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

[Stephane Eranian]

On Tue, Oct 8, 2013 at 9:41 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * 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.
>
That's a possibility. They are also pieces in the perf tool itself.
We could certainly make the attr->mmap2 bit disappear.

I think it boils down to how can we uniquely identify virtual
mapping to the same physical data either via shmat(), files, VM_CLONE.
We had all covered but the last case with the ino approach. We don't have
a solution for VM_CLONE yet.
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