[PATCH] highmem: catch illegal nesting

Catch illegally nested kmap_atomic()s even if the page that is mapped by
the 'inner' instance is from lowmem.

This avoids spuriously zapped kmap-atomic ptes and turns hard to find
crashes into clear asserts at the bug site.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/i386/mm/highmem.c

@@ -33,13 +33,14 @@ void *kmap_atomic(struct page *page, enum km_type type)
 
 	/* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
 	pagefault_disable();
+
+	idx = type + KM_TYPE_NR*smp_processor_id();
+	BUG_ON(!pte_none(*(kmap_pte-idx)));
+
 	if (!PageHighMem(page))
 		return page_address(page);
 
-	idx = type + KM_TYPE_NR*smp_processor_id();
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
-	if (!pte_none(*(kmap_pte-idx)))
-		BUG();
 	set_pte(kmap_pte-idx, mk_pte(page, kmap_prot));
 
 	return (void*) vaddr;