From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id 2D1F76B0005 for ; Fri, 29 Jun 2018 02:51:35 -0400 (EDT) Received: by mail-wr0-f197.google.com with SMTP id s15-v6so4111741wrn.16 for ; Thu, 28 Jun 2018 23:51:35 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id j125-v6si680705wmb.201.2018.06.28.23.51.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 28 Jun 2018 23:51:33 -0700 (PDT) Received: from pps.filterd (m0098393.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w5T6n5Xr126630 for ; Fri, 29 Jun 2018 02:51:31 -0400 Received: from e06smtp04.uk.ibm.com (e06smtp04.uk.ibm.com [195.75.94.100]) by mx0a-001b2d01.pphosted.com with ESMTP id 2jwe65c5be-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Fri, 29 Jun 2018 02:51:31 -0400 Received: from localhost by e06smtp04.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 29 Jun 2018 07:51:29 +0100 Subject: Re: [PATCH/RFC] mm: do not drop unused pages when userfaultd is running References: <20180628123916.96106-1-borntraeger@de.ibm.com> From: Christian Borntraeger Date: Fri, 29 Jun 2018 08:51:23 +0200 MIME-Version: 1.0 In-Reply-To: <20180628123916.96106-1-borntraeger@de.ibm.com> Content-Language: en-US Message-Id: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org, linux-s390@vger.kernel.org Cc: kvm@vger.kernel.org, Janosch Frank , David Hildenbrand , Cornelia Huck , linux-kernel@vger.kernel.org, Martin Schwidefsky , Andrea Arcangeli On 06/28/2018 02:39 PM, 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. > > Cc: Martin Schwidefsky > Cc: Andrea Arcangeli > Cc: stable@vger.kernel.org > Signed-off-by: Christian Borntraeger > --- > mm/rmap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 6db729dc4c50..3f3a72aa99f2 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1481,7 +1481,7 @@ 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) && !vma->vm_userfaultfd_ctx.ctx) { FWIW, this needs a fix for !CONFIG_USERFAULTFD. Still: more opinions on the patch itself? > /* > * The guest indicated that the page content is of no > * interest anymore. Simply discard the pte, vmscan >