I have implemented highmem for ARM.  To catch wrong usage of __pa() and 
__va() with out of range values, I added a range check when 
CONFIG_DEBUG_HIGHMEM is set.

One issue is that bootmem.h uses __pa(MAX_DMA_ADDRESS). However 
MAX_DMA_ADDRESS on ARM is defined as 0xffffffff because there is usually 
no restriction on the maximum DMA-able address.

RMK suggested that those places should be using ISA_DMA_THRESHOLD 
instead -- here's an excert of our conversation on this topic:

quoted text
> I suspect all those places which are doing __pa(MAX_DMA_ADDRESS) really
> want to be using ISA_DMA_THRESHOLD - that's something to raise on LKML
> if it's causing problems.
> 
> > > Given the definition I've just given, if there's no restrictions on
> > > what can be DMA'd from, defining 'MAX_DMA_ADDRESS' to a constant well
> > > in excess of the maximum amount of RAM is quite reasonable.  Since most
> > > ARM platforms don't have any restrictions on memory that can be DMA'd,
> > > the existing code is perfectly reasonable and correct.
> > 
> > It falls apart when the range check on values passed to __pa() is 
> > enabled in a highmem context.
> 
> As I say, having it in excess of the real amount of RAM fitted is not a
> problem.  Unfortunately, it makes the range check fall apart but I suspect
> that's because of inappropriate use.

So what about this patch?

diff --git a/include/linux/bootmem.h b/include/linux/bootmem.h
index 95837bf..7a97ffe 100644
--- a/include/linux/bootmem.h
+++ b/include/linux/bootmem.h
@@ -96,21 +96,21 @@ extern void *__alloc_bootmem_low_node(pg_data_t *pgdat,
 				      unsigned long goal);
 #ifndef CONFIG_HAVE_ARCH_BOOTMEM_NODE
 #define alloc_bootmem(x) \
-	__alloc_bootmem(x, SMP_CACHE_BYTES, __pa(MAX_DMA_ADDRESS))
+	__alloc_bootmem(x, SMP_CACHE_BYTES, ISA_DMA_THRESHOLD)
 #define alloc_bootmem_nopanic(x) \
-	__alloc_bootmem_nopanic(x, SMP_CACHE_BYTES, __pa(MAX_DMA_ADDRESS))
+	__alloc_bootmem_nopanic(x, SMP_CACHE_BYTES, ISA_DMA_THRESHOLD)
 #define alloc_bootmem_low(x) \
 	__alloc_bootmem_low(x, SMP_CACHE_BYTES, 0)
 #define alloc_bootmem_pages(x) \
-	__alloc_bootmem(x, PAGE_SIZE, __pa(MAX_DMA_ADDRESS))
+	__alloc_bootmem(x, PAGE_SIZE, ISA_DMA_THRESHOLD)
 #define alloc_bootmem_pages_nopanic(x) \
-	__alloc_bootmem_nopanic(x, PAGE_SIZE, __pa(MAX_DMA_ADDRESS))
+	__alloc_bootmem_nopanic(x, PAGE_SIZE, ISA_DMA_THRESHOLD)
 #define alloc_bootmem_low_pages(x) \
 	__alloc_bootmem_low(x, PAGE_SIZE, 0)
 #define alloc_bootmem_node(pgdat, x) \
-	__alloc_bootmem_node(pgdat, x, SMP_CACHE_BYTES, __pa(MAX_DMA_ADDRESS))
+	__alloc_bootmem_node(pgdat, x, SMP_CACHE_BYTES, ISA_DMA_THRESHOLD)
 #define alloc_bootmem_pages_node(pgdat, x) \
-	__alloc_bootmem_node(pgdat, x, PAGE_SIZE, __pa(MAX_DMA_ADDRESS))
+	__alloc_bootmem_node(pgdat, x, PAGE_SIZE, ISA_DMA_THRESHOLD)
 #define alloc_bootmem_low_pages_node(pgdat, x) \
 	__alloc_bootmem_low_node(pgdat, x, PAGE_SIZE, 0)
 #endif /* !CONFIG_HAVE_ARCH_BOOTMEM_NODE */
--
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