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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 42363C43461 for ; Fri, 9 Apr 2021 12:48:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 20878610FB for ; Fri, 9 Apr 2021 12:48:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233755AbhDIMs5 (ORCPT ); Fri, 9 Apr 2021 08:48:57 -0400 Received: from mx2.suse.de ([195.135.220.15]:37322 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231756AbhDIMs1 (ORCPT ); Fri, 9 Apr 2021 08:48:27 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1617972493; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=/cklqRtaAoBxj6T8zaOYaeYthIiew+7URzqJWSDV+A4=; b=nH5uQ/hzVZLcswwNjsuRUB8vnnaKjiKfQtjZmowow1wDF0qUEGQCeCAR60WiOWkCTvKm+n 4v2khlPRms4RgZrOmITu1wvBJGjG/gAETYXCvKez8A9Hpip+O40YRbRoFBFiLY0yCMJWQs HrwzP2axM2tCz+jYqaZ8ocm//DUGev0= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 3916DAFD8; Fri, 9 Apr 2021 12:48:13 +0000 (UTC) Date: Fri, 9 Apr 2021 14:48:12 +0200 From: Michal Hocko To: Mel Gorman Cc: Andrew Morton , Linux-MM , LKML , Oscar Salvador , "Michael S. Tsirkin" , David Hildenbrand , Vlastimil Babka , Alexander Duyck , Minchan Kim Subject: Re: [PATCH] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove Message-ID: References: <20210409120957.GM3697@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 09-04-21 14:42:58, Michal Hocko wrote: > On Fri 09-04-21 13:09:57, Mel Gorman wrote: > > zone_pcp_reset allegedly protects against a race with drain_pages > > using local_irq_save but this is bogus. local_irq_save only operates > > on the local CPU. If memory hotplug is running on CPU A and drain_pages > > is running on CPU B, disabling IRQs on CPU A does not affect CPU B and > > offers no protection. > > Yes, the synchronization aspect is bogus indeed. > > > This patch reorders memory hotremove such that the PCP structures > > relevant to the zone are no longer reachable by the time the structures > > are freed. With this reordering, no protection is required to prevent > > a use-after-free and the IRQs can be left enabled. zone_pcp_reset is > > renamed to zone_pcp_destroy to make it clear that the per-cpu structures > > are deleted when the function returns. > > Wouldn't it be much easier to simply not destroy/reset pcp of an empty > zone at all? The whole point of this exercise seems to be described in > 340175b7d14d5. setup_zone_pageset can check for an already allocated pcp > and simply reinitialize it. I meant this diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 754026a9164d..7169342f5474 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1810,7 +1818,6 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages) init_per_zone_wmark_min(); if (!populated_zone(zone)) { - zone_pcp_reset(zone); build_all_zonelists(NULL); } else zone_pcp_update(zone); diff --git a/mm/page_alloc.c b/mm/page_alloc.c index e6a602e82860..b0fdda77e570 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -6496,7 +6496,13 @@ void __meminit setup_zone_pageset(struct zone *zone) struct per_cpu_pageset *p; int cpu; - zone->pageset = alloc_percpu(struct per_cpu_pageset); + /* + * zone could have gone completely offline during memory hotplug + * when the pgdat is left behind for simplicity. On a next onlining + * we do not need to reallocate pcp state. + */ + if (!zone->pageset) + zone->pageset = alloc_percpu(struct per_cpu_pageset); for_each_possible_cpu(cpu) { p = per_cpu_ptr(zone->pageset, cpu); pageset_init(p); @@ -8803,25 +8809,6 @@ void zone_pcp_enable(struct zone *zone) mutex_unlock(&pcp_batch_high_lock); } -void zone_pcp_reset(struct zone *zone) -{ - unsigned long flags; - int cpu; - struct per_cpu_pageset *pset; - - /* avoid races with drain_pages() */ - local_irq_save(flags); - if (zone->pageset != &boot_pageset) { - for_each_online_cpu(cpu) { - pset = per_cpu_ptr(zone->pageset, cpu); - drain_zonestat(zone, pset); - } - free_percpu(zone->pageset); - zone->pageset = &boot_pageset; - } - local_irq_restore(flags); -} - #ifdef CONFIG_MEMORY_HOTREMOVE /* * All pages in the range must be in a single zone, must not contain holes, -- Michal Hocko SUSE Labs