asm-generic: Provide kmap_size.h

kmap_types.h is a misnomer because the old atomic MAP based array does not
exist anymore and the whole indirection of architectures including
kmap_types.h is inconinstent and does not allow to provide guard page
debugging for this misfeature.

Add a common header file which defines the mapping stack size for all
architectures. Will be used when converting architectures over to a
generic kmap_local/atomic implementation.

The array size is chosen with the following constraints in mind:

    - The deepest nest level in one context is 3 according to code
      inspection.

    - The worst case nesting for the upcoming reemptible version would be:

      2 maps in task context and a fault inside
      2 maps in the fault handler
      3 maps in softirq
      2 maps in interrupt

So a total of 16 is sufficient and probably overestimated.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095857.078043987@linutronix.de

include/asm-generic/Kbuild

@@ -31,6 +31,7 @@ mandatory-y += irq_regs.h
 mandatory-y += irq_work.h
 mandatory-y += kdebug.h
 mandatory-y += kmap_types.h
+mandatory-y += kmap_size.h
 mandatory-y += kprobes.h
 mandatory-y += linkage.h
 mandatory-y += local.h

include/asm-generic/kmap_size.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_KMAP_SIZE_H
+#define _ASM_GENERIC_KMAP_SIZE_H
+
+/* For debug this provides guard pages between the maps */
+#ifdef CONFIG_DEBUG_HIGHMEM
+# define KM_MAX_IDX	33
+#else
+# define KM_MAX_IDX	16
+#endif
+
+#endif