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=-7.3 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 2D393C433C1 for ; Wed, 31 Mar 2021 06:41:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F149C619D6 for ; Wed, 31 Mar 2021 06:41:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233812AbhCaGlU (ORCPT ); Wed, 31 Mar 2021 02:41:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:49961 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233694AbhCaGlT (ORCPT ); Wed, 31 Mar 2021 02:41:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617172878; 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=qLJOO2atgx+JHmVUPVt2Vsd5PrFDH/M5+OfaqMOeD1c=; b=agyaYKVLqZKJg6cxT0ptA2dAtqmauD6XwcJOkyNPhaDv/k4vrDv9JfCokxIMcphmvhdwB0 arSHZngTzcFGCF9x1ZmumUq1Ae1TQ6WcZPYLAfytnihTnXY1C2H0F1CCQE61TIkvkBy5YP D6BDrhVjQLCxbIhXqhfPMw66x0C4E0Q= 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-13-r5TpdFcWMEWwxQsq_6ijwQ-1; Wed, 31 Mar 2021 02:41:09 -0400 X-MC-Unique: r5TpdFcWMEWwxQsq_6ijwQ-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 4CBDF9CC00; Wed, 31 Mar 2021 06:41:04 +0000 (UTC) Received: from [10.36.113.60] (ovpn-113-60.ams2.redhat.com [10.36.113.60]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2BB485D740; Wed, 31 Mar 2021 06:41:02 +0000 (UTC) To: Alistair Popple Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, daniel.vetter@ffwll.ch, dan.j.williams@intel.com, gregkh@linuxfoundation.org, jhubbard@nvidia.com, jglisse@redhat.com, linux-mm@kvack.org References: <20210326012035.3853-1-apopple@nvidia.com> <3158185.bARUjMUeyn@nvdebian> <12736273.ONR6GAMRWp@nvdebian> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v2] kernel/resource: Fix locking in request_free_mem_region Message-ID: <2e312099-bf47-831a-5d0e-3e95053cdb3f@redhat.com> Date: Wed, 31 Mar 2021 08:41:00 +0200 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: <12736273.ONR6GAMRWp@nvdebian> 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 31.03.21 08:19, Alistair Popple wrote: > On Tuesday, 30 March 2021 8:13:32 PM AEDT David Hildenbrand wrote: >> External email: Use caution opening links or attachments >> >> >> On 29.03.21 03:37, Alistair Popple wrote: >>> On Friday, 26 March 2021 7:57:51 PM AEDT David Hildenbrand wrote: >>>> On 26.03.21 02:20, Alistair Popple wrote: >>>>> request_free_mem_region() is used to find an empty range of physical >>>>> addresses for hotplugging ZONE_DEVICE memory. It does this by iterating >>>>> over the range of possible addresses using region_intersects() to see if >>>>> the range is free. >>>> >>>> Just a high-level question: how does this iteract with memory >>>> hot(un)plug? IOW, how defines and manages the "range of possible >>>> addresses" ? >>> >>> Both the driver and the maximum physical address bits available define the >>> range of possible addresses for device private memory. From >>> __request_free_mem_region(): >>> >>> end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1); >>> addr = end - size + 1UL; >>> >>> There is no lower address range bound here so it is effectively zero. The > code >>> will try to allocate the highest possible physical address first and > continue >>> searching down for a free block. Does that answer your question? >> >> Oh, sorry, the fist time I had a look I got it wrong - I thought (1UL << >> MAX_PHYSMEM_BITS) would be the lower address limit. That looks indeed >> problematic to me. >> >> You might end up reserving an iomem region that could be used e.g., by >> memory hotplug code later. If someone plugs a DIMM or adds memory via >> different approaches (virtio-mem), memory hotplug (via add_memory()) >> would fail. >> >> You never should be touching physical memory area reserved for memory >> hotplug, i.e., via SRAT. >> >> What is the expectation here? > > Most drivers call request_free_mem_region() with iomem_resource as the base. > So zone device private pages currently tend to get allocated from the top of > that. Okay, but you could still "steal" iomem space that does not belong to you, and the firmware will be unaware of that (e.g., it might hotplug a DIMM in these spots). This is really nasty (although I guess as you allocate top down, it will happen rarely). > > By definition ZONE_DEVICE private pages are unaddressable from the CPU. So in > terms of expectation I think all that is really required for ZONE_DEVICE > private pages (at least for Nouveau) is a valid range of physical addresses > that allow page_to_pfn() and pfn_to_page() to work correctly. To make this > work drivers add the pages via memremap_pages() -> pagemap_range() -> > add_pages(). So you'd actually want some region above the hotpluggable/addressable range -- e.g., above MAX_PHYSMEM_BITS. The maximum number of sections we can have is define by #define SECTIONS_SHIFT (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS) You'd e.g., want an extra space like (to be improved) #define DEVMEM_BITS 1 #define SECTIONS_SHIFT (MAX_PHYSMEM_BITS + DEVMEM_BITS - SECTION_SIZE_BITS) And do the search only within that range. -- Thanks, David / dhildenb