revert "highmem: catch illegal nesting"

Revert

	commit 656dad312f
	Author: Ingo Molnar <mingo@elte.hu>
	Date:   Sat Feb 10 01:46:36 2007 -0800

	[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.

Problem is, a get_zeroed_page(GFP_KERNEL) from interrupt context will trigger
this check if non-irq code on this CPU holds a KM_USER0 mapping.  But that
get_zeroed_page() will never be altering the kmap slot anyway due to the
GFP_KERNEL.

Cc: Christoph Lameter <clameter@sgi.com>
Cc: 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

@@ -34,17 +34,16 @@ void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
 	/* 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);
+	BUG_ON(!pte_none(*(kmap_pte-idx)));
 	set_pte(kmap_pte-idx, mk_pte(page, prot));
 	arch_flush_lazy_mmu_mode();
 
-	return (void*) vaddr;
+	return (void *)vaddr;
 }
 
 void *kmap_atomic(struct page *page, enum km_type type)