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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT 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 C20E9C282CE for ; Mon, 22 Apr 2019 12:23:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9CAA320881 for ; Mon, 22 Apr 2019 12:23:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727245AbfDVMX0 (ORCPT ); Mon, 22 Apr 2019 08:23:26 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55006 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726098AbfDVMXZ (ORCPT ); Mon, 22 Apr 2019 08:23:25 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D666CC0495A3; Mon, 22 Apr 2019 12:23:24 +0000 (UTC) Received: from xz-x1 (ovpn-12-23.pek2.redhat.com [10.72.12.23]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 41BBC1A8F4; Mon, 22 Apr 2019 12:23:15 +0000 (UTC) Date: Mon, 22 Apr 2019 20:23:12 +0800 From: Peter Xu To: Jerome Glisse Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand , Hugh Dickins , Maya Gokhale , Pavel Emelyanov , Johannes Weiner , Martin Cracauer , Shaohua Li , Andrea Arcangeli , Mike Kravetz , Denis Plotnikov , Mike Rapoport , Marty McFadden , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" Subject: Re: [PATCH v3 17/28] userfaultfd: wp: support swap and page migration Message-ID: <20190422122312.GB25896@xz-x1> References: <20190320020642.4000-1-peterx@redhat.com> <20190320020642.4000-18-peterx@redhat.com> <20190418205907.GL3288@redhat.com> <20190419074220.GG13323@xz-x1> <20190419150802.GB3311@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190419150802.GB3311@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 22 Apr 2019 12:23:25 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 19, 2019 at 11:08:02AM -0400, Jerome Glisse wrote: > On Fri, Apr 19, 2019 at 03:42:20PM +0800, Peter Xu wrote: > > On Thu, Apr 18, 2019 at 04:59:07PM -0400, Jerome Glisse wrote: > > > On Wed, Mar 20, 2019 at 10:06:31AM +0800, Peter Xu wrote: > > > > For either swap and page migration, we all use the bit 2 of the entry to > > > > identify whether this entry is uffd write-protected. It plays a similar > > > > role as the existing soft dirty bit in swap entries but only for keeping > > > > the uffd-wp tracking for a specific PTE/PMD. > > > > > > > > Something special here is that when we want to recover the uffd-wp bit > > > > from a swap/migration entry to the PTE bit we'll also need to take care > > > > of the _PAGE_RW bit and make sure it's cleared, otherwise even with the > > > > _PAGE_UFFD_WP bit we can't trap it at all. > > > > > > > > Note that this patch removed two lines from "userfaultfd: wp: hook > > > > userfault handler to write protection fault" where we try to remove the > > > > VM_FAULT_WRITE from vmf->flags when uffd-wp is set for the VMA. This > > > > patch will still keep the write flag there. > > > > > > > > Reviewed-by: Mike Rapoport > > > > Signed-off-by: Peter Xu > > > > > > Some missing thing see below. > > > > > > [...] > > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > index 6405d56debee..c3d57fa890f2 100644 > > > > --- a/mm/memory.c > > > > +++ b/mm/memory.c > > > > @@ -736,6 +736,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > > > pte = swp_entry_to_pte(entry); > > > > if (pte_swp_soft_dirty(*src_pte)) > > > > pte = pte_swp_mksoft_dirty(pte); > > > > + if (pte_swp_uffd_wp(*src_pte)) > > > > + pte = pte_swp_mkuffd_wp(pte); > > > > set_pte_at(src_mm, addr, src_pte, pte); > > > > } > > > > } else if (is_device_private_entry(entry)) { > > > > > > You need to handle the is_device_private_entry() as the migration case > > > too. > > > > Hi, Jerome, > > > > Yes I can simply add the handling, but I'd confess I haven't thought > > clearly yet on how userfault-wp will be used with HMM (and that's > > mostly because my unfamiliarity so far with HMM). Could you give me > > some hint on a most general and possible scenario? > > device private is just a temporary state with HMM you can have thing > like GPU or FPGA migrate some anonymous page to their local memory > because it is use by the GPU or the FPGA. The GPU or FPGA behave like > a CPU from mm POV so if it wants to write it will fault and go through > the regular CPU page fault. > > That said it can still migrate a page that is UFD write protected just > because the device only care about reading. So if you have a UFD pte > to a regular page that get migrated to some device memory you want to > keep the UFD WP flags after the migration (in both direction when going > to device memory and from coming back from it). > > As far as UFD is concern this is just another page, it just does not > have a valid pte entry because CPU can not access such memory. But from > mm point of view it just another page. I see the point. Thanks for explaining that! -- Peter Xu