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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 5D916C04AAC for ; Tue, 21 May 2019 02:08:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2BB9921479 for ; Tue, 21 May 2019 02:08:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727567AbfEUCIo (ORCPT ); Mon, 20 May 2019 22:08:44 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:55934 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726335AbfEUCIo (ORCPT ); Mon, 20 May 2019 22:08:44 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CC1BB341; Mon, 20 May 2019 19:08:43 -0700 (PDT) Received: from [10.162.42.136] (p8cg001049571a15.blr.arm.com [10.162.42.136]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B102A3F718; Mon, 20 May 2019 19:08:41 -0700 (PDT) Subject: Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing virtual address To: Dan Williams , Jerome Glisse Cc: Andrew Morton , Linux Kernel Mailing List , Linux MM , Laurent Dufour References: <1558089514-25067-1-git-send-email-anshuman.khandual@arm.com> <20190517145050.2b6b0afdaab5c3c69a4b153e@linux-foundation.org> <20190520192721.GA4049@redhat.com> From: Anshuman Khandual Message-ID: <502ec91a-4019-39c7-537d-f86a7348ca40@arm.com> Date: Tue, 21 May 2019 07:38:53 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/21/2019 01:03 AM, Dan Williams wrote: > On Mon, May 20, 2019 at 12:27 PM Jerome Glisse wrote: >> >> On Mon, May 20, 2019 at 11:07:38AM +0530, Anshuman Khandual wrote: >>> On 05/18/2019 03:20 AM, Andrew Morton wrote: >>>> On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual wrote: >>>> >>>>> The presence of struct page does not guarantee linear mapping for the pfn >>>>> physical range. Device private memory which is non-coherent is excluded >>>>> from linear mapping during devm_memremap_pages() though they will still >>>>> have struct page coverage. Just check for device private memory before >>>>> giving out virtual address for a given pfn. >>>> >>>> I was going to give my standard "what are the user-visible runtime >>>> effects of this change?", but... >>>> >>>>> All these helper functions are all pfn_t related but could not figure out >>>>> another way of determining a private pfn without looking into it's struct >>>>> page. pfn_t_to_virt() is not getting used any where in mainline kernel.Is >>>>> it used by out of tree drivers ? Should we then drop it completely ? >>>> >>>> Yeah, let's kill it. >>>> >>>> But first, let's fix it so that if someone brings it back, they bring >>>> back a non-buggy version. >>> >>> Makes sense. >>> >>>> >>>> So... what (would be) the user-visible runtime effects of this change? >>> >>> I am not very well aware about the user interaction with the drivers which >>> hotplug and manage ZONE_DEVICE memory in general. Hence will not be able to >>> comment on it's user visible runtime impact. I just figured this out from >>> code audit while testing ZONE_DEVICE on arm64 platform. But the fix makes >>> the function bit more expensive as it now involve some additional memory >>> references. >> >> A device private pfn can never leak outside code that does not understand it >> So this change is useless for any existing users and i would like to keep the >> existing behavior ie never leak device private pfn. > > The issue is that only an HMM expert might know that such a pfn can > never leak, in other words the pfn concept from a code perspective is > already leaked / widespread. Ideally any developer familiar with a pfn > and the core-mm pfn helpers need only worry about pfn semantics > without being required to go audit HMM users. Agreed.