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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 95990C3279B for ; Mon, 2 Jul 2018 21:06:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4CED224617 for ; Mon, 2 Jul 2018 21:06:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4CED224617 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753439AbeGBVGm (ORCPT ); Mon, 2 Jul 2018 17:06:42 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:35950 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752686AbeGBVGk (ORCPT ); Mon, 2 Jul 2018 17:06:40 -0400 Received: from akpm3.svl.corp.google.com (unknown [104.133.9.92]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id 38895DA7; Mon, 2 Jul 2018 21:06:39 +0000 (UTC) Date: Mon, 2 Jul 2018 14:06:38 -0700 From: Andrew Morton To: Christian Borntraeger Cc: linux-mm@kvack.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org, Janosch Frank , David Hildenbrand , Cornelia Huck , linux-kernel@vger.kernel.org, Martin Schwidefsky , Andrea Arcangeli , Mike Rapoport Subject: Re: [PATCHi v2] mm: do not drop unused pages when userfaultd is running Message-Id: <20180702140638.eb3edfaa611ba9fa018f92eb@linux-foundation.org> In-Reply-To: <20180702075049.9157-1-borntraeger@de.ibm.com> References: <20180702075049.9157-1-borntraeger@de.ibm.com> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2 Jul 2018 09:50:49 +0200 Christian Borntraeger wrote: > KVM guests on s390 can notify the host of unused pages. This can result > in pte_unused callbacks to be true for KVM guest memory. > > If a page is unused (checked with pte_unused) we might drop this page > instead of paging it. This can have side-effects on userfaultd, when the > page in question was already migrated: > > The next access of that page will trigger a fault and a user fault > instead of faulting in a new and empty zero page. As QEMU does not > expect a userfault on an already migrated page this migration will fail. > > The most straightforward solution is to ignore the pte_unused hint if a > userfault context is active for this VMA. > > ... > > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -64,6 +64,7 @@ > #include > #include > #include > +#include > > #include > > @@ -1481,7 +1482,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, > set_pte_at(mm, address, pvmw.pte, pteval); > } > > - } else if (pte_unused(pteval)) { > + } else if (pte_unused(pteval) && !userfaultfd_armed(vma)) { > /* > * The guest indicated that the page content is of no > * interest anymore. Simply discard the pte, vmscan A reader of this code will wonder why we're checking userfaultfd_armed(). So the writer of this code should add a comment which explains this to them ;) Please.