[PATCH] vmtrunc: bug if page_mapped

If unmap_mapping_range (and mapping->truncate_count) are doing their jobs
right, truncate_complete_page should never find the page mapped: add BUG_ON
for our immediate testing, but this patch should probably not go to mainline -
a mapped page here is not a catastrophe.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/truncate.c

@@ -45,6 +45,7 @@ static inline void truncate_partial_page(struct page *page, unsigned partial)
 static void
 truncate_complete_page(struct address_space *mapping, struct page *page)
 {
+	BUG_ON(page_mapped(page));
 	if (page->mapping != mapping)
 		return;