linux-kernel - Re: [RFC] perf: mmap2 not covering VM

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Date:	Wed, 9 Oct 2013 12:39:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	David Ahern <dsahern@...il.com>,
	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>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC] perf: mmap2 not covering VM_CLONE regions

On Wed, Oct 09, 2013 at 11:59:06AM +0200, Ingo Molnar wrote:
> PeterZ didn't like exposing the physical RAM address, right?

More than not like; its useless and broken, see below.

> Peter: could we expose the page frame number instead? (Maybe mix it with a 
> random seed through the hash-mixer and expose that.) User-space will thus 
> be able to discover whether two pages are shared or not, but not extract 
> other information.

No, all physical stuff is useless, a page can get moved about through
swap/ksm/thp/compaction/etc.

> The global MM ID thing looked rather ugly as well.

I'm still having the argument with Stephane -- although it fell off-list
-- on why this wouldn't work.

AFAICT the mm_id should work for CLONE_VM, as CLONE_VM inherits the
entire address space so all should be identical when mm_id match.

> That way we could drop the inode info as well? That feels a bit hacky too.

No, the inode stuff is required for shared pages, its the only way to
track them; and barring a filename (which we've so far used for this)
the dev:ino:gen thing is the only way to uniquely identify files -- it
even works where filenames don't, eg. filesystem namespaces and weird
things like shm that don't have stable filenames.
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