highmem: Make DEBUG_HIGHMEM functional

For some obscure reason when CONFIG_DEBUG_HIGHMEM is enabled the stack
depth is increased from 20 to 41. But the only thing DEBUG_HIGHMEM does is
to enable a few BUG_ON()'s in the mapping code.

That's a leftover from the historical mapping code which had fixed entries
for various purposes. DEBUG_HIGHMEM inserted guard mappings between the map
types. But that got all ditched when kmap_atomic() switched to a stack
based map management. Though the WITH_KM_FENCE magic survived without being
functional. All the thing does today is to increase the stack depth.

Add a working implementation to the generic kmap_local* implementation.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095857.268258322@linutronix.de

mm/highmem.c

@@ -376,9 +376,19 @@ EXPORT_SYMBOL(kunmap_high);
 
 static DEFINE_PER_CPU(int, __kmap_local_idx);
 
+/*
+ * With DEBUG_HIGHMEM the stack depth is doubled and every second
+ * slot is unused which acts as a guard page
+ */
+#ifdef CONFIG_DEBUG_HIGHMEM
+# define KM_INCR	2
+#else
+# define KM_INCR	1
+#endif
+
 static inline int kmap_local_idx_push(void)
 {
-	int idx = __this_cpu_inc_return(__kmap_local_idx) - 1;
+	int idx = __this_cpu_add_return(__kmap_local_idx, KM_INCR) - 1;
 
 	WARN_ON_ONCE(in_irq() && !irqs_disabled());
 	BUG_ON(idx >= KM_MAX_IDX);
@@ -392,7 +402,7 @@ static inline int kmap_local_idx(void)
 
 static inline void kmap_local_idx_pop(void)
 {
-	int idx = __this_cpu_dec_return(__kmap_local_idx);
+	int idx = __this_cpu_sub_return(__kmap_local_idx, KM_INCR);
 
 	BUG_ON(idx < 0);
 }