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=-2.5 required=3.0 tests=MAILING_LIST_MULTI,SPF_PASS, 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 235E8C43441 for ; Wed, 14 Nov 2018 09:01:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E8ECD22419 for ; Wed, 14 Nov 2018 09:01:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E8ECD22419 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 S1732499AbeKNTD4 (ORCPT ); Wed, 14 Nov 2018 14:03:56 -0500 Received: from mx2.suse.de ([195.135.220.15]:58722 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727595AbeKNTD4 (ORCPT ); Wed, 14 Nov 2018 14:03:56 -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 69964ADD3; Wed, 14 Nov 2018 09:01:35 +0000 (UTC) Date: Wed, 14 Nov 2018 10:01:34 +0100 From: Michal Hocko To: David Hildenbrand Cc: Baoquan He , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, aarcange@redhat.com Subject: Re: Memory hotplug softlock issue Message-ID: <20181114090134.GG23419@dhcp22.suse.cz> References: <20181114070909.GB2653@MiWiFi-R3L-srv> <5a6c6d6b-ebcd-8bfa-d6e0-4312bfe86586@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5a6c6d6b-ebcd-8bfa-d6e0-4312bfe86586@redhat.com> 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 14-11-18 09:18:09, David Hildenbrand wrote: > On 14.11.18 08:09, Baoquan He wrote: > > Hi, > > > > Tested memory hotplug on a bare metal system, hot removing always > > trigger a lock. Usually need hot plug/unplug several times, then the hot > > removing will hang there at the last block. Surely with memory pressure > > added by executing "stress -m 200". > > > > Will attach the log partly. Any idea or suggestion, appreciated. > > > > Thanks > > Baoquan > > > > Code seems to be waiting for the mem_hotplug_lock in read. > We hold mem_hotplug_lock in write whenever we online/offline/add/remove > memory. There are two ways to trigger offlining of memory: > > 1. Offlining via "cat offline > /sys/devices/system/memory/memory0/state" > > This always properly took the mem_hotplug_lock. Nothing changed > > 2. Offlining via "cat 0 > /sys/devices/system/memory/memory0/online" > > This didn't take the mem_hotplug_lock and I fixed that for this release. This discrepancy should go. > So if you were testing with 1., you should have seen the same error > before this release (unless there is something else now broken in this > release). > > > The real question is, however, why offlining of the last block doesn't > succeed. In __offline_pages() we basically have an endless loop (while > holding the mem_hotplug_lock in write). Now I consider this piece of > code very problematic (we should automatically fail after X > attempts/after X seconds, we should not ignore -ENOMEM), and we've had > other BUGs whereby we would run into an endless loop here (e.g. related > to hugepages I guess). We used to have number of retries previous and it was too fragile. If you need a timeout then you can easily do that from userspace. Just do timeout $TIME echo 0 > $MEM_PATH/online I have seen an issue when the migration cannot make a forward progress because of a glibc page with a reference count bumping up and down. Most probable explanation is the faultaround code. I am working on this and will post a patch soon. In any case the migration should converge and if it doesn't do then there is a bug lurking somewhere. Failing on ENOMEM is a questionable thing. I haven't seen that happening wildly but if it is a case then I wouldn't be opposed. > You mentioned memory pressure, if our host is under memory pressure we > can easily trigger running into an endless loop there, because we > basically ignore -ENOMEM e.g. when we cannot get a page to migrate some > memory to be offlined. I assume this is the case here. > do_migrate_range() could be the bad boy if it keeps failing forever and > we keep retrying. My hotplug debugging patches [1] should help to tell us. [1] http://lkml.kernel.org/r/20181107101830.17405-1-mhocko@kernel.org -- Michal Hocko SUSE Labs