Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set without mapping

[Guenter Roeck <linux@roeck-us.net>]

On 6/11/24 12:28, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> 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@goodmis.org>
> ---
>   kernel/trace/ring_buffer.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 28853966aa9a..78beaccf9c8c 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5224,6 +5224,9 @@ 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 +6170,7 @@ 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);
>   	}
> @@ -6359,12 +6362,13 @@ 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);
> +

Picky again. Is that a leftover from something ? I don't see an immediate reason
for the added newline.

>   	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;
> +		cpu_buffer->mapped++;
>   		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>   	} else {
>   		kfree(cpu_buffer->subbuf_ids);
> @@ -6403,7 +6407,8 @@ 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;
> +	WARN_ON_ONCE(!cpu_buffer->mapped);
> +	cpu_buffer->mapped--;

This will wrap to UINT_MAX if it was 0. Is that intentional ?

>   
>   	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>