linux-kernel - Re: [PATCH v9 1/2] ring-buffer: Introducing ring-buffer mapping functions

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Message-ID: <20231221125813.673293a2@gandalf.local.home>
Date: Thu, 21 Dec 2023 12:58:13 -0500
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, kernel-team@...roid.com
Subject: Re: [PATCH v9 1/2] ring-buffer: Introducing ring-buffer mapping
 functions

On Thu, 21 Dec 2023 17:35:22 +0000
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> @@ -5999,6 +6078,307 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);
>  

The kernel developers have agreed to allow loop variables to be declared in
loops. This will simplify these macros:



> +#define subbuf_page(off, start) \
> +	virt_to_page((void *)(start + (off << PAGE_SHIFT)))
> +
> +#define foreach_subbuf_page(off, sub_order, start, page)	\
> +	for (off = 0, page = subbuf_page(0, start);		\
> +	     off < (1 << sub_order);				\
> +	     off++, page = subbuf_page(off, start))

#define foreach_subbuf_page(sub_order, start, page)		\
	for (int __off = 0, page = subbuf_page(0, (start));	\
	     __off < (1 << (sub_order));			\
	     __off++, page = subbuf_page(__off, (start)))

And parameters as r-values should always be wrapped in parenthesis.

> +
> +static inline void subbuf_map_prepare(unsigned long subbuf_start, int order)
> +{
> +	struct page *page;
> +	int subbuf_off;

Then you can remove the "subbuf_off".

> +
> +	/*
> +	 * When allocating order > 0 pages, only the first struct page has a
> +	 * refcount > 1. Increasing the refcount here ensures none of the struct
> +	 * page composing the sub-buffer is freeed when the mapping is closed.

Nice catch by the way ;-)

-- Steve

> +	 */
> +	foreach_subbuf_page(subbuf_off, order, subbuf_start, page)
> +		page_ref_inc(page);
> +}
> +
> +static inline void subbuf_unmap(unsigned long subbuf_start, int order)
> +{
> +	struct page *page;
> +	int subbuf_off;
> +
> +	foreach_subbuf_page(subbuf_off, order, subbuf_start, page) {
> +		page_ref_dec(page);
> +		page->mapping = NULL;
> +	}
> +}
> +