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=-5.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 84EDCC388F9 for ; Wed, 11 Nov 2020 18:00:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 210D82072C for ; Wed, 11 Nov 2020 18:00:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="FWxvl3S8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726973AbgKKSAE (ORCPT ); Wed, 11 Nov 2020 13:00:04 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:37949 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726821AbgKKSAB (ORCPT ); Wed, 11 Nov 2020 13:00:01 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605117600; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=E8xfs94NnyMJClh1BOZT8JsRuv0OOa40sFS9VpKsYwg=; b=FWxvl3S8oEAFG6ImA+BuKYMzkbxqIThXriBttnE15g+VnmTQk0ZBBuesyRGkp5N0bn/fxB gNV2CQ5GWcmLsbUxgymV1PH47Rqej4SlAJeMRwOVW7sYCKFfyEpH4EnCeTlX6PdWSS6ffj IhkNYVH7IR3SYo3PhCjGxnKzlwJlG50= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-291-56ub6ghkOQuJmGQcS-LWCg-1; Wed, 11 Nov 2020 12:59:57 -0500 X-MC-Unique: 56ub6ghkOQuJmGQcS-LWCg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 627AD1084D7C; Wed, 11 Nov 2020 17:59:41 +0000 (UTC) Received: from [10.36.114.151] (ovpn-114-151.ams2.redhat.com [10.36.114.151]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4A3CC5DA6A; Wed, 11 Nov 2020 17:59:39 +0000 (UTC) Subject: Re: [PATCH v3 7/7] mm, page_alloc: disable pcplists during memory offline To: Vlastimil Babka , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Michal Hocko , Pavel Tatashin , Oscar Salvador , Joonsoo Kim , Michal Hocko References: <20201111092812.11329-1-vbabka@suse.cz> <20201111092812.11329-8-vbabka@suse.cz> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <3317cf76-458c-8873-0e1a-4129745f0fda@redhat.com> Date: Wed, 11 Nov 2020 18:59:38 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20201111092812.11329-8-vbabka@suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11.11.20 10:28, Vlastimil Babka wrote: > Memory offlining relies on page isolation to guarantee a forward > progress because pages cannot be reused while they are isolated. But the > page isolation itself doesn't prevent from races while freed pages are > stored on pcp lists and thus can be reused. This can be worked around by > repeated draining of pcplists, as done by commit 968318261221 > ("mm/memory_hotplug: drain per-cpu pages again during memory offline"). > > David and Michal would prefer that this race was closed in a way that callers > of page isolation who need stronger guarantees don't need to repeatedly drain. > David suggested disabling pcplists usage completely during page isolation, > instead of repeatedly draining them. > > To achieve this without adding special cases in alloc/free fastpath, we can use > the same approach as boot pagesets - when pcp->high is 0, any pcplist addition > will be immediately flushed. > > The race can thus be closed by setting pcp->high to 0 and draining pcplists > once, before calling start_isolate_page_range(). The draining will serialize > after processes that already disabled interrupts and read the old value of > pcp->high in free_unref_page_commit(), and processes that have not yet disabled > interrupts, will observe pcp->high == 0 when they are rescheduled, and skip > pcplists. This guarantees no stray pages on pcplists in zones where isolation > happens. > > This patch thus adds zone_pcp_disable() and zone_pcp_enable() functions that > page isolation users can call before start_isolate_page_range() and after > unisolating (or offlining) the isolated pages. > > Also, drain_all_pages() is optimized to only execute on cpus where pcplists are > not empty. The check can however race with a free to pcplist that has not yet > increased the pcp->count from 0 to 1. Thus make the drain optionally skip the > racy check and drain on all cpus, and use this option in zone_pcp_disable(). ^ there it is, can you move that comment into the code? People (e.g., me :) ) staring at that will be confused otherwise. -- Thanks, David / dhildenb