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, 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 125ECC433DB for ; Wed, 3 Feb 2021 03:50:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 561A464F61 for ; Wed, 3 Feb 2021 03:50:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 561A464F61 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 8CAE06B0006; Tue, 2 Feb 2021 22:50:16 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 87C1B6B006C; Tue, 2 Feb 2021 22:50:16 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 76ACC6B006E; Tue, 2 Feb 2021 22:50:16 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0129.hostedemail.com [216.40.44.129]) by kanga.kvack.org (Postfix) with ESMTP id 5CBDF6B0006 for ; Tue, 2 Feb 2021 22:50:16 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 1F425181AEF0B for ; Wed, 3 Feb 2021 03:50:16 +0000 (UTC) X-FDA: 77775578832.30.paint53_1105437275d0 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id 0CF6A180B3C8B for ; Wed, 3 Feb 2021 03:50:15 +0000 (UTC) X-HE-Tag: paint53_1105437275d0 X-Filterd-Recvd-Size: 6148 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf46.hostedemail.com (Postfix) with ESMTP for ; Wed, 3 Feb 2021 03:50:15 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 88E77ED1; Tue, 2 Feb 2021 19:50:14 -0800 (PST) Received: from [192.168.0.130] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BFE703F694; Tue, 2 Feb 2021 19:50:10 -0800 (PST) Subject: Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory To: David Hildenbrand , Will Deacon Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Catalin Marinas , Ard Biesheuvel , Mark Rutland , James Morse , Robin Murphy , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Dan Williams , Mike Rapoport References: <1612239114-28428-1-git-send-email-anshuman.khandual@arm.com> <1612239114-28428-2-git-send-email-anshuman.khandual@arm.com> <20210202123215.GA16868@willie-the-truck> <20210202123524.GB16868@willie-the-truck> <20210202125152.GC16868@willie-the-truck> <4d8f5156-8628-5531-1485-322ad92aa15c@redhat.com> From: Anshuman Khandual Message-ID: <0e649f28-4d54-319d-f876-8a93870cda7f@arm.com> Date: Wed, 3 Feb 2021 09:20:39 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <4d8f5156-8628-5531-1485-322ad92aa15c@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2/2/21 6:26 PM, David Hildenbrand wrote: > On 02.02.21 13:51, Will Deacon wrote: >> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote: >>> On 02.02.21 13:35, Will Deacon wrote: >>>> On Tue, Feb 02, 2021 at 12:32:15PM +0000, Will Deacon wrote: >>>>> On Tue, Feb 02, 2021 at 09:41:53AM +0530, Anshuman Khandual wrote: >>>>>> pfn_valid() validates a pfn but basically it checks for a valid st= ruct page >>>>>> backing for that pfn. It should always return positive for memory = ranges >>>>>> backed with struct page mapping. But currently pfn_valid() fails f= or all >>>>>> ZONE_DEVICE based memory types even though they have struct page m= apping. >>>>>> >>>>>> pfn_valid() asserts that there is a memblock entry for a given pfn= without >>>>>> MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based = memory is >>>>>> that they do not have memblock entries. Hence memblock_is_map_memo= ry() will >>>>>> invariably fail via memblock_search() for a ZONE_DEVICE based addr= ess. This >>>>>> eventually fails pfn_valid() which is wrong. memblock_is_map_memor= y() needs >>>>>> to be skipped for such memory ranges. As ZONE_DEVICE memory gets h= otplugged >>>>>> into the system via memremap_pages() called from a driver, their r= espective >>>>>> memory sections will not have SECTION_IS_EARLY set. >>>>>> >>>>>> Normal hotplug memory will never have MEMBLOCK_NOMAP set in their = memblock >>>>>> regions. Because the flag MEMBLOCK_NOMAP was specifically designed= and set >>>>>> for firmware reserved memory regions. memblock_is_map_memory() can= just be >>>>>> skipped as its always going to be positive and that will be an opt= imization >>>>>> for the normal hotplug memory. Like ZONE_DEVICE based memory, all = normal >>>>>> hotplugged memory too will not have SECTION_IS_EARLY set for their= sections >>>>>> >>>>>> Skipping memblock_is_map_memory() for all non early memory section= s would >>>>>> fix pfn_valid() problem for ZONE_DEVICE based memory and also impr= ove its >>>>>> performance for normal hotplug memory as well. >>>>> >>>>> Hmm. Although I follow your logic, this does seem to rely on an awf= ul lot of >>>>> assumptions to continue to hold true as the kernel evolves. In part= icular, >>>>> how do we ensure that early sections are always fully backed with >>>> >>>> Sorry, typo here:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^^^ should be = *non-early* sections. >>> >>> It might be a good idea to have a look at generic >>> include/linux/mmzone.h:pfn_valid() >> >> The generic implementation already makes assumptions that aren't true = on >> arm64, so that's why we've ended up with our own implementation. But t= he >> patches here put us in a position where I worry that pfn_valid() may r= eturn >> 'true' in future for cases where the underlying struct page is either >> non-existent or bogus, and debugging those failures really sucks. We h= ad a >> raft of those back when NOMAP was introduced and I don't want to re-li= ve >> that experience. >=20 > Yeah, and I agree when it comes to boot mem. However, the way generic m= emory hotplug/memremap infrastructure (->!early sections) works does not = allow for such special cases you mention and would break quite some other= code if messed up. So I wouldn't worry about that part too much for now. Agreed. >=20 >> >>> As I expressed already, long term we should really get rid of the arm= 64 >>> variant and rather special-case the generic one. Then we won't go out= of >>> sync - just as it happened with ZONE_DEVICE handling here. >> >> Why does this have to be long term? This ZONE_DEVICE stuff could be th= e >> carrot on the stick :) >=20 > Yes, I suggested to do it now, but Anshuman convinced me that doing a s= imple fix upfront might be cleaner --- for example when it comes to backp= orting :) Right. The current pfn_valid() breaks for ZONE_DEVICE memory and this fix= es the problem in the present context which can be easily backported if requ= ired. Changing or rather overhauling the generic code with new configs as propo= sed earlier (which I am planning to work on subsequently) would definitely be= an improvement for the current pfn_valid() situation in terms of maintainabi= lity but then it should not stop us from fixing the problem now.