[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 <Alan.Brunelle@hp.com>
Cc: Mike Travis <travis@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 22 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 68d91c8233f4..ed92864d1325 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -577,35 +577,29 @@ config SWIOTLB
 
 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"
@@ -996,17 +990,10 @@ config NUMA_EMU
           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"
@@ -1014,7 +1001,6 @@ config NODES_SHIFT
         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