[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>

diff --git a/arch/i386/mm/highmem.c b/arch/i386/mm/highmem.c
index e0fa6cb..bb2de10 100644 (file)
--- a/arch/i386/mm/highmem.c
+++ b/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;