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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 CE558C64E7B for ; Mon, 30 Nov 2020 11:02:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 4AFB020855 for ; Mon, 30 Nov 2020 11:02:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4AFB020855 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 71A7B6B0036; Mon, 30 Nov 2020 06:02:06 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6CE188D0003; Mon, 30 Nov 2020 06:02:06 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 593018D0001; Mon, 30 Nov 2020 06:02:06 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0249.hostedemail.com [216.40.44.249]) by kanga.kvack.org (Postfix) with ESMTP id 40F336B0036 for ; Mon, 30 Nov 2020 06:02:06 -0500 (EST) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 0B756180AD83B for ; Mon, 30 Nov 2020 11:02:06 +0000 (UTC) X-FDA: 77540795052.15.fact92_5c07a55273a1 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin15.hostedemail.com (Postfix) with ESMTP id CDD431814B0C1 for ; Mon, 30 Nov 2020 11:02:05 +0000 (UTC) X-HE-Tag: fact92_5c07a55273a1 X-Filterd-Recvd-Size: 5432 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf18.hostedemail.com (Postfix) with ESMTP for ; Mon, 30 Nov 2020 11:02:05 +0000 (UTC) Received: from gaia (unknown [95.146.230.165]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 730F02087C; Mon, 30 Nov 2020 11:02:02 +0000 (UTC) Date: Mon, 30 Nov 2020 11:01:59 +0000 From: Catalin Marinas To: Anshuman Khandual Cc: Christophe Leroy , linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, steven.price@arm.com, gerald.schaefer@linux.ibm.com, vgupta@synopsys.com, paul.walmsley@sifive.com Subject: Re: [PATCH 1/2] mm/debug_vm_pgtable/basic: Add validation for dirtiness after write protect Message-ID: <20201130110159.GB3902@gaia> References: <1606453584-15399-1-git-send-email-anshuman.khandual@arm.com> <1606453584-15399-2-git-send-email-anshuman.khandual@arm.com> <20201127094421.GA25070@gaia> <9d9e5c8b-08f6-9ed4-074c-3dafc8fa3717@arm.com> <20201130093841.GA3902@gaia> <171a4e71-b1ab-3ff5-7088-54781d960b2a@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <171a4e71-b1ab-3ff5-7088-54781d960b2a@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Mon, Nov 30, 2020 at 04:28:20PM +0530, Anshuman Khandual wrote: > On 11/30/20 3:08 PM, Catalin Marinas wrote: > > On Mon, Nov 30, 2020 at 09:55:00AM +0530, Anshuman Khandual wrote: > >> On 11/27/20 3:14 PM, Catalin Marinas wrote: > >>> On Fri, Nov 27, 2020 at 09:22:24AM +0100, Christophe Leroy wrote: > >>>> Le 27/11/2020 =E0 06:06, Anshuman Khandual a =E9crit=A0: > >>>>> This adds validation tests for dirtiness after write protect conv= ersion for > >>>>> each page table level. This is important for platforms such as ar= m64 that > >>>>> removes the hardware dirty bit while making it an write protected= one. This > >>>>> also fixes pxx_wrprotect() related typos in the documentation fil= e. > >>>> > >>>>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c > >>>>> index c05d9dcf7891..a5be11210597 100644 > >>>>> --- a/mm/debug_vm_pgtable.c > >>>>> +++ b/mm/debug_vm_pgtable.c > >>>>> @@ -70,6 +70,7 @@ static void __init pte_basic_tests(unsigned lon= g pfn, pgprot_t prot) > >>>>> WARN_ON(pte_young(pte_mkold(pte_mkyoung(pte)))); > >>>>> WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte)))); > >>>>> WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte)))); > >>>>> + WARN_ON(pte_dirty(pte_wrprotect(pte))); > >>>> > >>>> Wondering what you are testing here exactly. > >>>> > >>>> Do you expect that if PTE has the dirty bit, it gets cleared by > >>>> pte_wrprotect() ? > >>>> > >>>> Powerpc doesn't do that, it only clears the RW bit but the dirty > >>>> bit remains if it is set, until you call pte_mkclean() explicitely= . > >>> > >>> Arm64 has an unusual way of setting a hardware dirty "bit", it actu= ally > >>> clears the PTE_RDONLY bit. The pte_wrprotect() sets the PTE_RDONLY = bit > >>> back and we can lose the dirty information. Will found this and pos= ted > >>> patches to fix the arm64 pte_wprotect() to set a software PTE_DIRTY= if > >>> !PTE_RDONLY (we do this for ptep_set_wrprotect() already). My conce= rn > >>> was that we may inadvertently make a fresh/clean pte dirty with suc= h > >>> change, hence the suggestion for the test. > >>> > >>> That said, I think we also need a test in the other direction, > >>> pte_wrprotect() should preserve any dirty information: > >>> > >>> WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); > >> > >> This seems like a generic enough principle which all platforms shoul= d > >> adhere to. But the proposed test WARN_ON(pte_dirty(pte_wrprotect(pte= ))) > >> might fail on some platforms if the page table entry came in as a di= rty > >> one and pte_wrprotect() is not expected to alter the dirty state. > >=20 > > Ah, so do we have architectures where entries in protection_map[] are > > already dirty? If those are valid, maybe the check should be: >=20 > Okay, I did not imply that actually. The current position for these new > tests in respective pxx_basic_tests() functions is right at the end and > hence the pxx might have already gone through some changes from the tim= e > it was originally created with pfn_pxx(). The entry here is not startin= g > from the beginning. It is not expected as well, per design. So dirty bi= t > might or might not be there depending on all the previous test sequence= s > leading upto these new ones. >=20 > IIUC, Christophe mentioned the fact that on platforms like powerpc, dir= ty > bit just remains unchanged during pte_wprotect(). So the current test > WARN_ON(pte_dirty(pte_wrprotect(pte))) will not work on powerpc if the > previous tests leading upto that point has got the dirty bit set. This = is > irrespective of how it was created with pfn_pte() from protection_map[] > originally at the beginning. [...] > To achieve this, we could move the test right at the beginning just aft= er > the pxx gets created from protection_map[], with a comment explaining t= he > rationale.=20 OK, this makes sense. Thanks for the clarification. --=20 Catalin