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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 74643C433E7 for ; Thu, 8 Oct 2020 09:49:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2849320725 for ; Thu, 8 Oct 2020 09:49:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729304AbgJHJth (ORCPT ); Thu, 8 Oct 2020 05:49:37 -0400 Received: from pegase1.c-s.fr ([93.17.236.30]:44728 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725887AbgJHJtf (ORCPT ); Thu, 8 Oct 2020 05:49:35 -0400 Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 4C6RHv6vbCz9v0Jb; Thu, 8 Oct 2020 11:49:31 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id mnZbLhC9ScyS; Thu, 8 Oct 2020 11:49:31 +0200 (CEST) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 4C6RHv5nYlz9v0JY; Thu, 8 Oct 2020 11:49:31 +0200 (CEST) Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id A96338B832; Thu, 8 Oct 2020 11:49:32 +0200 (CEST) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id xNCjMtQPDKXQ; Thu, 8 Oct 2020 11:49:32 +0200 (CEST) Received: from [192.168.4.90] (unknown [192.168.4.90]) by messagerie.si.c-s.fr (Postfix) with ESMTP id A45508B82F; Thu, 8 Oct 2020 11:49:29 +0200 (CEST) From: Christophe Leroy Subject: mm: Question about the use of 'accessed' flags and pte_young() helper To: linux-mm , "linux-kernel@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "Aneesh Kumar K.V" , Nicholas Piggin Message-ID: <31ef1305-1fd4-8159-a2ca-e9968a568ff0@csgroup.eu> Date: Thu, 8 Oct 2020 11:49:23 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.1 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In a 10 years old commit (https://github.com/linuxppc/linux/commit/d069cb4373fe0d451357c4d3769623a7564dfa9f), powerpc 8xx has made the handling of PTE accessed bit conditional to CONFIG_SWAP. Since then, this has been extended to some other powerpc variants. That commit means that when CONFIG_SWAP is not selected, the accessed bit is not set by SW TLB miss handlers, leading to pte_young() returning garbage, or should I say possibly returning false allthough a page has been accessed since its access flag was reset. Looking at various mm/ places, pte_young() is used independent of CONFIG_SWAP Is it still valid the not manage accessed flags when CONFIG_SWAP is not selected ? If yes, should pte_young() always return true in that case ? While we are at it, I'm wondering whether powerpc should redefine arch_faults_on_old_pte() On some variants of powerpc, accessed flag is managed by HW. On others, it is managed by SW TLB miss handlers via page fault handling. Thanks Christophe