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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 AC3C3C433FF for ; Tue, 13 Aug 2019 13:30:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8B73620578 for ; Tue, 13 Aug 2019 13:30:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729151AbfHMNai (ORCPT ); Tue, 13 Aug 2019 09:30:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33128 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728526AbfHMNah (ORCPT ); Tue, 13 Aug 2019 09:30:37 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 341AC30EA182; Tue, 13 Aug 2019 13:30:37 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.136]) by smtp.corp.redhat.com (Postfix) with SMTP id 3CC49608A5; Tue, 13 Aug 2019 13:30:35 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 13 Aug 2019 15:30:36 +0200 (CEST) Date: Tue, 13 Aug 2019 15:30:34 +0200 From: Oleg Nesterov To: "Kirill A. Shutemov" Cc: Song Liu , Linux Kernel Mailing List , Linux MM , Andrew Morton , Matthew Wilcox , "Kirill A. Shutemov" , Kernel Team , William Kucharski , "srikar@linux.vnet.ibm.com" Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP Message-ID: <20190813133034.GA6971@redhat.com> References: <20190807233729.3899352-1-songliubraving@fb.com> <20190807233729.3899352-6-songliubraving@fb.com> <20190808163303.GB7934@redhat.com> <770B3C29-CE8F-4228-8992-3C6E2B5487B6@fb.com> <20190809152404.GA21489@redhat.com> <3B09235E-5CF7-4982-B8E6-114C52196BE5@fb.com> <4D8B8397-5107-456B-91FC-4911F255AE11@fb.com> <20190812121144.f46abvpg6lvxwwzs@box> <20190812132257.GB31560@redhat.com> <20190812144045.tkvipsyit3nccvuk@box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190812144045.tkvipsyit3nccvuk@box> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Tue, 13 Aug 2019 13:30:37 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/12, Kirill A. Shutemov wrote: > > On Mon, Aug 12, 2019 at 03:22:58PM +0200, Oleg Nesterov wrote: > > On 08/12, Kirill A. Shutemov wrote: > > > > > > On Fri, Aug 09, 2019 at 06:01:18PM +0000, Song Liu wrote: > > > > + if (pte_none(*pte) || !pte_present(*pte)) > > > > + continue; > > > > > > You don't need to check both. Present is never none. > > > > Agreed. > > > > Kirill, while you are here, shouldn't retract_page_tables() check > > vma->anon_vma (and probably do mm_find_pmd) under vm_mm->mmap_sem? > > > > Can't it race with, say, do_cow_fault? > > vma->anon_vma can race, but it doesn't matter. False-negative is fine. > It's attempt to avoid taking mmap_sem where it can be not productive. I guess I misunderstood the purpose of this check or your answer... Let me reword my question. Why can retract_page_tables() safely do pmdp_collapse_flush(vma) without additional checks similar to what collapse_pte_mapped_thp() does? I thought that retract_page_tables() checks vma->anon_vma to ensure that this vma doesn't have a cow'ed PageAnon() page. And I still can't understand why can't it race with __handle_mm_fault() paths. Suppose that shmem_file was mmaped with PROT_READ|WRITE, MAP_PRIVATE. To simplify, suppose that a non-THP page was already faulted in, pte_present() == T. Userspace writes to this page. Why __handle_mm_fault()->handle_pte_fault()->do_wp_page()->wp_page_copy() can not cow this page and update pte after the vma->anon_vma chech and before down_write_trylock(mmap_sem) ? Oleg.