mm/highmem: Provide and use CONFIG_DEBUG_KMAP_LOCAL

CONFIG_KMAP_LOCAL can be enabled by x86/32bit even if CONFIG_HIGHMEM is not
enabled for temporary MMIO space mappings.

Provide it as a seperate config option which depends on CONFIG_KMAP_LOCAL
and let CONFIG_DEBUG_HIGHMEM select it.

This won't increase the debug coverage of this significantly but it paves
the way to do so.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204006.869487226@linutronix.de

include/asm-generic/kmap_size.h

@@ -3,7 +3,7 @@
 #define _ASM_GENERIC_KMAP_SIZE_H
 
 /* For debug this provides guard pages between the maps */
-#ifdef CONFIG_DEBUG_HIGHMEM
+#ifdef CONFIG_DEBUG_KMAP_LOCAL
 # define KM_MAX_IDX	33
 #else
 # define KM_MAX_IDX	16

lib/Kconfig.debug

@@ -849,9 +849,17 @@ config DEBUG_PER_CPU_MAPS
 
 	  Say N if unsure.
 
+config DEBUG_KMAP_LOCAL
+	bool "Debug kmap_local temporary mappings"
+	depends on DEBUG_KERNEL && KMAP_LOCAL
+	help
+	  This option enables additional error checking for the kmap_local
+	  infrastructure.  Disable for production use.
+
 config DEBUG_HIGHMEM
 	bool "Highmem debugging"
 	depends on DEBUG_KERNEL && HIGHMEM
+	select DEBUG_KMAP_LOCAL
 	help
 	  This option enables additional error checking for high memory
 	  systems.  Disable for production systems.

mm/highmem.c

@@ -368,10 +368,10 @@ EXPORT_SYMBOL(kunmap_high);
 static DEFINE_PER_CPU(int, __kmap_local_idx);
 
 /*
- * With DEBUG_HIGHMEM the stack depth is doubled and every second
+ * With DEBUG_KMAP_LOCAL the stack depth is doubled and every second
  * slot is unused which acts as a guard page
  */
-#ifdef CONFIG_DEBUG_HIGHMEM
+#ifdef CONFIG_DEBUG_KMAP_LOCAL
 # define KM_INCR	2
 #else
 # define KM_INCR	1