linux-kernel - Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping functions

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Message-ID: <ZCQussSh5h/GSwh6@google.com>
Date:   Wed, 29 Mar 2023 13:27:30 +0100
From:   Vincent Donnefort <vdonnefort@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping
 functions

On Wed, Mar 29, 2023 at 08:07:58AM -0400, Steven Rostedt wrote:
> On Wed, 29 Mar 2023 07:03:53 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > struct ring_buffer_meta_page_header {
> > #if __BITS_PER_LONG == 64
> > 	__u64	entries;
> > 	__u64	overrun;
> > #else
> > 	__u32	entries;
> > 	__u32	overrun;
> > #endif
> > 	__u32	pages_touched;
> > 	__u32	meta_page_size;
> > 	__u32	reader_page;	/* page ID for the reader page */
> > 	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
> > };
> > 
> > BTW, shouldn't the nr_data_pages take into account the reader page? As it
> > is part of the array we traverse isn't it?
> 
> Ah, I guess nr_data_pages is the length of the index mapping, not the
> array of pages mapped?

Yes correct, data_pages[nr_data_pages] and the reader_page being excluded...
which might not be the easiest interface, as the size of the buffer to read
depends on if the reader_page has data to be read or not.

> 
> -- Steve