From e5a80ff0a9d48da992d62b6e25b25eb255b4c20e Mon Sep 17 00:00:00 2001 From: Thomas Gleixner 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 Signed-off-by: Sebastian Andrzej Siewior --- 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