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Date:   Tue, 5 Oct 2021 12:10:55 -0500
From:   Mike Christie <michael.christie@...cle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     geert@...ux-m68k.org, vverma@...italocean.com, hdanton@...a.com,
        hch@...radead.org, stefanha@...hat.com, jasowang@...hat.com,
        mst@...hat.com, sgarzare@...hat.com,
        virtualization@...ts.linux-foundation.org, axboe@...nel.dk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 5/9] fork: add helper to clone a process

On 10/5/21 7:50 AM, Christian Brauner wrote:
> 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@...cle.com>
>> Acked-by: Christian Brauner <christian.brauner@...ntu.com>
>> ---
>>  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.
> 

I'm not sure how to handle this comment, because I might have misread
an older comment or made it up in my head.

KERN_WORK_USER is only set on the kernel_clone_args, so at this point we
don't have that struct available anymore.

I didn't add a new PF_KTHREAD_WORK_USER flag to sched.h, because I thought
I had got a review comment to not add another PF flag for this. However, I
can't seem to find that comment now so I'm not sure if maybe I misread a
comment or made it up.

If it's ok I could add a PF_KTHREAD_WORK_USER, then do a:

WARN_ON(!(tsk->flags & PF_KTHREAD_WORK_USER)

so future developers get loud feedback they are doing the
wrong thing right away.

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