[tip: core/mm] highmem: Make DEBUG_HIGHMEM functional

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the core/mm branch of tip:

Commit-ID:     389755c250814185938f5b04334a4f0184c30647
Gitweb:        https://git.kernel.org/tip/389755c250814185938f5b04334a4f0184c30647
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 03 Nov 2020 10:27:19 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Fri, 06 Nov 2020 23:14:55 +01:00

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@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201103095857.268258322@xxxxxxxxxxxxx

---
 mm/highmem.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/highmem.c b/mm/highmem.c
index bb4ce13..67d2d59 100644
--- a/mm/highmem.c
+++ b/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);
 }