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

[Stephane Eranian]

On Mon, Sep 30, 2013 at 6:15 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.
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