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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 CC355C282C3 for ; Wed, 23 Jan 2019 02:17:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9C2EF20868 for ; Wed, 23 Jan 2019 02:17:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726972AbfAWCR4 (ORCPT ); Tue, 22 Jan 2019 21:17:56 -0500 Received: from mx1.redhat.com ([209.132.183.28]:49602 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726867AbfAWCR4 (ORCPT ); Tue, 22 Jan 2019 21:17:56 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 40471C05FF7E; Wed, 23 Jan 2019 02:17:55 +0000 (UTC) Received: from xz-x1 (ovpn-12-57.pek2.redhat.com [10.72.12.57]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B2690600CD; Wed, 23 Jan 2019 02:17:47 +0000 (UTC) Date: Wed, 23 Jan 2019 10:17:45 +0800 From: Peter Xu To: Jerome Glisse Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Maya Gokhale , Johannes Weiner , Martin Cracauer , Denis Plotnikov , Shaohua Li , Andrea Arcangeli , Pavel Emelyanov , Mike Kravetz , Marty McFadden , Mike Rapoport , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" , Rik van Riel Subject: Re: [PATCH RFC 06/24] userfaultfd: wp: support write protection for userfault vma range Message-ID: <20190123021745.GB2970@xz-x1> References: <20190121075722.7945-1-peterx@redhat.com> <20190121075722.7945-7-peterx@redhat.com> <20190121140535.GD3344@redhat.com> <20190122093935.GF14907@xz-x1> <20190122170223.GC3188@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190122170223.GC3188@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Wed, 23 Jan 2019 02:17:55 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 22, 2019 at 12:02:24PM -0500, Jerome Glisse wrote: > On Tue, Jan 22, 2019 at 05:39:35PM +0800, Peter Xu wrote: > > On Mon, Jan 21, 2019 at 09:05:35AM -0500, Jerome Glisse wrote: > > > > [...] > > > > > > + change_protection(dst_vma, start, start + len, newprot, > > > > + !enable_wp, 0); > > > > > > So setting dirty_accountable bring us to that code in mprotect.c: > > > > > > if (dirty_accountable && pte_dirty(ptent) && > > > (pte_soft_dirty(ptent) || > > > !(vma->vm_flags & VM_SOFTDIRTY))) { > > > ptent = pte_mkwrite(ptent); > > > } > > > > > > My understanding is that you want to set write flag when enable_wp > > > is false and you want to set the write flag unconditionaly, right ? > > > > Right. > > > > > > > > If so then you should really move the change_protection() flags > > > patch before this patch and add a flag for setting pte write flags. > > > > > > Otherwise the above is broken at it will only set the write flag > > > for pte that were dirty and i am guessing so far you always were > > > lucky because pte were all dirty (change_protection will preserve > > > dirtyness) when you write protected them. > > > > > > So i believe the above is broken or at very least unclear if what > > > you really want is to only set write flag to pte that have the > > > dirty flag set. > > > > You are right, if we build the tree until this patch it won't work for > > all the cases. It'll only work if the page was at least writable > > before and also it's dirty (as you explained). Sorry to be unclear > > about this, maybe I should at least mention that in the commit message > > but I totally forgot it. > > > > All these problems are solved in later on patches, please feel free to > > have a look at: > > > > mm: merge parameters for change_protection() > > userfaultfd: wp: apply _PAGE_UFFD_WP bit > > userfaultfd: wp: handle COW properly for uffd-wp > > > > Note that even in the follow up patches IMHO we can't directly change > > the write permission since the page can be shared by other processes > > (e.g., the zero page or COW pages). But the general idea is the same > > as you explained. > > > > I tried to avoid squashing these stuff altogether as explained > > previously. Also, this patch can be seen as a standalone patch to > > introduce the new interface which seems to make sense too, and it is > > indeed still working in many cases so I see the latter patches as > > enhancement of this one. Please let me know if you still want me to > > have all these stuff squashed, or if you'd like me to squash some of > > them. > > Yeah i have look at those after looking at this one. You should just > re-order the patch this one first and then one that add new flag, > then ones that add the new userfaultfd feature. Otherwise you are > adding a userfaultfd feature that is broken midway ie it is added > broken and then you fix it. Some one bisecting thing might get hurt > by that. It is better to add and change everything you need and then > add the new feature so that the new feature will work as intended. > > So no squashing just change the order ie add the userfaultfd code > last. Yes this makes sense, I'll do that in v2. Thanks for the suggestion! -- Peter Xu