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Date: Wed, 24 Sep 2008 09:24:46 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
cc: Martin Bligh <mbligh@google.com>, Steven Rostedt <rostedt@goodmis.org>,
       linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@linux-foundation.org>, prasad@linux.vnet.ibm.com,
       Mathieu Desnoyers <compudj@krystal.dyndns.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, David Wilder <dwilder@us.ibm.com>,
       hch@lst.de, Tom Zanussi <zanussi@comcast.net>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer
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On Wed, 24 Sep 2008, Peter Zijlstra wrote:
> 
> So when we reserve we get a pointer into page A, but our reserve length
> will run over into page B. A write() method will know how to check for
> this and break up the memcpy to copy up-to the end of A and continue
> into B.

I would suggest just not allowing page straddling.

Yeah, it would limit event size to less than a page, but seriously, do 
people really want more than that? If you have huge events, I suspect it 
would be a hell of a lot better to support some kind of indirection 
scheme than to force the ring buffer to handle insane cases.

Most people will want the events to be as _small_ as humanly possible. The 
normal event size should hopefully be in the 8-16 bytes, and I think the 
RFC patch is already broken because it allocates that insane 64-bit event 
counter for things. Who the hell wants a 64-bit event counter that much? 
That's broken.

		Linus