[x86] Clean up MAXSMP Kconfig, and limit NR_CPUS to 512

This fixes a regression that was indirectly caused by commit
1184dc2ffe2c8fb9afb766d870850f2c3165ef25 ("x86: modify Kconfig to allow
up to 4096 cpus").

Allowing 4k CPU's is not practical at this time, because we still have a
number of places that have several 'cpumask_t's on the stack, and a
4k-bit cpumask is 512 bytes of stack-space for each such variable. This
literally caused functions like 'smp_call_function_mask' to have a 2.5kB
stack frame, and several functions to have 2kB stackframes.

With an 8kB stack total, smashing the stack was simply much too likely.
At least bugzilla entry

http://bugzilla.kernel.org/show_bug.cgi?id=11342

was due to this.

The earlier commit to not inline load_module() into sys_init_module()
fixed the particular symptoms of this that Alan Brunelle saw in that
bugzilla entry, but the huge stack waste by cpumask_t's was the more
direct cause.

Some day we'll have allocation helpers that allocate large CPU masks
dynamically, but in the meantime we simply cannot allow cpumasks this
large.

Cc: Alan D. Brunelle <[e-mail]>
Cc: Mike Travis <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/Kconfig

···
 
 config IOMMU_HELPER
 	def_bool (CALGARY_IOMMU || GART_IOMMU || SWIOTLB || AMD_IOMMU)
+
 config MAXSMP
 	bool "Configure Maximum number of SMP Processors and NUMA Nodes"
-	depends on X86_64 && SMP
+	depends on X86_64 && SMP && BROKEN
 	default n
 	help
 	  Configure maximum number of CPUS and NUMA Nodes for this architecture.
 	  If unsure, say N.
 
-if MAXSMP
 config NR_CPUS
-	int
-	default "4096"
-endif
-
-if !MAXSMP
-config NR_CPUS
-	int "Maximum number of CPUs (2-4096)"
-	range 2 4096
+	int "Maximum number of CPUs (2-512)" if !MAXSMP
+	range 2 512
 	depends on SMP
+	default "4096" if MAXSMP
 	default "32" if X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000
 	default "8"
 	help
 	  This allows you to specify the maximum number of CPUs which this
-	  kernel will support.  The maximum supported value is 4096 and the
+	  kernel will support.  The maximum supported value is 512 and the
 	  minimum value which makes sense is 2.
 
 	  This is purely to save memory - each supported CPU adds
 	  approximately eight kilobytes to the kernel image.
-endif
 
 config SCHED_SMT
 	bool "SMT (Hyperthreading) scheduler support"
···
 	  into virtual nodes when booted with "numa=fake=N", where N is the
 	  number of nodes. This is only useful for debugging.
 
-if MAXSMP
-
 config NODES_SHIFT
-	int
-	default "9"
-endif
-
-if !MAXSMP
-config NODES_SHIFT
-	int "Maximum NUMA Nodes (as a power of 2)"
+	int "Maximum NUMA Nodes (as a power of 2)" if !MAXSMP
 	range 1 9   if X86_64
+	default "9" if MAXSMP
 	default "6" if X86_64
 	default "4" if X86_NUMAQ
 	default "3"
···
 	help
 	  Specify the maximum number of NUMA Nodes available on the target
 	  system.  Increases memory reserved to accomodate various tables.
-endif
 
 config HAVE_ARCH_BOOTMEM_NODE
 	def_bool y