Re: [PATCH 0/8] perf: add ability to sample physical data addresses

Peter Zijlstra
2013-07-30T08:38:14Z

On Tue, Jul 30, 2013 at 10:02:01AM +0200, Stephane Eranian wrote:
> > Ahh. We don't put the useful bits in the mmap event; we'll need to fix
> > that too then ;-)
> >
> > Doing so is going to be a bit of a bother since we use the tail of
> > PERF_RECORD_MMAP for filenames and thus aren't particularly extensible.
> >
> > This would mean doing something like PERF_RECORD_MMAP2 and some means
> > for userspace to requrest the new events instead of the old one.
> >
> Tracking mmaps even for shmat() won't cover the paging cases. When you page a
> page back in, it most likely gets a different physical page. How would
> we track that
> case too using the same approach?

It doesn't matter. Even if a page ends up being a different physical
page, it will always be the same sb:inode:pgoffset. You should be able
to always uniquely identify a (shared) page by that triplet.

So if we create a net MMAP record that includes the device (substitute
for the superblock) and inode information we should be good.