From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753078AbcERJbM (ORCPT ); Wed, 18 May 2016 05:31:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:33448 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752772AbcERJbK (ORCPT ); Wed, 18 May 2016 05:31:10 -0400 Subject: Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page To: Mel Gorman , Naoya Horiguchi References: <1463470975-29972-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20160518092100.GB2527@techsingularity.net> Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Naoya Horiguchi From: Vlastimil Babka Message-ID: <573C365B.6020807@suse.cz> Date: Wed, 18 May 2016 11:31:07 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2 MIME-Version: 1.0 In-Reply-To: <20160518092100.GB2527@techsingularity.net> Content-Type: text/plain; charset=iso-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/18/2016 11:21 AM, Mel Gorman wrote: > On Tue, May 17, 2016 at 04:42:55PM +0900, Naoya Horiguchi wrote: >> There's a race window between checking page->flags and unpoisoning, which >> taints kernel with "BUG: Bad page state". That's overkill. It's safer to >> use bad_flags to detect hwpoisoned page. >> > > I'm not quite getting this one. Minimally, instead of = __PG_HWPOISON, it > should have been (bad_flags & __PG_POISON). As Vlastimil already pointed > out, __PG_HWPOISON can be 0. What I'm not getting is why this fixes the > race. The current race is > > 1. Check poison, set bad_flags > 2. poison clears in parallel > 3. Check page->flag state in bad_page and trigger warning > > The code changes it to > > 1. Check poison, set bad_flags > 2. poison clears in parallel > 3. Check bad_flags and trigger warning I think you got step 3 here wrong. It's "skip the warning since we have set bad_flags to hwpoison and bad_flags didn't change due to parallel unpoison". Perhaps the question is why do we need to split the handling between check_new_page_bad() and bad_page() like this? It might have been different in the past, but seems like at this point we only look for hwpoison from check_new_page_bad(). But a cleanup can come later. > There is warning either way. What did I miss? >