Subject: [patch V3 07/37] highmem: Make DEBUG_HIGHMEM functional

From: Thomas Gleixner <[email-masked]>
Date: Tue, 03 Nov 2020 10:27:19 +0100

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 <[email-masked]>
---
V3: New patch
---
 mm/highmem.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -374,9 +374,19 @@ 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);
@@ -390,7 +400,7 @@ 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);
 }