Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755616AbaFPTgQ (ORCPT ); Mon, 16 Jun 2014 15:36:16 -0400 Received: from g2t1383g.austin.hp.com ([15.217.136.92]:41278 "EHLO g2t1383g.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753365AbaFPTgP (ORCPT ); Mon, 16 Jun 2014 15:36:15 -0400 From: Waiman Long To: Andrew Morton , Mel Gorman , Rik van Riel , Ingo Molnar , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Scott J Norton , Waiman Long Subject: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map Date: Mon, 16 Jun 2014 15:35:48 -0400 Message-Id: <1402947348-60655-1-git-send-email-Waiman.Long@hp.com> X-Mailer: git-send-email 1.7.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In the __split_huge_page_map() function, the check for page_mapcount(page) is invariant within the for loop. Because of the fact that the macro is implemented using atomic_read(), the redundant check cannot be optimized away by the compiler leading to unnecessary read to the page structure. This patch move the invariant bug check out of the loop so that it will be done only once. Signed-off-by: Waiman Long --- mm/huge_memory.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index b4b1feb..b8bb16c 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1744,6 +1744,8 @@ static int __split_huge_page_map(struct page *page, if (pmd) { pgtable = pgtable_trans_huge_withdraw(mm, pmd); pmd_populate(mm, &_pmd, pgtable); + if (pmd_write(*pmd)) + BUG_ON(page_mapcount(page) != 1); haddr = address; for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) { @@ -1753,8 +1755,6 @@ static int __split_huge_page_map(struct page *page, entry = maybe_mkwrite(pte_mkdirty(entry), vma); if (!pmd_write(*pmd)) entry = pte_wrprotect(entry); - else - BUG_ON(page_mapcount(page) != 1); if (!pmd_young(*pmd)) entry = pte_mkold(entry); if (pmd_numa(*pmd)) -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/