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,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 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 81803C433DF for ; Tue, 18 Aug 2020 12:28:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5EB96207D3 for ; Tue, 18 Aug 2020 12:28:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726773AbgHRM2C (ORCPT ); Tue, 18 Aug 2020 08:28:02 -0400 Received: from lhrrgout.huawei.com ([185.176.76.210]:2615 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726745AbgHRM16 (ORCPT ); Tue, 18 Aug 2020 08:27:58 -0400 Received: from lhreml710-chm.china.huawei.com (unknown [172.18.7.107]) by Forcepoint Email with ESMTP id DD78C3F249352D8EDAFF; Tue, 18 Aug 2020 13:27:56 +0100 (IST) Received: from localhost (10.52.121.15) by lhreml710-chm.china.huawei.com (10.201.108.61) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.1913.5; Tue, 18 Aug 2020 13:27:56 +0100 Date: Tue, 18 Aug 2020 13:26:25 +0100 From: Jonathan Cameron To: Anshuman Khandual CC: , , , , , Mark Rutland , Marc Zyngier , "Suzuki Poulose" , Subject: Re: [PATCH 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics Message-ID: <20200818132625.00003d05@Huawei.com> In-Reply-To: <8db455b6-8fe5-b552-119f-4abab0cc8501@arm.com> References: <1597655984-15428-1-git-send-email-anshuman.khandual@arm.com> <1597655984-15428-2-git-send-email-anshuman.khandual@arm.com> <20200818101301.000027ef@Huawei.com> <8db455b6-8fe5-b552-119f-4abab0cc8501@arm.com> Organization: Huawei Technologies Research and Development (UK) Ltd. X-Mailer: Claws Mail 3.17.4 (GTK+ 2.24.32; i686-w64-mingw32) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.52.121.15] X-ClientProxiedBy: lhreml723-chm.china.huawei.com (10.201.108.74) To lhreml710-chm.china.huawei.com (10.201.108.61) X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 18 Aug 2020 15:11:58 +0530 Anshuman Khandual wrote: > On 08/18/2020 02:43 PM, Jonathan Cameron wrote: > > On Mon, 17 Aug 2020 14:49:43 +0530 > > Anshuman Khandual wrote: > > > >> pmd_present() and pmd_trans_huge() are expected to behave in the following > >> manner during various phases of a given PMD. It is derived from a previous > >> detailed discussion on this topic [1] and present THP documentation [2]. > >> > >> pmd_present(pmd): > >> > >> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd) > >> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(pmd) > >> > >> pmd_trans_huge(pmd): > >> > >> - Returns true if pmd refers to system RAM and is a trans huge mapping > >> > >> ------------------------------------------------------------------------- > >> | PMD states | pmd_present | pmd_trans_huge | > >> ------------------------------------------------------------------------- > >> | Mapped | Yes | Yes | > >> ------------------------------------------------------------------------- > >> | Splitting | Yes | Yes | > >> ------------------------------------------------------------------------- > >> | Migration/Swap | No | No | > >> ------------------------------------------------------------------------- > >> > >> The problem: > >> > >> PMD is first invalidated with pmdp_invalidate() before it's splitting. This > >> invalidation clears PMD_SECT_VALID as below. > >> > >> PMD Split -> pmdp_invalidate() -> pmd_mkinvalid -> Clears PMD_SECT_VALID > >> > >> Once PMD_SECT_VALID gets cleared, it results in pmd_present() return false > >> on the PMD entry. It will need another bit apart from PMD_SECT_VALID to re- > >> affirm pmd_present() as true during the THP split process. To comply with > >> above mentioned semantics, pmd_trans_huge() should also check pmd_present() > >> first before testing presence of an actual transparent huge mapping. > >> > >> The solution: > >> > >> Ideally PMD_TYPE_SECT should have been used here instead. But it shares the > >> bit position with PMD_SECT_VALID which is used for THP invalidation. Hence > >> it will not be there for pmd_present() check after pmdp_invalidate(). > >> > >> A new software defined PMD_PRESENT_INVALID (bit 59) can be set on the PMD > >> entry during invalidation which can help pmd_present() return true and in > >> recognizing the fact that it still points to memory. > >> > >> This bit is transient. During the split process it will be overridden by a > >> page table page representing normal pages in place of erstwhile huge page. > >> Other pmdp_invalidate() callers always write a fresh PMD value on the entry > >> overriding this transient PMD_PRESENT_INVALID bit, which makes it safe. > >> > >> [1]: https://lkml.org/lkml/2018/10/17/231 > >> [2]: https://www.kernel.org/doc/Documentation/vm/transhuge.txt > > > > Hi Anshuman, > > > > One query on this. From my reading of the ARM ARM, bit 59 is not > > an ignored bit. The exact requirements for hardware to be using > > it are a bit complex though. > > > > It 'might' be safe to use it for this, but if so can we have a comment > > explaining why. Also more than possible I'm misunderstanding things! > > We are using this bit 59 only when the entry is not active from MMU > perspective i.e PMD_SECT_VALID is clear. > Understood. I guess we ran out of bits that were always ignored so had to start using ones that are ignored in this particular state. Jonathan