From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id 4B1E66B025E for ; Wed, 18 May 2016 05:31:10 -0400 (EDT) Received: by mail-wm0-f72.google.com with SMTP id w143so25313055wmw.3 for ; Wed, 18 May 2016 02:31:10 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id f141si9807999wmf.102.2016.05.18.02.31.08 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 18 May 2016 02:31:09 -0700 (PDT) Subject: Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page References: <1463470975-29972-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20160518092100.GB2527@techsingularity.net> From: Vlastimil Babka Message-ID: <573C365B.6020807@suse.cz> Date: Wed, 18 May 2016 11:31:07 +0200 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: owner-linux-mm@kvack.org List-ID: To: Mel Gorman , Naoya Horiguchi Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Naoya Horiguchi 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? > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org