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=-5.3 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 E5D0AC636CB for ; Sun, 18 Jul 2021 06:35:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AA08E61183 for ; Sun, 18 Jul 2021 06:35:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230249AbhGRGid (ORCPT ); Sun, 18 Jul 2021 02:38:33 -0400 Received: from foss.arm.com ([217.140.110.172]:34198 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229480AbhGRGic (ORCPT ); Sun, 18 Jul 2021 02:38:32 -0400 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 862066D; Sat, 17 Jul 2021 23:35:34 -0700 (PDT) Received: from [10.163.66.155] (unknown [10.163.66.155]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 585543F766; Sat, 17 Jul 2021 23:35:32 -0700 (PDT) Subject: Re: [PATCH 00/12] mm/debug_vm_pgtable: Enhancements To: Gavin Shan , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, shan.gavin@gmail.com, chuhu@redhat.com References: <20210706061748.161258-1-gshan@redhat.com> <42a26202-10f7-e744-3fc5-c9e5a7445193@arm.com> <30a938df-f5c8-910c-8ddc-52a2d2a0a11e@arm.com> From: Anshuman Khandual Message-ID: <0d0e438a-fe12-fb06-fe98-12ad43e35096@arm.com> Date: Sun, 18 Jul 2021 12:06:22 +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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/15/21 10:47 AM, Gavin Shan wrote: > Hi Anshuman, > > On 7/14/21 3:26 PM, Anshuman Khandual wrote: >> On 7/13/21 6:50 AM, Gavin Shan wrote: >>> On 7/12/21 2:14 PM, Anshuman Khandual wrote: >>>> Though I have not jumped into the details for all individual >>>> patches here but still have some high level questions below. >>>> >>>> On 7/6/21 11:47 AM, Gavin Shan wrote: >>>>> There are couple of issues with current implementations and this series >>>>> tries to resolve the issues: >>>>> >>>>>     (a) All needed information are scattered in variables, passed to various >>>>>         test functions. The code is organized in pretty much relaxed fashion. >>>> All these variables are first prepared in debug_vm_pgtable(), before >>>> getting passed into respective individual test functions. Also these >>>> test functions receive only the required number of variables not all. >>>> Adding a structure that captures all test parameters at once before >>>> passing them down will be unnecessary. I am still wondering what will >>>> be the real benefit of this large code churn ? >>>> >>> >>> Thanks for your review. There are couple of reasons to have "struct vm_pgtable_debug". >>> >>> (1) With the struct, the old and new implementation can coexist. In this way, >>>      the patches in this series can be stacked up easily. >> >> Makes sense. >> >>> (2) I think passing single struct to individual test functions improves the >>>      code readability. Besides, it also makes the empty stubs simplified. >> >> Empty stub simplified - reduced argument set in the empty stubs ? >> > > Yes. > >>> (3) The code can be extended easily if we need in future. >> >> Agreed. >> >>> >>>>> >>>>>     (b) The page isn't allocated from buddy during page table entry modifying >>>>>         tests. The page can be invalid, conflicting to the implementations >>>>>         of set_{pud, pmd, pte}_at() on ARM64. The target page is accessed >>>>>         so that the iCache can be flushed when execution permission is given >>>>>         on ARM64. Besides, the target page can be unmapped and access to >>>>>         it causes kernel crash. >>>> >>>> Using 'start_kernel' based method for struct page usage, enabled this >>>> test to run on platforms which might not have enough memory required >>>> for various individual test functions. This method is not a problem for >>>> tests that just need an aligned pfn (which creates a page table entry) >>>> not a real struct page. >>>> >>>> But not allocating and owning the struct page might be problematic for >>>> tests that expect a real struct page and transform its state via set_ >>>> {pud, pmd, pte}_at() functions as reported here. >>>> >>> >>> Yeah, I totally agree. The series follows what you explained: Except the >>> test cases where set_{pud, pmd, pte}_at() is used, the allocated page >>> is used. For other test cases, 'start_kernel' based PFN is used as before. >>> >>>>> >>>>> "struct vm_pgtable_debug" is introduced to address issue (a). For issue >>>>> (b), the used page is allocated from buddy in page table entry modifying >>>>> tests. The corresponding tets will be skipped if we fail to allocate the >>>>> (huge) page. For other test cases, the original page around to kernel >>>>> symbol (@start_kernel) is still used. >>>> >>>> For all basic pfn requiring tests, existing 'start_kernel' based method >>>> should continue but allocate a struct page for other tests which change >>>> the passed struct page. Skipping the tests when allocation fails is the >>>> right thing to do. >>>> >>> >>> Yes, it's exactly what this series does. Hope you can jump into the details >>> when you get a chance :) >> >> I have already started looking into the series. But still wondering if >> the huge page memory allocation change and the arm64 specific page fix >> should be completed first, before getting into the new structure based >> arguments (in a separate series). Although the end result would still >> remain the same, the transition there would be better I guess. Do you >> see any challenges in achieving that ? >> > > Thanks for your time to review in details. As I can understand, the reason > to have the fix for easy backporting to stable-kernel and I didn't do that > because of couple of facts: (1) The changes included in this series only > affects only one source file, so backporting the whole series isn't hard. > (2) There will be more redundant code if we include the fix before switching > to "struct vm_pgtable_debug". It's unnecessary. Okay. > > So lets keep the patch layout we had if you agree. Actually, the issues are > found during the testing with RHEL downstream kernel. Once it's settled down, > I will backport the whole series to RHEL downstream kernel. Okay, then lets keep this proposed layout and address the issues here. - Anshuman