mm / dtrace: Allow DTrace to entirely disable page faults.

mm / dtrace: Allow DTrace to entirely disable page faults.

In some very limited circumstances (largely restricted to get_user() calls, but
not entirely) DTrace needs to be able to probe userspace in irq and trap
context, in which page faults are prohibited or otherwise impossible because we
cannot guarantee that certain locks (like the mmap_sem) are not already taken by
the time DTrace is invoked.

In a similar fashion to the existing CPU_DTRACE_NOFAULT machinery, which allows
DTrace to prohibit invalid address faults, we introduce a CPU_DTRACE_NOPF value
in the per-CPU DTrace flag variable, which, when set by the DTrace module,
causes all page faults to set CPU_DTRACE_PF_TRAPPED in the flag variable: the
page fault is then ignored (and the current instruction skipped so that it is
not immediately retriggered).

Clearly, ignoring page faults in random pieces of kernel code would be very
risky: this machinery is only used in situations such as get_user() in which the
kernel code does nothing with the page other than to extract data from it and
pass it back to the DTrace module.

The impact on the core x86 fault path when CONFIG_DTRACE is set is one
unlikely() conditional and one function call: when CONFIG_DTRACE is not on the
impact is zero.  (Eliminating the function call and directly testing the DTrace
per-CPU flag is possible as a future optimization, but was considered too
invasive for now.)

Orabug: 18412802
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>
Conflicts:
	arch/x86/mm/fault.c