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=-7.5 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 044AFC433DB for ; Fri, 26 Mar 2021 14:54:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C5542619E4 for ; Fri, 26 Mar 2021 14:54:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230209AbhCZOyL (ORCPT ); Fri, 26 Mar 2021 10:54:11 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:44124 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229871AbhCZOxv (ORCPT ); Fri, 26 Mar 2021 10:53:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1616770430; 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=sniOXWpl3SzSr30LS38MLQ1sgZ4a8SSZHY7ekXG97xE=; b=M0fvLAofx4IhbmUBfEihFvah/wMQVbekVERevDtfMhkYIvvXin5MALbvqL08B2CmMheSt6 Ipzuyi0w+hHVspjLFbNAoi0Cm5cvqk57K7+gu9zauzRIa9mJvxFKS8zLcAIKyCL1xKCknd 92og8BqgxtnvZQ7UsOcxyFI0h1j9u48= 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-526-G0sXW3nxNQeM3W9axMD_oA-1; Fri, 26 Mar 2021 10:53:46 -0400 X-MC-Unique: G0sXW3nxNQeM3W9axMD_oA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 33087A40C0; Fri, 26 Mar 2021 14:53:45 +0000 (UTC) Received: from [10.36.112.81] (ovpn-112-81.ams2.redhat.com [10.36.112.81]) by smtp.corp.redhat.com (Postfix) with ESMTP id ED113772E0; Fri, 26 Mar 2021 14:53:42 +0000 (UTC) To: Michal Hocko Cc: Oscar Salvador , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <40fac999-2d28-9205-23f0-516fa9342bbe@redhat.com> <92fe19d0-56ac-e929-a9c1-d6a4e0da39d1@redhat.com> <5be95091-b4ac-8e05-4694-ac5c65f790a4@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: <0e735ea7-b3d9-615e-6bba-fa3a16883226@redhat.com> Date: Fri, 26 Mar 2021 15:53:41 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26.03.21 15:38, Michal Hocko wrote: > On Fri 26-03-21 09:52:58, David Hildenbrand wrote: > [...] >> Something else to note: >> >> >> We'll not call the memory notifier (e.g., MEM_ONLINE) for the vmemmap. The >> result is that >> >> 1. We won't allocate extended struct pages for the range. Don't think this >> is really problematic (pages are never allocated/freed, so I guess we don't >> care - like ZONE_DEVICE code). > > Agreed. I do not think we need them. Future might disagree but let's > handle it when we have a clear demand. > >> 2. We won't allocate kasan shadow memory. We most probably have to do it >> explicitly via kasan_add_zero_shadow()/kasan_remove_zero_shadow(), see >> mm/memremap.c:pagemap_range() > > I think this is similar to the above. Does kasan has to know about > memory which will never be used for anything? IIRC, kasan will track read/writes to the vmemmap as well. So it could theoretically detect if we read from the vmemmap before writing (initializing) it IIUC. This is also why mm/memremap.c does a kasan_add_zero_shadow() before the move_pfn_range_to_zone()->memmap_init_range() for the whole region, including altmap space. Now, I am no expert on KASAN, what would happen in case we have access to non-tracked memory. commit 0207df4fa1a869281ddbf72db6203dbf036b3e1a Author: Andrey Ryabinin Date: Fri Aug 17 15:47:04 2018 -0700 kernel/memremap, kasan: make ZONE_DEVICE with work with KASAN indicates that kasan will crash the system on "non-existent shadow memory" > >> Further a locking rework might be necessary. We hold the device hotplug >> lock, but not the memory hotplug lock. E.g., for get_online_mems(). Might >> have to move that out online_pages. > > Could you be more explicit why this locking is needed? What it would > protect from for vmemmap pages? > One example is in mm/kmemleak.c:kmemleak_scan(), where we scan the vmemmap for pointers. We don't want the vmemmap to get unmapped while we are working on it (-> fault). -- Thanks, David / dhildenb