linux-kernel - Re: [PATCH 0/8] perf: add ability to sample physical data addresses

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Message-ID: <20130730083758.GH3008@twins.programming.kicks-ass.net>
Date:	Tue, 30 Jul 2013 10:37:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [PATCH 0/8] perf: add ability to sample physical data addresses

On Tue, Jul 30, 2013 at 10:02:01AM +0200, Stephane Eranian wrote:
> > Ahh. We don't put the useful bits in the mmap event; we'll need to fix
> > that too then ;-)
> >
> > Doing so is going to be a bit of a bother since we use the tail of
> > PERF_RECORD_MMAP for filenames and thus aren't particularly extensible.
> >
> > This would mean doing something like PERF_RECORD_MMAP2 and some means
> > for userspace to requrest the new events instead of the old one.
> >
> Tracking mmaps even for shmat() won't cover the paging cases. When you page a
> page back in, it most likely gets a different physical page. How would
> we track that
> case too using the same approach?

It doesn't matter. Even if a page ends up being a different physical
page, it will always be the same sb:inode:pgoffset. You should be able
to always uniquely identify a (shared) page by that triplet.

So if we create a net MMAP record that includes the device (substitute
for the superblock) and inode information we should be good.
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