Re: [PATCH v5 1/4] ring-buffer: add ring_buffer_memory_size()

[Vincent Donnefort <vdonnefort@google.com>]

On Thu, Nov 05, 2026 at 10:02:18PM +0800, Xiang Gao wrote:
> Add a helper to report the page memory consumed by a ring buffer for a
> given CPU. It accounts for the data sub-buffers, the reader page, the
> cached read page, and the mmap metadata page. Unlike
> ring_buffer_capacity(), which reports the usable data capacity, this
> reports the full pages allocated to the buffer.
> 
> The function is generic and returns a size for any buffer type, so
> callers that need the memory footprint of remote or persistent buffers
> can use it as well.
> 
> Signed-off-by: Xiang Gao <gaoxiang17@xiaomi.com>
> ---
>  include/linux/ring_buffer.h |  1 +
>  kernel/trace/ring_buffer.c  | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index 649b2276b557..e83e94623e76 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -174,6 +174,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter);
>  bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter);
>  
>  unsigned long ring_buffer_capacity(struct trace_buffer *buffer, int cpu);
> +unsigned long ring_buffer_memory_size(struct trace_buffer *buffer, int cpu);
>  unsigned long ring_buffer_max_event_size(struct trace_buffer *buffer);
>  
>  void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu);
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 1fd28a72e6ee..0396ec671998 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -6566,6 +6566,43 @@ unsigned long ring_buffer_capacity(struct trace_buffer *buffer, int cpu)
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_capacity);
>  
> +/**
> + * ring_buffer_memory_size - return the memory used by the buffer (in bytes)
> + * @buffer: The ring buffer.
> + * @cpu: The CPU to get ring buffer memory from.
> + *
> + * Returns the page memory consumed by @cpu, including the data sub-buffers,
> + * the reader page, the cached read page, and the mmap metadata page. Unlike
> + * ring_buffer_capacity(), which reports the usable data capacity, this
> + * accounts for the full pages allocated to the buffer.
> + */
> +unsigned long ring_buffer_memory_size(struct trace_buffer *buffer, int cpu)

memory_size() may be confusing because we don't account for all the other
kmalloc() allocations. 

I have also just realised, do we really need yet another accessor? With the
existing ones:

  nr_pages = ring_buffer_capacity(buffer, cpu) / (ring_buffer_subbuf_size_get(buffer) - BUF_PAGE_HDR_SIZE);

(Alhough there's a race with ring_buffer_subbuf_size_set() to be resolved!!!)

And if the caller knows if a cpu_buffer is a remote, or a persistent buffer, it
can just add nr_pages++.

This only leaves free_page unaccounted for. But does that really matter if the
other solution would be to have a very racy free_page accounting?

-- 
Vincent

> +{
> +	struct ring_buffer_per_cpu *cpu_buffer;
> +	unsigned long subbuf_size;
> +	unsigned long size;
> +
> +	if (!cpumask_test_cpu(cpu, buffer->cpumask))
> +		return 0;
> +
> +	cpu_buffer = buffer->buffers[cpu];
> +	subbuf_size = PAGE_SIZE << READ_ONCE(buffer->subbuf_order);
> +
> +	/* Data sub-buffers plus the reader page. */
> +	size = (READ_ONCE(cpu_buffer->nr_pages) + 1) * subbuf_size;
> +
> +	/* The cached read page, if present, is a full sub-buffer page. */
> +	if (READ_ONCE(cpu_buffer->free_page.data))
> +		size += subbuf_size;
> +
> +	/* The mmap metadata page is a single system page. */
> +	if (READ_ONCE(cpu_buffer->meta_page))
> +		size += PAGE_SIZE;
> +
> +	return size;
> +}
> +EXPORT_SYMBOL_GPL(ring_buffer_memory_size);
> +
>  /**
>   * ring_buffer_max_event_size - return the max data size of an event
>   * @buffer: The ring buffer.
> -- 
> 2.34.1
>