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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 9900FC4321A for ; Fri, 28 Jun 2019 03:46:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 77F55214DA for ; Fri, 28 Jun 2019 03:46:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727271AbfF1DqM (ORCPT ); Thu, 27 Jun 2019 23:46:12 -0400 Received: from foss.arm.com ([217.140.110.172]:39750 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726748AbfF1DqL (ORCPT ); Thu, 27 Jun 2019 23:46:11 -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 EEEA72B; Thu, 27 Jun 2019 20:46:10 -0700 (PDT) Received: from [10.162.40.144] (p8cg001049571a15.blr.arm.com [10.162.40.144]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 965093F706; Thu, 27 Jun 2019 20:46:08 -0700 (PDT) Subject: Re: [RFC 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics To: Zi Yan Cc: linux-mm@kvack.org, Catalin Marinas , Will Deacon , Mark Rutland , Marc Zyngier , Suzuki Poulose , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <1561639696-16361-1-git-send-email-anshuman.khandual@arm.com> <1561639696-16361-2-git-send-email-anshuman.khandual@arm.com> <7F685152-7C6C-4E99-99DF-03DDD03D6094@nvidia.com> From: Anshuman Khandual Message-ID: <5c490be8-5ac1-0a3a-32cf-d4e692fc59b5@arm.com> Date: Fri, 28 Jun 2019 09:16:35 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <7F685152-7C6C-4E99-99DF-03DDD03D6094@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/27/2019 09:01 PM, Zi Yan wrote: > On 27 Jun 2019, at 8:48, 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_mknotpresent -> 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(). >> >> PTE_SPECIAL never gets used for PMD mapping i.e there is no pmd_special(). >> Hence this bit can be set on the PMD entry during invalidation which can >> help in making pmd_present() return true and in recognizing the fact that >> it still points to memory. >> >> This bit is transient. During the split is process it will be overridden >> by a page table page representing the normal pages in place of erstwhile >> huge page. Other pmdp_invalidate() callers always write a fresh PMD value >> on the entry overriding this transient PTE_SPECIAL making it safe. In the >> past former pmd_[mk]splitting() functions used PTE_SPECIAL. >> >> [1]: https://lkml.org/lkml/2018/10/17/231 > > Just want to point out that lkml.org link might not be stable. > This one would be better: https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/ Sure will update the link in the commit. Thanks !