From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752928AbcERJw4 (ORCPT ); Wed, 18 May 2016 05:52:56 -0400 Received: from outbound-smtp03.blacknight.com ([81.17.249.16]:47438 "EHLO outbound-smtp03.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752572AbcERJwy (ORCPT ); Wed, 18 May 2016 05:52:54 -0400 Date: Wed, 18 May 2016 10:52:51 +0100 From: Mel Gorman To: Vlastimil Babka Cc: Naoya Horiguchi , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Naoya Horiguchi Subject: Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page Message-ID: <20160518095251.GD2527@techsingularity.net> References: <1463470975-29972-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20160518092100.GB2527@techsingularity.net> <573C365B.6020807@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <573C365B.6020807@suse.cz> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 18, 2016 at 11:31:07AM +0200, Vlastimil Babka wrote: > 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". > I think the benefit is marginal. The race means that the patch will trigger a warning that might have been missed before due to a parallel unpoison but that's not necessary a Good Thing. It's inherently race-prone. Naoya, if you fix the check to (bad_flags & __PG_POISON) then I'll add my ack but I'm not convinced it's a real problem. -- Mel Gorman SUSE Labs