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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 BF4F0C433B4 for ; Fri, 9 Apr 2021 13:42:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8E8C2610D1 for ; Fri, 9 Apr 2021 13:42:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233705AbhDINmk (ORCPT ); Fri, 9 Apr 2021 09:42:40 -0400 Received: from outbound-smtp50.blacknight.com ([46.22.136.234]:57653 "EHLO outbound-smtp50.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233655AbhDINmg (ORCPT ); Fri, 9 Apr 2021 09:42:36 -0400 Received: from mail.blacknight.com (pemlinmail06.blacknight.ie [81.17.255.152]) by outbound-smtp50.blacknight.com (Postfix) with ESMTPS id D71E0FAD4C for ; Fri, 9 Apr 2021 14:42:22 +0100 (IST) Received: (qmail 15210 invoked from network); 9 Apr 2021 13:42:22 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 9 Apr 2021 13:42:22 -0000 Date: Fri, 9 Apr 2021 14:42:21 +0100 From: Mel Gorman To: Michal Hocko 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: <20210409134221.GO3697@techsingularity.net> References: <20210409120957.GM3697@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 09, 2021 at 02:48:12PM +0200, Michal Hocko wrote: > 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 > It might be simplier but if the intention is to free as much memory as possible during hot-remove, it seems wasteful to leave the per-cpu structures behind if we do not have to. If a problem with my patch can be spotted then I'm happy to go with an alternative fix but there are two minor issues with your proposed fix. > 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); Should be "if (zone->pageset != &boot_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); > -} > - zone_pcp_reset still needs to exist to drain the remaining vmstats or it'll break 5a883813845a ("memory-hotplug: fix zone stat mismatch"). -- Mel Gorman SUSE Labs