mm: thp: turn compound_head() into BUG_ON(!PageTail) in get_huge_page_tail()

get_huge_page_tail()->compound_head() looks confusing.  Every caller
must check PageTail(page), otherwise atomic_inc(&page->_mapcount) is
simply wrong if this page is compound-trans-head.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Jones <davej@redhat.com>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -445,13 +445,12 @@ static inline bool compound_tail_refcounted(struct page *page)
 static inline void get_huge_page_tail(struct page *page)
 {
 	/*
-	 * __split_huge_page_refcount() cannot run
-	 * from under us.
-	 * In turn no need of compound_trans_head here.
+	 * __split_huge_page_refcount() cannot run from under us.
 	 */
+	VM_BUG_ON(!PageTail(page));
 	VM_BUG_ON(page_mapcount(page) < 0);
 	VM_BUG_ON(atomic_read(&page->_count) != 0);
-	if (compound_tail_refcounted(compound_head(page)))
+	if (compound_tail_refcounted(page->first_page))
 		atomic_inc(&page->_mapcount);
 }