From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B302BC0044C for ; Wed, 7 Nov 2018 07:35:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 43EB520862 for ; Wed, 7 Nov 2018 07:35:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 43EB520862 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729989AbeKGRFA (ORCPT ); Wed, 7 Nov 2018 12:05:00 -0500 Received: from mx2.suse.de ([195.135.220.15]:53820 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726298AbeKGRFA (ORCPT ); Wed, 7 Nov 2018 12:05:00 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id DE9E7AC6C; Wed, 7 Nov 2018 07:35:49 +0000 (UTC) Date: Wed, 7 Nov 2018 08:35:48 +0100 From: Michal Hocko To: Balbir Singh Cc: Andrew Morton , Baoquan He , Oscar Salvador , linux-mm@kvack.org, LKML Subject: Re: [PATCH] mm, memory_hotplug: check zone_movable in has_unmovable_pages Message-ID: <20181107073548.GU27423@dhcp22.suse.cz> References: <20181106095524.14629-1-mhocko@kernel.org> <20181106203518.GC9042@350D> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181106203518.GC9042@350D> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 07-11-18 07:35:18, Balbir Singh wrote: > On Tue, Nov 06, 2018 at 10:55:24AM +0100, Michal Hocko wrote: > > From: Michal Hocko > > > > Page state checks are racy. Under a heavy memory workload (e.g. stress > > -m 200 -t 2h) it is quite easy to hit a race window when the page is > > allocated but its state is not fully populated yet. A debugging patch to > > dump the struct page state shows > > : [ 476.575516] has_unmovable_pages: pfn:0x10dfec00, found:0x1, count:0x0 > > : [ 476.582103] page:ffffea0437fb0000 count:1 mapcount:1 mapping:ffff880e05239841 index:0x7f26e5000 compound_mapcount: 1 > > : [ 476.592645] flags: 0x5fffffc0090034(uptodate|lru|active|head|swapbacked) > > > > Note that the state has been checked for both PageLRU and PageSwapBacked > > already. Closing this race completely would require some sort of retry > > logic. This can be tricky and error prone (think of potential endless > > or long taking loops). > > > > Workaround this problem for movable zones at least. Such a zone should > > only contain movable pages. 15c30bc09085 ("mm, memory_hotplug: make > > has_unmovable_pages more robust") has told us that this is not strictly > > true though. Bootmem pages should be marked reserved though so we can > > move the original check after the PageReserved check. Pages from other > > zones are still prone to races but we even do not pretend that memory > > hotremove works for those so pre-mature failure doesn't hurt that much. > > > > Reported-and-tested-by: Baoquan He > > Acked-by: Baoquan He > > Fixes: "mm, memory_hotplug: make has_unmovable_pages more robust") > > Signed-off-by: Michal Hocko > > --- > > > > Hi, > > this has been reported [1] and we have tried multiple things to address > > the issue. The only reliable way was to reintroduce the movable zone > > check into has_unmovable_pages. This time it should be safe also for > > the bug originally fixed by 15c30bc09085. > > > > [1] http://lkml.kernel.org/r/20181101091055.GA15166@MiWiFi-R3L-srv > > mm/page_alloc.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 863d46da6586..c6d900ee4982 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -7788,6 +7788,14 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count, > > if (PageReserved(page)) > > goto unmovable; > > > > + /* > > + * If the zone is movable and we have ruled out all reserved > > + * pages then it should be reasonably safe to assume the rest > > + * is movable. > > + */ > > + if (zone_idx(zone) == ZONE_MOVABLE) > > + continue; > > + > > /* > > > There is a WARN_ON() in case of failure at the end of the routine, > is that triggered when we hit the bug? If we're adding this patch, > the WARN_ON needs to go as well. No the warning should stay in case we encounter reserved pages in zone movable. > The check seems to be quite aggressive and in a loop that iterates > pages, but has nothing to do with the page, did you mean to make > the check > > zone_idx(page_zone(page)) == ZONE_MOVABLE Does it make any difference? Can we actually encounter a page from a different zone here? > it also skips all checks for pinned pages and other checks Yes, this is intentional and the comment tries to explain why. I wish we could be add a more specific checks for movable pages - e.g. detect long term pins that would prevent migration - but we do not have any facility for that. Please note that the worst case of a false positive is a repeated migration failure and user has a way to break out of migration by a signal. -- Michal Hocko SUSE Labs