From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754563Ab1A0N4T (ORCPT ); Thu, 27 Jan 2011 08:56:19 -0500 Received: from casper.infradead.org ([85.118.1.10]:40627 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751269Ab1A0N4R convert rfc822-to-8bit (ORCPT ); Thu, 27 Jan 2011 08:56:17 -0500 Subject: Re: One (possible) x86 get_user_pages bug From: Peter Zijlstra To: Xiaowei Yang Cc: Nick Piggin , Jan Beulich , Kenneth Lee , wangzhenguo@huawei.com, linqaingmin , fanhenglong@huawei.com, Wu Fengguang , linux-kernel@vger.kernel.org, Kaushik Barde In-Reply-To: <4D416D9A.9010603@huawei.com> References: <4D416D9A.9010603@huawei.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Thu, 27 Jan 2011 14:56:58 +0100 Message-ID: <1296136618.15234.187.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2011-01-27 at 21:05 +0800, Xiaowei Yang wrote: > > However, from the comments embedded in gup.c, it seems deliberate to > avoid the lock in the fast path. The question is: if so, how to avoid > the above scenario? Something like the below comes to mind... but I must say I haven't fully considered the problem yet.. --- arch/x86/mm/gup.c | 9 +++++---- 1 files changed, 5 insertions(+), 4 deletions(-) diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c index dbe34b9..6527933 100644 --- a/arch/x86/mm/gup.c +++ b/arch/x86/mm/gup.c @@ -89,10 +89,11 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned long addr, } VM_BUG_ON(!pfn_valid(pte_pfn(pte))); page = pte_page(pte); - get_page(page); - SetPageReferenced(page); - pages[*nr] = page; - (*nr)++; + if (get_page_unless_zero(page)) { + SetPageReferenced(page); + pages[*nr] = page; + (*nr)++; + } } while (ptep++, addr += PAGE_SIZE, addr != end); pte_unmap(ptep - 1);