linux-kernel - Re: [PATCH v19 RESEND 3/5] tracing: Allow user-space mapping of the ring-buffer

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Date: Wed, 3 Apr 2024 10:13:52 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: mhiramat@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
 kernel-team@...roid.com, linux-mm@...ck.org
Subject: Re: [PATCH v19 RESEND 3/5] tracing: Allow user-space mapping of the
 ring-buffer

On Fri, 29 Mar 2024 14:40:55 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> > +static vm_fault_t tracing_buffers_mmap_fault(struct vm_fault *vmf)
> > +{
> > +	return VM_FAULT_SIGBUS;
> > +}  
> 
> If this is all it does, I don't believe it's needed.
> 
> > +
> > +#ifdef CONFIG_TRACER_MAX_TRACE
> > +static int get_snapshot_map(struct trace_array *tr)
> > +{
> > +	int err = 0;
> > +
> > +	/*
> > +	 * Called with mmap_lock held. lockdep would be unhappy if we would now
> > +	 * take trace_types_lock. Instead use the specific
> > +	 * snapshot_trigger_lock.
> > +	 */
> > +	spin_lock(&tr->snapshot_trigger_lock);
> > +
> > +	if (tr->snapshot || tr->mapped == UINT_MAX)
> > +		err = -EBUSY;
> > +	else
> > +		tr->mapped++;
> > +
> > +	spin_unlock(&tr->snapshot_trigger_lock);
> > +
> > +	/* Wait for update_max_tr() to observe iter->tr->mapped */
> > +	if (tr->mapped == 1)
> > +		synchronize_rcu();
> > +
> > +	return err;
> > +
> > +}
> > +static void put_snapshot_map(struct trace_array *tr)
> > +{
> > +	spin_lock(&tr->snapshot_trigger_lock);
> > +	if (!WARN_ON(!tr->mapped))
> > +		tr->mapped--;
> > +	spin_unlock(&tr->snapshot_trigger_lock);
> > +}
> > +#else
> > +static inline int get_snapshot_map(struct trace_array *tr) { return 0; }
> > +static inline void put_snapshot_map(struct trace_array *tr) { }
> > +#endif
> > +
> > +static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
> > +{
> > +	struct ftrace_buffer_info *info = vma->vm_file->private_data;
> > +	struct trace_iterator *iter = &info->iter;
> > +
> > +	WARN_ON(ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file));
> > +	put_snapshot_map(iter->tr);
> > +}
> > +
> > +static void tracing_buffers_mmap_open(struct vm_area_struct *vma) { }  
> 
> Same for the open.
> 
> 
> > +
> > +static const struct vm_operations_struct tracing_buffers_vmops = {
> > +	.open		= tracing_buffers_mmap_open,
> > +	.close		= tracing_buffers_mmap_close,
> > +	.fault		= tracing_buffers_mmap_fault,
> > +};  
> 
> I replaced this with:
> 
> static const struct vm_operations_struct tracing_buffers_vmops = {
> 	.close		= tracing_buffers_mmap_close,
> };
> 
> And it appears to work just fine. The mm code handles the NULL cases for
> .open and .fault.
> 
> Is there any reason to do something different than the mm defaults?

Hi Vincent,

Do you plan on sending out a v20 series?

-- Steve