vhost_task: Allow vhost layer to use copy_process

Qemu will create vhost devices in the kernel which perform network, SCSI,
etc IO and management operations from worker threads created by the
kthread API. Because the kthread API does a copy_process on the kthreadd
thread, the vhost layer has to use kthread_use_mm to access the Qemu
thread's memory and cgroup_attach_task_all to add itself to the Qemu
thread's cgroups, and it bypasses the RLIMIT_NPROC limit which can result
in VMs creating more threads than the admin expected.

This patch adds a new struct vhost_task which can be used instead of
kthreads. They allow the vhost layer to use copy_process and inherit
the userspace process's mm and cgroups, the task is accounted for
under the userspace's nproc count and can be seen in its process tree,
and other features like namespaces work and are inherited by default.

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>

MAINTAINERS

@@ -22176,7 +22176,9 @@ L:	virtualization@lists.linux-foundation.org
 L:	netdev@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git
+F:	kernel/vhost_task.c
 F:	drivers/vhost/
+F:	include/linux/sched/vhost_task.h
 F:	include/linux/vhost_iotlb.h
 F:	include/uapi/linux/vhost.h
 

drivers/vhost/Kconfig

@@ -13,9 +13,14 @@ config VHOST_RING
 	  This option is selected by any driver which needs to access
 	  the host side of a virtio ring.
 
+config VHOST_TASK
+	bool
+	default n
+
 config VHOST
 	tristate
 	select VHOST_IOTLB
+	select VHOST_TASK
 	help
 	  This option is selected by any driver which needs to access
 	  the core of vhost.

include/linux/sched/vhost_task.h

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_VHOST_TASK_H
+#define _LINUX_VHOST_TASK_H
+
+#include <linux/completion.h>
+
+struct task_struct;
+
+struct vhost_task {
+	int (*fn)(void *data);
+	void *data;
+	struct completion exited;
+	unsigned long flags;
+	struct task_struct *task;
+};
+
+struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
+				     const char *name);
+void vhost_task_start(struct vhost_task *vtsk);
+void vhost_task_stop(struct vhost_task *vtsk);
+bool vhost_task_should_stop(struct vhost_task *vtsk);
+
+#endif

kernel/Makefile

@@ -15,6 +15,7 @@ obj-y     = fork.o exec_domain.o panic.o \
 obj-$(CONFIG_USERMODE_DRIVER) += usermode_driver.o
 obj-$(CONFIG_MODULES) += kmod.o
 obj-$(CONFIG_MULTIUSER) += groups.o
+obj-$(CONFIG_VHOST_TASK) += vhost_task.o
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace internal ftrace files

kernel/vhost_task.c

@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2021 Oracle Corporation
+ */
+#include <linux/slab.h>
+#include <linux/completion.h>
+#include <linux/sched/task.h>
+#include <linux/sched/vhost_task.h>
+#include <linux/sched/signal.h>
+
+enum vhost_task_flags {
+	VHOST_TASK_FLAGS_STOP,
+};
+
+static int vhost_task_fn(void *data)
+{
+	struct vhost_task *vtsk = data;
+	int ret;
+
+	ret = vtsk->fn(vtsk->data);
+	complete(&vtsk->exited);
+	do_exit(ret);
+}
+
+/**
+ * vhost_task_stop - stop a vhost_task
+ * @vtsk: vhost_task to stop
+ *
+ * Callers must call vhost_task_should_stop and return from their worker
+ * function when it returns true;
+ */
+void vhost_task_stop(struct vhost_task *vtsk)
+{
+	pid_t pid = vtsk->task->pid;
+
+	set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
+	wake_up_process(vtsk->task);
+	/*
+	 * Make sure vhost_task_fn is no longer accessing the vhost_task before
+	 * freeing it below. If userspace crashed or exited without closing,
+	 * then the vhost_task->task could already be marked dead so
+	 * kernel_wait will return early.
+	 */
+	wait_for_completion(&vtsk->exited);
+	/*
+	 * If we are just closing/removing a device and the parent process is
+	 * not exiting then reap the task.
+	 */
+	kernel_wait4(pid, NULL, __WCLONE, NULL);
+	kfree(vtsk);
+}
+EXPORT_SYMBOL_GPL(vhost_task_stop);
+
+/**
+ * vhost_task_should_stop - should the vhost task return from the work function
+ * @vtsk: vhost_task to stop
+ */
+bool vhost_task_should_stop(struct vhost_task *vtsk)
+{
+	return test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
+}
+EXPORT_SYMBOL_GPL(vhost_task_should_stop);
+
+/**
+ * vhost_task_create - create a copy of a process to be used by the kernel
+ * @fn: thread stack
+ * @arg: data to be passed to fn
+ * @name: the thread's name
+ *
+ * This returns a specialized task for use by the vhost layer or NULL on
+ * failure. The returned task is inactive, and the caller must fire it up
+ * through vhost_task_start().
+ */
+struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
+				     const char *name)
+{
+	struct kernel_clone_args args = {
+		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM,
+		.exit_signal	= 0,
+		.fn		= vhost_task_fn,
+		.name		= name,
+		.user_worker	= 1,
+		.no_files	= 1,
+		.ignore_signals	= 1,
+	};
+	struct vhost_task *vtsk;
+	struct task_struct *tsk;
+
+	vtsk = kzalloc(sizeof(*vtsk), GFP_KERNEL);
+	if (!vtsk)
+		return NULL;
+	init_completion(&vtsk->exited);
+	vtsk->data = arg;
+	vtsk->fn = fn;
+
+	args.fn_arg = vtsk;
+
+	tsk = copy_process(NULL, 0, NUMA_NO_NODE, &args);
+	if (IS_ERR(tsk)) {
+		kfree(vtsk);
+		return NULL;
+	}
+
+	vtsk->task = tsk;
+	return vtsk;
+}
+EXPORT_SYMBOL_GPL(vhost_task_create);
+
+/**
+ * vhost_task_start - start a vhost_task created with vhost_task_create
+ * @vtsk: vhost_task to wake up
+ */
+void vhost_task_start(struct vhost_task *vtsk)
+{
+	wake_up_new_task(vtsk->task);
+}
+EXPORT_SYMBOL_GPL(vhost_task_start);