[PATCH v2] KVM: Move VM's worker kthreads back to the original cgroups before exiting.

[Vipin Sharma <vipinsh@google.com>]

VM worker kthreads can linger in the VM process's cgroup for sometime
after KVM terminates the VM process.

KVM terminates the worker kthreads by calling kthread_stop() which waits
on the 'exited' completion, triggered by exit_mm(), via mm_release(),
during kthread's exit.  However, these kthreads are removed from the
cgroup using cgroup_exit() call which happens after exit_mm(). A VM
process can terminate between the time window of exit_mm() to
cgroup_exit(), leaving only worker kthreads in the cgroup.

Moving worker kthreads back to the original cgroup (kthreadd_task's
cgroup) makes sure that cgroup is empty as soon as the main VM process
is terminated.

kthreadd_task is not an exported symbol which causes build errors if KVM
is built as a loadable module. Both users (kvm_main & vhost) of
cgroup_attach_task_all(), have the same issue, therefore, using
kthreadd_task as a default option is chosen when the API is called with
NULL argument.

Signed-off-by: Vipin Sharma <vipinsh@google.com>
---

v2:
- Use kthreadd_task in the cgroup API to avoid build issue.

v1: https://lore.kernel.org/lkml/20211214050708.4040200-1-vipinsh@google.com/

 kernel/cgroup/cgroup-v1.c |  5 +++++
 virt/kvm/kvm_main.c       | 15 ++++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 81c9e0685948..81d4b2f2acf0 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -51,6 +51,8 @@ bool cgroup1_ssid_disabled(int ssid)
  * @from: attach to all cgroups of a given task
  * @tsk: the task to be attached
  *
+ * If @from is NULL then use kthreadd_task for finding the destination cgroups.
+ *
  * Return: %0 on success or a negative errno code on failure
  */
 int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
@@ -58,6 +60,9 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
 	struct cgroup_root *root;
 	int retval = 0;
 
+	if (!from)
+		from = kthreadd_task;
+
 	mutex_lock(&cgroup_mutex);
 	percpu_down_write(&cgroup_threadgroup_rwsem);
 	for_each_root(root) {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b0f7e6eb00ff..f7504578c374 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5785,7 +5785,7 @@ static int kvm_vm_worker_thread(void *context)
 	init_context = NULL;
 
 	if (err)
-		return err;
+		goto out;
 
 	/* Wait to be woken up by the spawner before proceeding. */
 	kthread_parkme();
@@ -5793,6 +5793,19 @@ static int kvm_vm_worker_thread(void *context)
 	if (!kthread_should_stop())
 		err = thread_fn(kvm, data);
 
+out:
+	/*
+	 * We need to move the kthread back to its original cgroups, so that it
+	 * doesn't linger in the cgroups of the user process after the user
+	 * process has already terminated.
+	 *
+	 * kthread_stop() waits on 'exited' completion condition which is set
+	 * in exit_mm(), via mm_release(), in do_exit(). However, kthread
+	 * is removed from cgroups in the cgroup_exit() which is called after
+	 * exit_mm(). This causes lingering of kthreads in cgroups after main
+	 * VM process has finished.
+	 */
+	WARN_ON(cgroup_attach_task_all(NULL, current));
 	return err;
 }
 

base-commit: 5e4e84f1124aa02643833b7ea40abd5a8e964388
-- 
2.34.1.307.g9b7440fafd-goog