From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752818AbcERJVI (ORCPT ); Wed, 18 May 2016 05:21:08 -0400 Received: from outbound-smtp04.blacknight.com ([81.17.249.35]:58277 "EHLO outbound-smtp04.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750870AbcERJVG (ORCPT ); Wed, 18 May 2016 05:21:06 -0400 Date: Wed, 18 May 2016 10:21:01 +0100 From: Mel Gorman To: Naoya Horiguchi Cc: Andrew Morton , Vlastimil Babka , 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: <20160518092100.GB2527@techsingularity.net> References: <1463470975-29972-1-git-send-email-n-horiguchi@ah.jp.nec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1463470975-29972-1-git-send-email-n-horiguchi@ah.jp.nec.com> 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 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 There is warning either way. What did I miss? -- Mel Gorman SUSE Labs