Re: [PATCH V3 5/9] fork: add helper to clone a process

[Christian Brauner]

On Mon, Oct 04, 2021 at 02:21:24PM -0500, Mike Christie wrote:
> The vhost layer has similar requirements as io_uring where its worker
> threads need to access the userspace thread's memory, want to inherit the
> parents's cgroups and namespaces, and be checked against the parent's
> RLIMITs. Right now, the vhost layer uses the kthread API which has
> kthread_use_mm for mem access, and those threads can use
> cgroup_attach_task_all for v1 cgroups, but there are no helpers for the
> other items.
> 
> This adds a helper to clone a process so we can inherit everything we
> want in one call. It's a more generic version of create_io_thread which
> will be used by the vhost layer and io_uring in later patches in this set.
> 
> Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
> Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> ---
>  include/linux/sched/task.h |  6 ++++-
>  kernel/fork.c              | 48 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 53 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index e165cc67fd3c..ba0499b6627c 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -87,7 +87,11 @@ extern void exit_files(struct task_struct *);
>  extern void exit_itimers(struct signal_struct *);
>  
>  extern pid_t kernel_clone(struct kernel_clone_args *kargs);
> -struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node);
> +struct task_struct *create_io_thread(int (*fn)(void *i), void *arg, int node);
> +struct task_struct *kernel_worker(int (*fn)(void *), void *arg, int node,
> +				  unsigned long clone_flags, u32 worker_flags);
> +__printf(2, 3)
> +void kernel_worker_start(struct task_struct *tsk, const char namefmt[], ...);
>  struct task_struct *fork_idle(int);
>  struct mm_struct *copy_init_mm(void);
>  extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 98264cf1d6a6..3f3fcabffa5f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2540,6 +2540,54 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
>  	return copy_process(NULL, 0, node, &args);
>  }
>  
> +/**
> + * kernel_worker - create a copy of a process to be used by the kernel
> + * @fn: thread stack
> + * @arg: data to be passed to fn
> + * @node: numa node to allocate task from
> + * @clone_flags: CLONE flags
> + * @worker_flags: KERN_WORKER flags
> + *
> + * This returns a created task, or an error pointer. The returned task is
> + * inactive, and the caller must fire it up through kernel_worker_start(). If
> + * this is an PF_IO_WORKER all singals but KILL and STOP are blocked.
> + */
> +struct task_struct *kernel_worker(int (*fn)(void *), void *arg, int node,
> +				  unsigned long clone_flags, u32 worker_flags)
> +{
> +	struct kernel_clone_args args = {
> +		.flags		= ((lower_32_bits(clone_flags) | CLONE_VM |
> +				   CLONE_UNTRACED) & ~CSIGNAL),
> +		.exit_signal	= (lower_32_bits(clone_flags) & CSIGNAL),
> +		.stack		= (unsigned long)fn,
> +		.stack_size	= (unsigned long)arg,
> +		.worker_flags	= KERN_WORKER_USER | worker_flags,
> +	};
> +
> +	return copy_process(NULL, 0, node, &args);
> +}
> +EXPORT_SYMBOL_GPL(kernel_worker);
> +
> +/**
> + * kernel_worker_start - Start a task created with kernel_worker
> + * @tsk: task to wake up
> + * @namefmt: printf-style format string for the thread name
> + * @arg: arguments for @namefmt
> + */
> +void kernel_worker_start(struct task_struct *tsk, const char namefmt[], ...)
> +{
> +	char name[TASK_COMM_LEN];
> +	va_list args;

You could think about reporting an error from this function if
KERN_WORK_USER isn't set or only call the below when KERN_WORK_USER is
set. Both options are fine.

> +
> +	va_start(args, namefmt);
> +	vsnprintf(name, sizeof(name), namefmt, args);
> +	set_task_comm(tsk, name);
> +	va_end(args);
> +
> +	wake_up_new_task(tsk);
> +}
> +EXPORT_SYMBOL_GPL(kernel_worker_start);
> +
>  /*
>   *  Ok, this is the main fork-routine.
>   *
> -- 
> 2.25.1
>