Commit 389755c2 authored Nov 03, 2020 by Thomas Gleixner

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

parent 298fa1ad

mm/highmem.c

+12 −2

Original line number	Diff line number	Diff line
		@@ -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);
		}

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