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>

1 files changed, 3 insertions, 4 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f95c71b7c1fd..58202c26c559 100644
--- a/include/linux/mm.h
+++ b/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);
 }