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

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Message-ID: <20130930161546.GG3081@twins.programming.kicks-ass.net>
Date:	Mon, 30 Sep 2013 18:15:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	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>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC] perf: mmap2 not covering VM_CLONE regions

On Mon, Sep 30, 2013 at 05:44:41PM +0200, Stephane Eranian wrote:
> Hi,
> 
> I was alerted by people trying to use the PERF_RECORD_MMAP2
> record to disambiguate virtual address mappings that there is a case
> where the record does not contain enough information.
> 
> As you know, the MMAP2 record adds the major, minor, ino number,
> inode generation numbers to a mapping. But it does that only for
> file or pseudo -file backed mappings. That covers file mmaps and also
> SYSV shared memory segments.
> 
> However there is a another kind of situation that arises in some
> multi-process benchmarks where a region of memory is cloned
> using VM_CLONE. As such, the virtual addresses match between
> the processes but the major, minor, inode, inode generation  fields
> are all zeroes because there is no inode associated with the mapping.
> Yet, it is important for the tool to know the mappings between the
> processes are pointing to the same physical data.
> 
> We need to cover this case and I am seeking for advice on how to
> best address this need given that we discarded using the plain physical
> address for disambiguation.

Urgh.. who in his bloody mind is playing VM_CLNOE games that is not
pthread_creatE() ?
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