linux-kernel - Re: [PATCH 1/2] perf: Add persistent event facilities

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Date:	Fri, 18 May 2012 15:37:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...64.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <borislav.petkov@....com>
Subject: Re: [PATCH 1/2] perf: Add persistent event facilities

On Fri, 2012-05-18 at 14:55 +0200, Borislav Petkov wrote:
> It is an interesting question how we handle that RO/RW thing?

RO buffers overwrite, RW buffers loose events.

We could allow both for persistent under the constraints:

 VM_SHARED - must imply !VM_WRITE since multiple people writing to the
             control page will get 'interesting' very fast.

 !VM_SHARED - must fail mmap() of the debugfs file if there's already
              an existing user.

Note that even RW buffers only allows writes to the control page, a
write to the actual data itself will generate a fault.
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