vhost: use vhost_tasks for worker threads

For vhost workers we use the kthread API which inherit's its values from
and checks against the kthreadd thread. This results in the wrong RLIMITs
being checked, so while tools like libvirt try to control the number of
threads based on the nproc rlimit setting we can end up creating more
threads than the user wanted.

This patch has us use the vhost_task helpers which will inherit its
values/checks from the thread that owns the device similar to if we did
a clone in userspace. The vhost threads will now be counted in the nproc
rlimits. And we get features like cgroups and mm sharing automatically,
so we can remove those calls.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

drivers/vhost/vhost.c

@@ -22,11 +22,11 @@
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/kthread.h>
-#include <linux/cgroup.h>
 #include <linux/module.h>
 #include <linux/sort.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/signal.h>
+#include <linux/sched/vhost_task.h>
 #include <linux/interval_tree_generic.h>
 #include <linux/nospec.h>
 #include <linux/kcov.h>
@@ -256,7 +256,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
 		 * test_and_set_bit() implies a memory barrier.
 		 */
 		llist_add(&work->node, &dev->worker->work_list);
-		wake_up_process(dev->worker->task);
+		wake_up_process(dev->worker->vtsk->task);
 	}
 }
 EXPORT_SYMBOL_GPL(vhost_work_queue);
@@ -336,17 +336,14 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 static int vhost_worker(void *data)
 {
 	struct vhost_worker *worker = data;
-	struct vhost_dev *dev = worker->dev;
 	struct vhost_work *work, *work_next;
 	struct llist_node *node;
 
-	kthread_use_mm(dev->mm);
-
 	for (;;) {
 		/* mb paired w/ kthread_stop */
 		set_current_state(TASK_INTERRUPTIBLE);
 
-		if (kthread_should_stop()) {
+		if (vhost_task_should_stop(worker->vtsk)) {
 			__set_current_state(TASK_RUNNING);
 			break;
 		}
@@ -368,7 +365,7 @@ static int vhost_worker(void *data)
 				schedule();
 		}
 	}
-	kthread_unuse_mm(dev->mm);
+
 	return 0;
 }
 
@@ -509,31 +506,6 @@ long vhost_dev_check_owner(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_check_owner);
 
-struct vhost_attach_cgroups_struct {
-	struct vhost_work work;
-	struct task_struct *owner;
-	int ret;
-};
-
-static void vhost_attach_cgroups_work(struct vhost_work *work)
-{
-	struct vhost_attach_cgroups_struct *s;
-
-	s = container_of(work, struct vhost_attach_cgroups_struct, work);
-	s->ret = cgroup_attach_task_all(s->owner, current);
-}
-
-static int vhost_attach_cgroups(struct vhost_dev *dev)
-{
-	struct vhost_attach_cgroups_struct attach;
-
-	attach.owner = current;
-	vhost_work_init(&attach.work, vhost_attach_cgroups_work);
-	vhost_work_queue(dev, &attach.work);
-	vhost_dev_flush(dev);
-	return attach.ret;
-}
-
 /* Caller should have device mutex */
 bool vhost_dev_has_owner(struct vhost_dev *dev)
 {
@@ -580,42 +552,36 @@ static void vhost_worker_free(struct vhost_dev *dev)
 
 	dev->worker = NULL;
 	WARN_ON(!llist_empty(&worker->work_list));
-	kthread_stop(worker->task);
+	vhost_task_stop(worker->vtsk);
 	kfree(worker);
 }
 
 static int vhost_worker_create(struct vhost_dev *dev)
 {
 	struct vhost_worker *worker;
-	struct task_struct *task;
+	struct vhost_task *vtsk;
+	char name[TASK_COMM_LEN];
 	int ret;
 
 	worker = kzalloc(sizeof(*worker), GFP_KERNEL_ACCOUNT);
 	if (!worker)
 		return -ENOMEM;
 
 	dev->worker = worker;
-	worker->dev = dev;
 	worker->kcov_handle = kcov_common_handle();
 	init_llist_head(&worker->work_list);
+	snprintf(name, sizeof(name), "vhost-%d", current->pid);
 
-	task = kthread_create(vhost_worker, worker, "vhost-%d", current->pid);
-	if (IS_ERR(task)) {
-		ret = PTR_ERR(task);
+	vtsk = vhost_task_create(vhost_worker, worker, name);
+	if (!vtsk) {
+		ret = -ENOMEM;
 		goto free_worker;
 	}
 
-	worker->task = task;
-	wake_up_process(task); /* avoid contributing to loadavg */
-
-	ret = vhost_attach_cgroups(dev);
-	if (ret)
-		goto stop_worker;
-
+	worker->vtsk = vtsk;
+	vhost_task_start(vtsk);
 	return 0;
 
-stop_worker:
-	kthread_stop(worker->task);
 free_worker:
 	kfree(worker);
 	dev->worker = NULL;

drivers/vhost/vhost.h

@@ -16,6 +16,7 @@
 #include <linux/irqbypass.h>
 
 struct vhost_work;
+struct vhost_task;
 typedef void (*vhost_work_fn_t)(struct vhost_work *work);
 
 #define VHOST_WORK_QUEUED 1
@@ -26,9 +27,8 @@ struct vhost_work {
 };
 
 struct vhost_worker {
-	struct task_struct	*task;
+	struct vhost_task	*vtsk;
 	struct llist_head	work_list;
-	struct vhost_dev	*dev;
 	u64			kcov_handle;
 };