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From e5a80ff0a9d48da992d62b6e25b25eb255b4c20e Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 3 Nov 2020 10:27:19 +0100
Subject: [PATCH 030/323] highmem: Make DEBUG_HIGHMEM functional
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.215-rt107.tar.xz
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>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
mm/highmem.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/mm/highmem.c b/mm/highmem.c
index bb4ce13ee7e77..67d2d5983cb06 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);
}
--
2.44.0
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