Re: [PATCH v5 01/13] ring-buffer: Allow mapped field to be set without mapping

[Steven Rostedt]

On Tue, 11 Jun 2024 22:16:43 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> 
> In preparation for having the ring buffer mapped to a dedicated location,
> which will have the same restrictions as user space memory mapped buffers,
> allow it to use the "mapped" field of the ring_buffer_per_cpu structure
> without having the user space meta page mapping.
> 
> When this starts using the mapped field, it will need to handle adding a
> user space mapping (and removing it) from a ring buffer that is using a
> dedicated memory range.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> ---
>  kernel/trace/ring_buffer.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 28853966aa9a..aa8eb878e0d4 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -491,6 +491,7 @@ struct ring_buffer_per_cpu {
>  	unsigned long			pages_removed;
>  
>  	unsigned int			mapped;
> +	unsigned int			user_mapped;	/* first user space mapping */

This actually needs to be a counter and not just save the mapped value
when it first gets set :-p

As the mappings could technically be incremented and decremented
differently.

I'll send a new patch. But since my long test keeps failing for subtle
things, I'm not going to post another series until the full test passes.

-- Steve


>  	struct mutex			mapping_lock;
>  	unsigned long			*subbuf_ids;	/* ID to subbuf VA */
>  	struct trace_buffer_meta	*meta_page;
> @@ -5224,6 +5225,9 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>  {
>  	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
>  
> +	if (!meta)
> +		return;
> +
>  	meta->reader.read = cpu_buffer->reader_page->read;
>  	meta->reader.id = cpu_buffer->reader_page->id;
>  	meta->reader.lost_events = cpu_buffer->lost_events;
> @@ -6167,7 +6171,7 @@ rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
>  
>  	mutex_lock(&cpu_buffer->mapping_lock);
>  
> -	if (!cpu_buffer->mapped) {
> +	if (!cpu_buffer->mapped || !cpu_buffer->meta_page) {
>  		mutex_unlock(&cpu_buffer->mapping_lock);
>  		return ERR_PTR(-ENODEV);
>  	}
> @@ -6194,7 +6198,7 @@ static int __rb_inc_dec_mapped(struct ring_buffer_per_cpu *cpu_buffer,
>  	if (inc && cpu_buffer->mapped == UINT_MAX)
>  		return -EBUSY;
>  
> -	if (WARN_ON(!inc && cpu_buffer->mapped == 0))
> +	if (WARN_ON(!inc && cpu_buffer->mapped < cpu_buffer->user_mapped))
>  		return -EINVAL;
>  
>  	mutex_lock(&cpu_buffer->buffer->mutex);
> @@ -6328,7 +6332,7 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
>  
>  	mutex_lock(&cpu_buffer->mapping_lock);
>  
> -	if (cpu_buffer->mapped) {
> +	if (cpu_buffer->user_mapped) {
>  		err = __rb_map_vma(cpu_buffer, vma);
>  		if (!err)
>  			err = __rb_inc_dec_mapped(cpu_buffer, true);
> @@ -6359,12 +6363,15 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
>  	 */
>  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>  	rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
> +
>  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  
>  	err = __rb_map_vma(cpu_buffer, vma);
>  	if (!err) {
>  		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> -		cpu_buffer->mapped = 1;
> +		/* This is the first time it is mapped externally */
> +		cpu_buffer->mapped++;
> +		cpu_buffer->user_mapped = cpu_buffer->mapped;
>  		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  	} else {
>  		kfree(cpu_buffer->subbuf_ids);
> @@ -6392,10 +6399,10 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
>  
>  	mutex_lock(&cpu_buffer->mapping_lock);
>  
> -	if (!cpu_buffer->mapped) {
> +	if (!cpu_buffer->user_mapped) {
>  		err = -ENODEV;
>  		goto out;
> -	} else if (cpu_buffer->mapped > 1) {
> +	} else if (cpu_buffer->mapped > cpu_buffer->user_mapped) {
>  		__rb_inc_dec_mapped(cpu_buffer, false);
>  		goto out;
>  	}
> @@ -6403,7 +6410,10 @@ int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
>  	mutex_lock(&buffer->mutex);
>  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>  
> -	cpu_buffer->mapped = 0;
> +	/* This is the last user space mapping */
> +	if (!WARN_ON_ONCE(cpu_buffer->mapped != cpu_buffer->user_mapped))
> +		cpu_buffer->mapped--;
> +	cpu_buffer->user_mapped = 0;
>  
>  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>