[PATCH v9 0/4] cgroups: add pids subsystem

[PATCH v9 0/4] cgroups: add pids subsystem

[Aleksa Sarai]

This is an updated version of v8 of the pids patchset[1], with a few
small fixes, namely:

* Remove the cgroup_fork_state abstraction and just store the ss_state
  (void *)[CGROUP_PREFORK_COUNT] on the stack.

* Only pass a (void **) to can_fork, and pass (void *) to
  {cancel_,}fork.

* Add a tagging macro to cgroup_subsys.h, so that the ss_state array can
  be CGROUP_PREFORK_COUNT in size.

[1]: https://lkml.org/lkml/2015/4/1/145

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        |  67 ++++++--
 include/linux/cgroup_subsys.h |  43 +++++
 init/Kconfig                  |  16 ++
 kernel/Makefile               |   1 +
 kernel/cgroup.c               | 182 +++++++++++++++------
 kernel/cgroup_freezer.c       |   2 +-
 kernel/cgroup_pids.c          | 367 ++++++++++++++++++++++++++++++++++++++++++
 kernel/fork.c                 |  19 ++-
 kernel/sched/core.c           |   2 +-
 9 files changed, 633 insertions(+), 66 deletions(-)
 create mode 100644 kernel/cgroup_pids.c

-- 
2.3.5

[PATCH v9 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)
-- 
2.3.5

[PATCH v9 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);
 
-- 
2.3.5

[PATCH v9 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.

Also add a tagging system for cgroup_subsys.h to allow for CGROUP_<TAG>
enumerations to be be defined and used (as well as CGROUP_<TAG>_COUNT).

This is in preparation for implementing the pids cgroup subsystem.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/cgroup.h        |  67 +++++++++++++++++++++-----
 include/linux/cgroup_subsys.h |  38 +++++++++++++++
 kernel/cgroup.c               | 107 ++++++++++++++++++++++++++++++++++++++----
 kernel/cgroup_freezer.c       |   2 +-
 kernel/fork.c                 |  19 +++++++-
 kernel/sched/core.c           |   2 +-
 6 files changed, 209 insertions(+), 26 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b9cb94c..51c7c6c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -25,6 +25,39 @@
 
 #ifdef CONFIG_CGROUPS
 
+#define _JOIN(left, right) left ## right
+#define JOIN(left, right) _JOIN(left, right)
+#define UNUSED_IDENT(_i) JOIN(JOIN(__unused_ident, __COUNTER__), _ ## _i)
+
+/* define the enumeration of all cgroup subsystems */
+enum cgroup_subsys_id {
+#define SUBSYS(_x) _x ## _cgrp_id,
+#include <linux/cgroup_subsys.h>
+#undef SUBSYS
+	CGROUP_SUBSYS_COUNT,
+};
+
+/* list of all tags for subsystems */
+enum cgroup_subsys_tag {
+#define TAG_ID(_t) CGROUP_ ## _t
+#define SUBSYS(_x) UNUSED_IDENT(_x),
+#define TAG(_t) TAG_ID(_t), \
+	UNUSED_IDENT(_t) = TAG_ID(_t) - 2,
+#define _COUNT(_t, _i) _i, \
+	CGROUP_ ## _t ## _COUNT = (TAG_ID(_t ## _END) - TAG_ID(_t ## _START)) + 1, \
+	UNUSED_IDENT(_t) = _i - 1,
+#define COUNT(_t) _COUNT(_t, UNUSED_IDENT(_t))
+#include <linux/cgroup_subsys.h>
+#undef COUNT
+#undef TAG
+#undef SUBSYS
+#undef TAG_ID
+};
+
+#undef UNUSED_IDENT
+#undef JOIN
+#undef _JOIN
+
 struct cgroup_root;
 struct cgroup_subsys;
 struct cgroup;
@@ -32,7 +65,12 @@ struct cgroup;
 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,
+			   void *ss_state[CGROUP_PREFORK_COUNT]);
+extern void cgroup_cancel_fork(struct task_struct *p,
+			       void *ss_state[CGROUP_PREFORK_COUNT]);
+extern void cgroup_post_fork(struct task_struct *p,
+			     void *old_ss_state[CGROUP_PREFORK_COUNT]);
 extern void cgroup_exit(struct task_struct *p);
 extern int cgroupstats_build(struct cgroupstats *stats,
 				struct dentry *dentry);
@@ -40,14 +78,6 @@ extern int cgroupstats_build(struct cgroupstats *stats,
 extern int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
 			    struct pid *pid, struct task_struct *tsk);
 
-/* define the enumeration of all cgroup subsystems */
-#define SUBSYS(_x) _x ## _cgrp_id,
-enum cgroup_subsys_id {
-#include <linux/cgroup_subsys.h>
-	CGROUP_SUBSYS_COUNT,
-};
-#undef SUBSYS
-
 /*
  * Per-subsystem/per-cgroup state maintained by the system.  This is the
  * fundamental structural building block that controllers deal with.
@@ -649,7 +679,11 @@ 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 +977,19 @@ struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
 
 #else /* !CONFIG_CGROUPS */
 
-struct cgroup_subsys_state;
-
 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,
+				  void *s[CGROUP_PREFORK_COUNT])
+{
+	return 0;
+}
+static inline void cgroup_cancel_fork(struct task_struct *p,
+				      void *s[CGROUP_SUBSYS_COUNT]) {}
+static inline void cgroup_post_fork(struct task_struct *p,
+				    void *s[CGROUP_PREFORK_COUNT]) {}
+
 static inline void cgroup_exit(struct task_struct *p) {}
 
 static inline int cgroupstats_build(struct cgroupstats *stats,
diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index e4a96fb..1a5a9aa 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -3,6 +3,21 @@
  *
  * DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS.
  */
+#ifndef SUBSYS
+#	define __TMP_SUBSYS
+#	define SUBSYS(_x)
+#endif
+
+#ifndef TAG
+#	define __TMP_TAG
+#	define TAG(_t)
+#endif
+
+#ifndef COUNT
+#	define __TMP_COUNT
+#	define COUNT(_t)
+#endif
+
 #if IS_ENABLED(CONFIG_CPUSETS)
 SUBSYS(cpuset)
 #endif
@@ -48,11 +63,34 @@ SUBSYS(hugetlb)
 #endif
 
 /*
+ * Subsystems that implement the can_fork() family of callbacks.
+ */
+TAG(PREFORK_START)
+TAG(PREFORK_END)
+COUNT(PREFORK)
+
+/*
  * The following subsystems are not supported on the default hierarchy.
  */
 #if IS_ENABLED(CONFIG_CGROUP_DEBUG)
 SUBSYS(debug)
 #endif
+
+#ifdef __TMP_SUBSYS
+#	undef __TMP_SUBSYS
+#	undef SUBSYS
+#endif
+
+#ifdef __TMP_TAG
+#	undef __TMP_TAG
+#	undef TAG
+#endif
+
+#ifdef __TMP_COUNT
+#	undef __TMP_COUNT
+#	undef COUNT
+#endif
+
 /*
  * DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS.
  */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index abd491f..c971885 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
@@ -5188,6 +5209,67 @@ 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,
+		    void *ss_state[CGROUP_PREFORK_COUNT])
+{
+	struct cgroup_subsys *ss;
+	int i, j, retval;
+
+	for_each_subsys_which(need_canfork_callback, ss, i) {
+		retval = ss->can_fork(child, &ss_state[i - CGROUP_PREFORK_START]);
+		if (retval)
+			goto out_revert;
+	}
+
+	return 0;
+
+out_revert:
+	for_each_subsys_which(need_cancelfork_callback, ss, j) {
+		void *state = NULL;
+
+		if (j >= i)
+			break;
+
+		if(CGROUP_PREFORK_START <= j && j <= CGROUP_PREFORK_END)
+			state = ss_state[j - CGROUP_PREFORK_START];
+
+		ss->cancel_fork(child, state);
+	}
+
+	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,
+			void *ss_state[CGROUP_PREFORK_COUNT])
+{
+	struct cgroup_subsys *ss;
+	int i;
+
+	for_each_subsys_which(need_cancelfork_callback, ss, i) {
+		void *state = NULL;
+
+		if(CGROUP_PREFORK_START <= i && i <= CGROUP_PREFORK_END)
+			state = ss_state[i - CGROUP_PREFORK_START];
+
+		ss->cancel_fork(child, state);
+	}
+}
+
+/**
  * cgroup_post_fork - called on a new task after adding it to the task list
  * @child: the task in question
  *
@@ -5197,7 +5279,8 @@ 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,
+		      void *old_ss_state[CGROUP_PREFORK_COUNT])
 {
 	struct cgroup_subsys *ss;
 	int i;
@@ -5241,8 +5324,14 @@ 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;
+
+		if(CGROUP_PREFORK_START <= i && i <= CGROUP_PREFORK_END)
+			state = old_ss_state[i - CGROUP_PREFORK_START];
+
+		ss->fork(child, state);
+	}
 }
 
 /**
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 92b98cc..f1b30ad 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..8281370 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;
+	void *ss_state[CGROUP_PREFORK_COUNT] = {};
 
 	if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
 		return ERR_PTR(-EINVAL);
@@ -1468,6 +1469,18 @@ 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, ss_state);
+	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 +1517,7 @@ 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 +1559,7 @@ 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, ss_state);
 	if (clone_flags & CLONE_THREAD)
 		threadgroup_change_end(current);
 	perf_event_fork(p);
@@ -1556,6 +1569,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	return p;
 
+bad_fork_cancel_cgroup:
+	cgroup_cancel_fork(p, ss_state);
 bad_fork_free_pid:
 	if (pid != &init_struct_pid)
 		free_pid(pid);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 62671f5..205b1cf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7988,7 +7988,7 @@ 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);
 }
-- 
2.3.5

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

On Sun, Apr 12, 2015 at 10:51:56AM +1000, Aleksa Sarai wrote:
> +/* list of all tags for subsystems */
> +enum cgroup_subsys_tag {
> +#define TAG_ID(_t) CGROUP_ ## _t
> +#define SUBSYS(_x) UNUSED_IDENT(_x),
> +#define TAG(_t) TAG_ID(_t), \
> +	UNUSED_IDENT(_t) = TAG_ID(_t) - 2,
> +#define _COUNT(_t, _i) _i, \
> +	CGROUP_ ## _t ## _COUNT = (TAG_ID(_t ## _END) - TAG_ID(_t ## _START)) + 1, \
> +	UNUSED_IDENT(_t) = _i - 1,
> +#define COUNT(_t) _COUNT(_t, UNUSED_IDENT(_t))
> +#include <linux/cgroup_subsys.h>
> +#undef COUNT
> +#undef TAG
> +#undef SUBSYS
> +#undef TAG_ID
> +};

Ummm... is this really necessary?  Can't we do something like the
following?

#define SUBSYS_TAG(tag)						\
	__##tag,						\
	tag = __##tag - 2,

-- 
tejun

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

Hi Tejun,

> Ummm... is this really necessary?  Can't we do something like the
> following?
>
> #define SUBSYS_TAG(tag)                                         \
>         __##tag,                                                \
>         tag = __##tag - 2,

Is which part necessary? That's what TAG() does. I can remove the
UNUSED_IDENT() if you want, that was just to make sure you don't get clashes
with other unused idents. COUNT() is so that we don't have to manually add
CGROUP_<TAG>_COUNT for every tag we add.

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

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

Hello,

On Tue, Apr 14, 2015 at 10:33:41AM +1000, Aleksa Sarai wrote:
> Hi Tejun,
> 
> > Ummm... is this really necessary?  Can't we do something like the
> > following?
> >
> > #define SUBSYS_TAG(tag)                                         \
> >         __##tag,                                                \
> >         tag = __##tag - 2,
> 
> Is which part necessary? That's what TAG() does. I can remove the

The complex macros.

> UNUSED_IDENT() if you want, that was just to make sure you don't get clashes
> with other unused idents. COUNT() is so that we don't have to manually add
> CGROUP_<TAG>_COUNT for every tag we add.

Let's please do the simple thing.  It just takes one two-liner macro.

Thanks.

-- 
tejun

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

Hi,

Do you also want me to completely drop the COUNT macro? IMO it makes
the CGROUP_<TAG>_COUNT consolidation much nicer.

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

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

On Thu, Apr 16, 2015 at 11:57:29PM +1000, Aleksa Sarai wrote:
> Do you also want me to completely drop the COUNT macro? IMO it makes
> the CGROUP_<TAG>_COUNT consolidation much nicer.

What's wrong with simply having start and end tags?

-- 
tejun

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

>> Do you also want me to completely drop the COUNT macro? IMO it makes
>> the CGROUP_<TAG>_COUNT consolidation much nicer.
>
> What's wrong with simply having start and end tags?

Because you'd have to write (CGROUP_TAG_END - CGROUP_TAG_START) every
time? It's a small addition and it makes referencing the range of a
tagged section much easier.

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

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

On Sat, Apr 18, 2015 at 06:35:41AM +1000, Aleksa Sarai wrote:
> >> Do you also want me to completely drop the COUNT macro? IMO it makes
> >> the CGROUP_<TAG>_COUNT consolidation much nicer.
> >
> > What's wrong with simply having start and end tags?
> 
> Because you'd have to write (CGROUP_TAG_END - CGROUP_TAG_START) every
> time? It's a small addition and it makes referencing the range of a
> tagged section much easier.

Wouldn't loops look more like

	for (subsys = CGROUP_TAG_START; subsys < CGROUP_TAG_END; subsys++)

And even if not, just define a separate macro for the length.  It's
not like we're gonna have a lot of tags.

Thanks.

-- 
tejun

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

>> >> Do you also want me to completely drop the COUNT macro? IMO it makes
>> >> the CGROUP_<TAG>_COUNT consolidation much nicer.
>> >
>> > What's wrong with simply having start and end tags?
>>
>> Because you'd have to write (CGROUP_TAG_END - CGROUP_TAG_START) every
>> time? It's a small addition and it makes referencing the range of a
>> tagged section much easier.
>
> Wouldn't loops look more like
>
>         for (subsys = CGROUP_TAG_START; subsys < CGROUP_TAG_END; subsys++)

Sorry, I meant for defining arrays. `state[CGROUP_TAG_END -
CGROUP_TAG_START]` is just more annoying to type and read than
`state[CGROUP_TAG_COUNT]`.

> And even if not, just define a separate macro for the length.  It's
> not like we're gonna have a lot of tags.

Do you mean like this?

#define SUBSYS_TAG_COUNT(_tag) (CGROUP_ ## _tag ## _END - CGROUP_ ##
_tag ## _START)

That's fine I guess, I just wanted to match CGROUP_SUBSYS_COUNT in
semantics, but I'll do that if you prefer it that way.

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

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Tejun Heo]

On Sat, Apr 18, 2015 at 06:48:55AM +1000, Aleksa Sarai wrote:
> Do you mean like this?
> 
> #define SUBSYS_TAG_COUNT(_tag) (CGROUP_ ## _tag ## _END - CGROUP_ ##
> _tag ## _START)
> 
> That's fine I guess, I just wanted to match CGROUP_SUBSYS_COUNT in
> semantics, but I'll do that if you prefer it that way.

Not even that, just do it manually.

#define CGROUP_TAGNAME_COUNT	(CGROUP_TAGNAME_END - CGROUP_TAGNAME_START)

At maximum, we're only gonna have a few of these.  No reason to be
smart about it.

-- 
tejun

Re: [PATCH v9 3/4] cgroups: allow a cgroup subsystem to reject a fork

[Aleksa Sarai]

>> Do you mean like this?
>>
>> #define SUBSYS_TAG_COUNT(_tag) (CGROUP_ ## _tag ## _END - CGROUP_ ##
>> _tag ## _START)
>>
>> That's fine I guess, I just wanted to match CGROUP_SUBSYS_COUNT in
>> semantics, but I'll do that if you prefer it that way.
>
> Not even that, just do it manually.
>
> #define CGROUP_TAGNAME_COUNT    (CGROUP_TAGNAME_END - CGROUP_TAGNAME_START)
>
> At maximum, we're only gonna have a few of these.  No reason to be
> smart about it.

Yeah, that's fair I guess.

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

[PATCH v9 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 |   5 +
 init/Kconfig                  |  16 ++
 kernel/Makefile               |   1 +
 kernel/cgroup_pids.c          | 367 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 389 insertions(+)
 create mode 100644 kernel/cgroup_pids.c

diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index 1a5a9aa..eb8bfb4 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -66,6 +66,11 @@ SUBSYS(hugetlb)
  * Subsystems that implement the can_fork() family of callbacks.
  */
 TAG(PREFORK_START)
+
+#if IS_ENABLED(CONFIG_CGROUP_PIDS)
+SUBSYS(pids)
+#endif
+
 TAG(PREFORK_END)
 COUNT(PREFORK)
 
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..5e244b3
--- /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,
+};
-- 
2.3.5