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@xxxxxxxxxx> --- 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
