Re: [PATCH 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Mar 20, 2023 at 09:45:16PM -0400, Steven Rostedt wrote:
> On Fri, 17 Mar 2023 14:33:09 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > Also, the meta-page being... a single page, this limits at the moment the
> > number of pages in the ring-buffer that can be mapped: ~3MB on a 4K pages
> > system.
> 
> I hate this limitation, so I fixed it ;-)

Thanks a lot for having a look. Do you mind if I fold this in my patch for a V2?

> 
> I added a meta_page_size field to the meta page, and user space can do:
> 
> 	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
> 	if (meta == MAP_FAILED)
> 		pdie("mmap");
> 
> 	map = meta;
> 	meta_len = map->meta_page_size;
> 
> 	if (meta_len > page_size) {
> 		munmap(meta, page_size);
> 		meta = mmap(NULL, meta_len, PROT_READ, MAP_SHARED, fd, 0);
> 		if (meta == MAP_FAILED)
> 			pdie("mmap");
> 		map = meta;
> 	}
> 
> This appears to work (but I'm still testing it).
> 
> -- Steve
> 
> diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> index 24bcec754a35..12f3f7ee33d9 100644
> --- a/include/uapi/linux/trace_mmap.h
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -18,6 +18,7 @@ struct ring_buffer_meta_page {
>  	__u32	reader_page;
>  	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
>  	__u32	data_page_head;	/* index of data_pages[] */
> +	__u32	meta_page_size;	/* size of the meta page */

Do we want a specific field here? That could be deduced from nr_data_pages()
quite easily?


>  	__u32	data_pages[];
>  };
>  
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 10a17e78cfe6..77c92e4a7adc 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -526,6 +526,7 @@ struct ring_buffer_per_cpu {
>  	u64				read_stamp;
>  
>  	int				mapped;
> +	int				meta_order;
>  	struct mutex			mapping_lock;
>  	unsigned long			*page_ids;	/* ID to addr */
>  	struct ring_buffer_meta_page	*meta_page;
> @@ -5898,7 +5899,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  EXPORT_SYMBOL_GPL(ring_buffer_read_page);
>  
>  #define META_PAGE_MAX_PAGES \
> -	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_page_head))) >> 2)
> +	((PAGE_SIZE - (offsetof(struct ring_buffer_meta_page, data_pages))) >> 2)
>

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