Re: [RFC] perf: mmap2 not covering VM_CLONE regions

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 30, 2013 at 06:48:55PM +0200, Stephane Eranian wrote:
> On Mon, Sep 30, 2013 at 6:15 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > 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() ?
> 
> Some matrix multiply benchmark, I guess.

So the problem is that we don't have a user visible address space
identifier; with CLONE_THREAD we have the thread group id that acts
like this. But for bare CLONE_VM usage there's nothing afaik.