From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) by kanga.kvack.org (Postfix) with ESMTP id 29FF66B0003 for ; Mon, 2 Jul 2018 03:51:01 -0400 (EDT) Received: by mail-ed1-f71.google.com with SMTP id w10-v6so5261433eds.7 for ; Mon, 02 Jul 2018 00:51:01 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id o1-v6si9747341edd.161.2018.07.02.00.50.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 02 Jul 2018 00:50:59 -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 w627i7qK059421 for ; Mon, 2 Jul 2018 03:50:57 -0400 Received: from e06smtp03.uk.ibm.com (e06smtp03.uk.ibm.com [195.75.94.99]) by mx0a-001b2d01.pphosted.com with ESMTP id 2jy9vvbb7c-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 02 Jul 2018 03:50:57 -0400 Received: from localhost by e06smtp03.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 2 Jul 2018 08:50:54 +0100 From: Christian Borntraeger Subject: [PATCHi v2] mm: do not drop unused pages when userfaultd is running Date: Mon, 2 Jul 2018 09:50:49 +0200 Message-Id: <20180702075049.9157-1-borntraeger@de.ibm.com> Content-Type: text/plain Content-Transfer-Encoding: 8bit MIME-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org, linux-s390@vger.kernel.org, Andrew Morton Cc: kvm@vger.kernel.org, Janosch Frank , David Hildenbrand , Cornelia Huck , linux-kernel@vger.kernel.org, Christian Borntraeger , Martin Schwidefsky , Andrea Arcangeli , Mike Rapoport 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: Mike Rapoport Signed-off-by: Christian Borntraeger --- rfc->v2: use userfaultfd_armed mm/rmap.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index 6db729dc4c50..e8fa564676b6 100644 --- 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 -- 2.17.0