linux-kernel - [PATCH -mm 2/2] mm: thp: turn compound_head() into BUG_ON(!PageTail) in get_huge_page

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Date:	Mon, 16 Dec 2013 21:19:52 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>
Subject: [PATCH -mm 2/2] 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@...hat.com>
---
 include/linux/mm.h |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 13bae9e..13495bd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -431,13 +431,12 @@ 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);
 }
 
-- 
1.5.5.1


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