[PATCH] 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 temrinates the VM process.

KVM terminates the worker kthreads by calling kthread_stop() which waits
on the signal generated by exit_mm() in do_exit() during kthread's exit.
However, these kthreads are removed from the cgroup using cgroup_exit()
call which happens after exit_mm() in do_exit(). 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.

Signed-off-by: Vipin Sharma <vipinsh@google.com>
---
 virt/kvm/kvm_main.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b0f7e6eb00ff..edd304a18f16 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 +5793,15 @@ 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 that has
+	 * already terminated. exit_mm() in do_exit() signals kthread_stop() to
+	 * return, whereas, removal of the task from the cgroups happens in
+	 * cgroup_exit() which happens after exit_mm().
+	 */
+	WARN_ON(cgroup_attach_task_all(kthreadd_task, current));
 	return err;
 }
 

base-commit: d8f6ef45a623d650f9b97e11553adb4978f6aa70
-- 
2.34.1.173.g76aa8bc2d0-goog