[Bug 203409] page fault in tcp_do_segment (r287759 suspected)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203409

--- Comment #1 from Andriy Gapon <avg at FreeBSD.org> ---
Created attachment 161483
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=161483&action=edit
fix / work-around

The attached trivial patch seems to make the issue go away.
But I am not sure if that's a correct fix as not firing the probe at all could
result in an incomplete event trail.

As a side note, in my opinion the use of mtod() with the SDT probes in
tcp_do_segment() is slightly against the recommended SDT usage.  Typically an
SDT probe's arguments are values that are actually used near the probe and thus
have a high chance of being in CPU registers or in the L1 cache.  In
tcp_do_segment() there does not seem to be any access to m_data, so the probes
have a bigger overhead because of the extra memory access.  m_data's value
might still be in the L1 cache, though.

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