clone3: allow spawning processes into cgroups

This adds support for creating a process in a different cgroup than its
parent. Callers can limit and account processes and threads right from
the moment they are spawned:
- A service manager can directly spawn new services into dedicated
  cgroups.
- A process can be directly created in a frozen cgroup and will be
  frozen as well.
- The initial accounting jitter experienced by process supervisors and
  daemons is eliminated with this.
- Threaded applications or even thread implementations can choose to
  create a specific cgroup layout where each thread is spawned
  directly into a dedicated cgroup.

This feature is limited to the unified hierarchy. Callers need to pass
a directory file descriptor for the target cgroup. The caller can
choose to pass an O_PATH file descriptor. All usual migration
restrictions apply, i.e. there can be no processes in inner nodes. In
general, creating a process directly in a target cgroup adheres to all
migration restrictions.

One of the biggest advantages of this feature is that CLONE_INTO_GROUP does
not need to grab the write side of the cgroup cgroup_threadgroup_rwsem.
This global lock makes moving tasks/threads around super expensive. With
clone3() this lock is avoided.

Cc: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: cgroups@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 138059eb730d..511af87f685e 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -33,6 +33,7 @@
 #include <linux/atomic.h>
 #include <linux/cgroup.h>
 #include <linux/slab.h>
+#include <linux/sched/task.h>
 
 #define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
 #define PIDS_MAX_STR "max"
@@ -214,13 +215,16 @@ static void pids_cancel_attach(struct cgroup_taskset *tset)
  * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
  * on cgroup_threadgroup_change_begin() held by the copy_process().
  */
-static int pids_can_fork(struct task_struct *task)
+static int pids_can_fork(struct task_struct *task, struct css_set *cset)
 {
 	struct cgroup_subsys_state *css;
 	struct pids_cgroup *pids;
 	int err;
 
-	css = task_css_check(current, pids_cgrp_id, true);
+	if (cset)
+		css = cset->subsys[pids_cgrp_id];
+	else
+		css = task_css_check(current, pids_cgrp_id, true);
 	pids = css_pids(css);
 	err = pids_try_charge(pids, 1);
 	if (err) {
@@ -235,12 +239,15 @@ static int pids_can_fork(struct task_struct *task)
 	return err;
 }
 
-static void pids_cancel_fork(struct task_struct *task)
+static void pids_cancel_fork(struct task_struct *task, struct css_set *cset)
 {
 	struct cgroup_subsys_state *css;
 	struct pids_cgroup *pids;
 
-	css = task_css_check(current, pids_cgrp_id, true);
+	if (cset)
+		css = cset->subsys[pids_cgrp_id];
+	else
+		css = task_css_check(current, pids_cgrp_id, true);
 	pids = css_pids(css);
 	pids_uncharge(pids, 1);
 }