linux-kernel - Re: [PATCH v10 Golden] Unified trace buffer

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Message-ID: <alpine.DEB.1.10.0809301350410.6224@gandalf.stny.rr.com>
Date:	Tue, 30 Sep 2008 13:56:36 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com, "Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v10 Golden] Unified trace buffer


On Tue, 30 Sep 2008, Peter Zijlstra wrote:
> 
> The problem with storing the page link information inside the page is
> that it doesnt transfer to another address space, so if you do indeed
> mmap these pages, then the link information is bogus.
> 
> Of course, in such a situation you could ignore these headers, but
> somehow that doesn't sound too apealing.


No that's not what I'm proposing. I'm proposing to allocate a page_header 
structure for every page we alloc, and make a link list of them.
In other words:


struct ring_buffer_per_cpu {
	[...]
	struct list_head pages;
	[...]
};

struct buffer_page {
	[...];
	void *page;
	struct list_head list;
	[...];
};

In ring_buffer_allocate_cpu:

	struct buffer_page *bpage;
	struct unsigned long addr;

	[...]

	for every page() {
		bpage = kzalloc(sizeof(*bpage), GFP_KERNEL);
		addr = get_free_page();
		bpage->page = (void *)addr;
		list_add(&bpage->list, &cpu_buffer->pages);
	}


Obviously need to add the error checking, but you get the idea. Here I do 
not need to change any of the later logic, because we are still dealing 
with the buffer_page. I only need to update way to index the page which is 
already encapsulated in its own function.

-- Steve


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