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=-5.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 7C4CDC43461 for ; Wed, 2 Sep 2020 17:51:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 60C382083B for ; Wed, 2 Sep 2020 17:51:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727850AbgIBRvt (ORCPT ); Wed, 2 Sep 2020 13:51:49 -0400 Received: from mx2.suse.de ([195.135.220.15]:46328 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726247AbgIBRvr (ORCPT ); Wed, 2 Sep 2020 13:51:47 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id DD109AD5C; Wed, 2 Sep 2020 17:51:46 +0000 (UTC) Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline To: Michal Hocko Cc: Pavel Tatashin , LKML , Andrew Morton , linux-mm References: <20200901124615.137200-1-pasha.tatashin@soleen.com> <20200902140851.GJ4617@dhcp22.suse.cz> <74f2341a-7834-3e37-0346-7fbc48d74df3@suse.cz> <20200902151306.GL4617@dhcp22.suse.cz> From: Vlastimil Babka Message-ID: Date: Wed, 2 Sep 2020 19:51:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20200902151306.GL4617@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/2/20 5:13 PM, Michal Hocko wrote: > On Wed 02-09-20 16:55:05, Vlastimil Babka wrote: >> On 9/2/20 4:26 PM, Pavel Tatashin wrote: >> > On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko wrote: >> >> >> >> > >> >> > Thread#1 - continue >> >> > free_unref_page_commit >> >> > migratetype = get_pcppage_migratetype(page); >> >> > // get old migration type >> >> > list_add(&page->lru, &pcp->lists[migratetype]); >> >> > // add new page to already drained pcp list >> >> > >> >> > Thread#2 >> >> > Never drains pcp again, and therefore gets stuck in the loop. >> >> > >> >> > The fix is to try to drain per-cpu lists again after >> >> > check_pages_isolated_cb() fails. >> >> >> >> But this means that the page is not isolated and so it could be reused >> >> for something else. No? >> > >> > The page is in a movable zone, has zero references, and the section is >> > isolated (i.e. set_pageblock_migratetype(page, MIGRATE_ISOLATE);) is >> > set. The page should be offlinable, but it is lost in a pcp list as >> > that list is never drained again after the first failure to migrate >> > all pages in the range. >> >> Yeah. To answer Michal's "it could be reused for something else" - yes, somebody >> could allocate it from the pcplist before we do the extra drain. But then it >> becomes "visible again" and the loop in __offline_pages() should catch it by >> scan_movable_pages() - do_migrate_range(). And this time the pageblock is >> already marked as isolated, so the page (freed by migration) won't end up on the >> pcplist again. > > So the page block is marked MIGRATE_ISOLATE but the allocation itself > could be used for non migrateable objects. Or does anything prevent that > from happening? In a movable zone, the allocation should not be used for non migrateable objects. E.g. if the zone was not ZONE_MOVABLE, the offlining could fail regardless of this race (analogically for migrating away from CMA pageblocks). > We really do depend on isolation to not allow reuse when offlining. This is not really different than if the page on pcplist was allocated just a moment before the offlining, thus isolation started. We ultimately rely on being able to migrate any allocated pages away during the isolation. This "freeing to pcplists" race doesn't fundamentally change anything in this regard. We just have to guarantee that pages on pcplists will be eventually flushed, to make forward progress, and there was a bug in this aspect.