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 D1B00C636C8 for ; Sun, 18 Jul 2021 06:35:37 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7BED361073 for ; Sun, 18 Jul 2021 06:35:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7BED361073 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 DD3258D00F4; Sun, 18 Jul 2021 02:35:37 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D5DA28D00EC; Sun, 18 Jul 2021 02:35:37 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B390F8D00F4; Sun, 18 Jul 2021 02:35:37 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0164.hostedemail.com [216.40.44.164]) by kanga.kvack.org (Postfix) with ESMTP id 880158D00EC for ; Sun, 18 Jul 2021 02:35:37 -0400 (EDT) Received: from smtpin28.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 118998249980 for ; Sun, 18 Jul 2021 06:35:36 +0000 (UTC) X-FDA: 78374747472.28.02501D7 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf03.hostedemail.com (Postfix) with ESMTP id 655C230001B9 for ; Sun, 18 Jul 2021 06:35:35 +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 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 Authentication-Results: imf03.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf03.hostedemail.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 655C230001B9 X-Stat-Signature: bdmc8djypd3z44dwj318czuyw6axa3y8 X-HE-Tag: 1626590135-190883 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 7/15/21 10:47 AM, Gavin Shan wrote: > Hi Anshuman, >=20 > 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 se= ries >>>>> tries to resolve the issues: >>>>> >>>>> =C2=A0=C2=A0=C2=A0 (a) All needed information are scattered in vari= ables, passed to various >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 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 wil= l >>>> be the real benefit of this large code churn ? >>>> >>> >>> Thanks for your review. There are couple of reasons to have "struct v= m_pgtable_debug". >>> >>> (1) With the struct, the old and new implementation can coexist. In t= his way, >>> =C2=A0=C2=A0=C2=A0=C2=A0 the patches in this series can be stacked up= easily. >> >> Makes sense. >> >>> (2) I think passing single struct to individual test functions improv= es the >>> =C2=A0=C2=A0=C2=A0=C2=A0 code readability. Besides, it also makes the= empty stubs simplified. >> >> Empty stub simplified - reduced argument set in the empty stubs ? >> >=20 > Yes. >=20 >>> (3) The code can be extended easily if we need in future. >> >> Agreed. >> >>> >>>>> >>>>> =C2=A0=C2=A0=C2=A0 (b) The page isn't allocated from buddy during p= age table entry modifying >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 tests. The page can be i= nvalid, conflicting to the implementations >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 of set_{pud, pmd, pte}_a= t() on ARM64. The target page is accessed >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 so that the iCache can b= e flushed when execution permission is given >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 on ARM64. Besides, the t= arget page can be unmapped and access to >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 it causes kernel crash. >>>> >>>> Using 'start_kernel' based method for struct page usage, enabled thi= s >>>> 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 entr= y) >>>> not a real struct page. >>>> >>>> But not allocating and owning the struct page might be problematic f= or >>>> 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 be= fore. >>> >>>>> >>>>> "struct vm_pgtable_debug" is introduced to address issue (a). For i= ssue >>>>> (b), the used page is allocated from buddy in page table entry modi= fying >>>>> tests. The corresponding tets will be skipped if we fail to allocat= e the >>>>> (huge) page. For other test cases, the original page around to kern= el >>>>> symbol (@start_kernel) is still used. >>>> >>>> For all basic pfn requiring tests, existing 'start_kernel' based met= hod >>>> should continue but allocate a struct page for other tests which cha= nge >>>> 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 d= etails >>> 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 ? >> >=20 > Thanks for your time to review in details. As I can understand, the rea= son > to have the fix for easy backporting to stable-kernel and I didn't do t= hat > because of couple of facts: (1) The changes included in this series onl= y > affects only one source file, so backporting the whole series isn't har= d. > (2) There will be more redundant code if we include the fix before swit= ching > to "struct vm_pgtable_debug". It's unnecessary. Okay. >=20 > 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