certain limit is reached.
+ *
+ * Since it is trivial to hit the task limit without hitting
+ * any kmemcg limits in place, PIDs are a fundamental resource.
+ * As such, PID exhaustion must be preventable in the scope of
+ * a cgroup hierarchy by allowing resource limiting of the
+ * number of tasks in a cgroup.
+ *
+ * In order to use the `pids` controller, set the maximum number
+ * of tasks in pids.max (this is not available in the root cgroup
+ * for obvious reasons). The number of processes currently
+ * in the cgroup is given by pids.current. Organisational operations
+ * are not blocked by cgroup policies, so it is possible to have
+ * pids.current > pids.max. However, fork()s will still not work.
+ *
+ * To set a cgroup to have no limit, set pids.max to "max". fork()
+ * will return -EBUSY if forking would cause a cgroup policy to be
+ * violated.
+ *
+ * pids.current tracks all child cgroup hierarchies, so
+ * parent/pids.current is a superset of parent/child/pids.current.
+ *
+ * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/threads.h>
+#include <linux/atomic.h>
+#include <linux/cgroup.h>
+#include <linux/slab.h>
+
+#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
+#define PIDS_MAX_STR "max"
+
+struct pids_cgroup {
+	struct cgroup_subsys_state	css;
+
+	/*
+	 * Use 64-bit types so that we can safely represent "max" as
+	 * (PID_MAX_LIMIT + 1).
+	 */
+	atomic64_t			counter;
+	int64_t				limit;
+};
+
+static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
+{
+	return container_of(css, struct pids_cgroup, css);
+}
+
+static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
+{
+	return css_pids(pids->css.parent);
+}
+
+static struct cgroup_subsys_state *
+pids_css_alloc(struct cgroup_subsys_state *parent)
+{
+	struct pids_cgroup *pids;
+
+	pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
+	if (!pids)
+		return ERR_PTR(-ENOMEM);
+
+	pids->limit = PIDS_MAX;
+	atomic64_set(&pids->counter, 0);
+	return &pids->css;
+}
+
+static void pids_css_free(struct cgroup_subsys_state *css)
+{
+	kfree(css_pids(css));
+}
+
+/**
+ * pids_cancel - uncharge the local pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to cancel
+ *
+ * This function will WARN if the pid count goes under 0,
+ * because such a case is a bug in the pids controller proper.
+ */
+static void pids_cancel(struct pids_cgroup *pids, int num)
+{
+	/*
+	 * A negative count (or overflow for that matter) is invalid,
+	 * and indicates a bug in the pids controller proper.
+	 */
+	WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
+}
+
+/**
+ * pids_uncharge - hierarchically uncharge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to uncharge
+ */
+static void pids_uncharge(struct pids_cgroup *pids, int num)
+{
+	struct pids_cgroup *p;
+
+	for (p = pids; p; p = parent_pids(p))
+		pids_cancel(p, num);
+}
+
+/**
+ * pids_charge - hierarchically charge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to charge
+ *
+ * This function does *not* follow the pid limit set. It cannot
+ * fail and the new pid count may exceed the limit, because
+ * organisational operations cannot fail in the unified hierarchy.
+ */
+static void pids_charge(struct pids_cgroup *pids, int num)
+{
+	struct pids_cgroup *p;
+
+	for (p = pids; p; p = parent_pids(p))
+		atomic64_add(num, &p->counter);
+}
+
+/**
+ * pids_try_charge - hierarchically try to charge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to charge
+ *
+ * This function follows the set limit. It will fail if the charge
+ * would cause the new value to exceed the hierarchical limit.
+ * Returns 0 if the charge succeded, otherwise -EAGAIN.
+ */
+static int pids_try_charge(struct pids_cgroup *pids, int num)
+{
+	struct pids_cgroup *p, *q;
+
+	for (p = pids; p; p = parent_pids(p)) {
+		int64_t new = atomic64_add_return(num, &p->counter);
+
+		/*
+		 * Since %new is capped to the maximum number of pid_t, if %p->limit is
+		 * %PIDS_MAX then we know that this test will never fail.
+		 */
+		if (new > p->limit)
+			goto revert;
+	}
+
+	return 0;
+
+revert:
+	for (q = pids; q != p; q = parent_pids(q))
+		pids_cancel(q, num);
+	pids_cancel(p, num);
+
+	return -EAGAIN;
+}
+
+static int pids_can_attach(struct cgroup_subsys_state *css,
+			   struct cgroup_taskset *tset)
+{
+	struct pids_cgroup *pids = css_pids(css);
+	struct task_struct *task;
+	int64_t num = 0;
+
+	cgroup_taskset_for_each(task, tset)
+		num++;
+
+	/*
+	 * Attaching to a cgroup is allowed to overcome the
+	 * the PID limit, so that organisation operations aren't
+	 * blocked by the `pids` cgroup controller.
+	 */
+	pids_charge(pids, num);
+	return 0;
+}
+
+static void pids_cancel_attach(struct cgroup_subsys_state *css,
+			       struct cgroup_taskset *tset)
+{
+	struct pids_cgroup *pids = css_pids(css);
+	struct task_struct *task;
+	int64_t num = 0;
+
+	cgroup_taskset_for_each(task, tset)
+		num++;
+
+	pids_uncharge(pids, num);
+}
+
+static void pids_detach(struct cgroup_subsys_state *old_css,
+			struct task_struct *task)
+{
+	struct pids_cgroup *old_pids = css_pids(old_css);
+
+	pids_uncharge(old_pids, 1);
+}
+
+static int pids_can_fork(struct task_struct *task, void **private)
+{
+	struct cgroup_subsys_state *css;
+	struct pids_cgroup *pids;
+	int retval;
+
+	/*
+	 * Use the "current" task_css for the pids subsystem as the tentative
+	 * css. It is possible we will charge the wrong hierarchy, in which
+	 * case we will forcefully revert/reapply the charge on the right
+	 * hierarchy after it is committed to the task proper.
+	 */
+	rcu_read_lock();
+	css = task_css(current, pids_cgrp_id);
+	if (!css_tryget_online(css)) {
+		retval = -EBUSY;
+		goto err_rcu_unlock;
+	}
+	rcu_read_unlock();
+	pids = css_pids(css);
+
+	retval = pids_try_charge(pids, 1);
+	if (retval)
+		goto err_css_put;
+
+	*private = css;
+	return 0;
+
+err_rcu_unlock:
+	rcu_read_unlock();
+err_css_put:
+	css_put(css);
+	return retval;
+}
+
+static void pids_cancel_fork(struct task_struct *task, void **private)
+{
+	struct cgroup_subsys_state *css = *private;
+	struct pids_cgroup *pids = css_pids(css);
+
+	pids_uncharge(pids, 1);
+	css_put(css);
+}
+
+static void pids_fork(struct task_struct *task, void **private)
+{
+	struct cgroup_subsys_state *css;
+	struct cgroup_subsys_state *old_css = *private;
+	struct pids_cgroup *pids;
+	struct pids_cgroup *old_pids = css_pids(old_css);
+
+	/*
+	 * Get the current task css. Since the task has already been exposed to the
+	 * system and had its cg_list updated, we know that we already have an
+	 * implicit reference through %task.
+	 */
+	rcu_read_lock();
+	css = task_css(task, pids_cgrp_id);
+	css_get(css);
+	rcu_read_unlock();
+
+	pids = css_pids(css);
+
+	/*
+	 * The association has changed, we have to revert and reapply the
+	 * charge/uncharge on the wrong hierarchy to the current one. Since
+	 * the association can only change due to an organisation event, its
+	 * okay for us to ignore the limit in this case.
+	 */
+	if(pids != old_pids) {
+		pids_uncharge(old_pids, 1);
+		pids_charge(pids, 1);
+	}
+
+	css_put(css);
+	css_put(old_css);
+}
+
+static void pids_exit(struct cgroup_subsys_state *css,
+		      struct cgroup_subsys_state *old_css,
+		      struct task_struct *task)
+{
+	struct pids_cgroup *pids = css_pids(old_css);
+
+	pids_uncharge(pids, 1);
+}
+
+static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
+			      size_t nbytes, loff_t off)
+{
+	struct cgroup_subsys_state *css = of_css(of);
+	struct pids_cgroup *pids = css_pids(css);
+	int64_t limit;
+	int err;
+
+	buf = strstrip(buf);
+	if (!strcmp(buf, PIDS_MAX_STR)) {
+		limit = PIDS_MAX;
+		goto set_limit;
+	}
+
+	err = kstrtoll(buf, 0, &limit);
+	if (err)
+		return err;
+
+	/* We use INT_MAX as the maximum value of pid_t. */
+	if (limit < 0 || limit > INT_MAX)
+		return -EINVAL;
+
+set_limit:
+	/*
+	 * Limit updates don't need to be mutex'd, since it isn't
+	 * critical that any racing fork()s follow the new limit.
+	 */
+	pids->limit = limit;
+	return nbytes;
+}
+
+static int pids_max_show(struct seq_file *sf, void *v)
+{
+	struct cgroup_subsys_state *css = seq_css(sf);
+	struct pids_cgroup *pids = css_pids(css);
+	int64_t limit = pids->limit;
+
+	if (limit == PIDS_MAX)
+		seq_printf(sf, "%s\n", PIDS_MAX_STR);
+	else
+		seq_printf(sf, "%lld\n", limit);
+
+	return 0;
+}
+
+static s64 pids_current_read(struct cgroup_subsys_state *css,
+			     struct cftype *cft)
+{
+	struct pids_cgroup *pids = css_pids(css);
+
+	return atomic64_read(&pids->counter);
+}
+
+static struct cftype files[] = {
+	{
+		.name = "max",
+		.write = pids_max_write,
+		.seq_show = pids_max_show,
+		.flags = CFTYPE_NOT_ON_ROOT,
+	},
+	{
+		.name = "current",
+		.read_s64 = pids_current_read,
+	},
+	{ }	/* terminate */
+};
+
+struct cgroup_subsys pids_cgrp_subsys = {
+	.css_alloc	= pids_css_alloc,
+	.css_free	= pids_css_free,
+	.can_attach	= pids_can_attach,
+	.cancel_attach	= pids_cancel_attach,
+	.detach		= pids_detach,
+	.can_fork	= pids_can_fork,
+	.cancel_fork	= pids_cancel_fork,
+	.fork		= pids_fork,
+	.exit		= pids_exit,
+	.legacy_cftypes	= files,
+	.early_init	= 0,
+};
-- 
2.3.4