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=-5.9 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 34E96C07E99 for ; Mon, 12 Jul 2021 04:14:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 15419610D0 for ; Mon, 12 Jul 2021 04:14:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231834AbhGLEQt (ORCPT ); Mon, 12 Jul 2021 00:16:49 -0400 Received: from foss.arm.com ([217.140.110.172]:44888 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229465AbhGLEQs (ORCPT ); Mon, 12 Jul 2021 00:16:48 -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 7661C6D; Sun, 11 Jul 2021 21:14:00 -0700 (PDT) Received: from [10.163.65.188] (unknown [10.163.65.188]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 430903F766; Sun, 11 Jul 2021 21:13:58 -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> From: Anshuman Khandual Message-ID: <42a26202-10f7-e744-3fc5-c9e5a7445193@arm.com> Date: Mon, 12 Jul 2021 09:44:46 +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: <20210706061748.161258-1-gshan@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Gavin, 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 ? > > (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. > > "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. - Anshuman