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

[Ingo Molnar]

* David Ahern <dsahern@xxxxxxxxx> wrote:

> Stephane:
> 
> On 9/30/13 9:44 AM, Stephane Eranian wrote:
> >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.
> 
> 
> If the current MMAP2 is not a complete solution for what you (Google) 
> need, should support be reverted before 3.12 is released? No sense in 
> making this part of the forever API if more work is needed on it.

Instead of a full revert we could just turn off the ABI portion minimally 
and not recognize it for now. Assuming a more complete solution is in the 
works for v3.13.

Thanks,

	Ingo
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