From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752786AbeBIUnD (ORCPT ); Fri, 9 Feb 2018 15:43:03 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:58878 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750853AbeBIUnC (ORCPT ); Fri, 9 Feb 2018 15:43:02 -0500 Date: Fri, 9 Feb 2018 12:43:00 -0800 From: Andrew Morton To: Yang Shi Cc: kirill.shutemov@linux.intel.com, gavin.dg@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs_one() Message-Id: <20180209124300.fc3468a72e0d223c0e4d4195@linux-foundation.org> In-Reply-To: <1518203521-81173-1-git-send-email-yang.shi@linux.alibaba.com> References: <1518203521-81173-1-git-send-email-yang.shi@linux.alibaba.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 10 Feb 2018 03:12:01 +0800 Yang Shi wrote: > For PTE-mapped THP, the compound THP has not been split to normal 4K > pages yet, the whole THP is considered referenced if any one of sub > page is referenced. > > When walking PTE-mapped THP by pvmw, all relevant PTEs will be checked > to retrieve referenced bit. But, the current code just returns the > result of the last PTE. If the last PTE has not referenced, the > referenced flag will be cleared. > > Just did logical OR for referenced to get the correct result. > > Reported-by: Gang Deng > Suggested-by: Kirill A. Shutemov > Signed-off-by: Yang Shi > --- > v2: adopted the suggestion from Kirill. Not use "||=" style to keep checkpatch > quiet, otherwise it reports ERROR: spaces required around that '||' > > mm/page_idle.c | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) > > diff --git a/mm/page_idle.c b/mm/page_idle.c > index 0a49374..a4baec9 100644 > --- a/mm/page_idle.c > +++ b/mm/page_idle.c > @@ -65,11 +65,15 @@ static bool page_idle_clear_pte_refs_one(struct page *page, > while (page_vma_mapped_walk(&pvmw)) { > addr = pvmw.address; > if (pvmw.pte) { > - referenced = ptep_clear_young_notify(vma, addr, > - pvmw.pte); > + /* > + * For PTE-mapped THP, one sub page is referenced, > + * the whole THP is referenced. > + */ > + referenced = referenced || ptep_clear_young_notify(vma, > + addr, pvmw.pte); That doesn't work. If `referenced' is already true, ptep_clear_young_notify() will not be called. if (ptep_clear_young_notify(...)) referenced = true; would suit. It makes me wonder what difference this bug makes and why that difference was not noted in your testing. Any theories about that? How can we design a test which *will* make this error apparent?