sparc64: Restrict number of processes

sparc64: Restrict number of processes

Orabug: 24523680

If the number of processes exceeds the number of supported context IDs
then there are random segfaults seen in the user programs.

The data collected when debugging this bug showed that two processes
with the same context IDs were present in the TLB shared between them
thus resulting in incorrect translations. Since the bug occurs after the
kernel hits the max context ID supported by the processor, the context
wraparound code found in get_new_mmu_context(...) and
smp_new_mmu_context_version_client(...) are under suspicion.

The plan is that this will get fixed when we implement "context domain"
feature for sparc in the kernel.  For now this patch temporarily
restricts the number of processes allowed by the kernel based on the
number of context IDs supported by the processor. This way we never reach
that condition of having incorrect translations.

For non root users fork will fail if the number of existing processes is
greater than (max_user_nctx - 100). Where max_user_nctx is the maximum
number of context IDs supported by the processor. For root user the fork
will fail if the the number of existing processes reaches max_user_nctx.
Extra context IDs are given to root to recover the system if users reach
their limit and cannot recover the system (i.e cannot even execute
'kill' to reduce the number of processes).

Signed-off-by: Sanath Kumar <sanath.s.kumar@oracle.com>
Reviewed-by: Rob Gardner <rob.gardner@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>