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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,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 96183C43613 for ; Thu, 20 Jun 2019 02:25:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 75D2B2084A for ; Thu, 20 Jun 2019 02:25:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731500AbfFTCZA (ORCPT ); Wed, 19 Jun 2019 22:25:00 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60856 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726370AbfFTCY7 (ORCPT ); Wed, 19 Jun 2019 22:24:59 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7B2A685538; Thu, 20 Jun 2019 02:24:59 +0000 (UTC) Received: from xz-x1.redhat.com (ovpn-12-78.pek2.redhat.com [10.72.12.78]) by smtp.corp.redhat.com (Postfix) with ESMTP id C410C1001DC3; Thu, 20 Jun 2019 02:24:46 +0000 (UTC) From: Peter Xu To: linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: David Hildenbrand , Hugh Dickins , Maya Gokhale , Jerome Glisse , Pavel Emelyanov , Johannes Weiner , peterx@redhat.com, Martin Cracauer , Denis Plotnikov , Shaohua Li , Andrea Arcangeli , Mike Kravetz , Marty McFadden , Mike Rapoport , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" Subject: [PATCH v5 22/25] userfaultfd: wp: UFFDIO_REGISTER_MODE_WP documentation update Date: Thu, 20 Jun 2019 10:20:05 +0800 Message-Id: <20190620022008.19172-23-peterx@redhat.com> In-Reply-To: <20190620022008.19172-1-peterx@redhat.com> References: <20190620022008.19172-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Thu, 20 Jun 2019 02:24:59 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Martin Cracauer Adds documentation about the write protection support. Signed-off-by: Martin Cracauer Signed-off-by: Andrea Arcangeli [peterx: rewrite in rst format; fixups here and there] Reviewed-by: Jerome Glisse Reviewed-by: Mike Rapoport Signed-off-by: Peter Xu --- Documentation/admin-guide/mm/userfaultfd.rst | 51 ++++++++++++++++++++ 1 file changed, 51 insertions(+) diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst index 5048cf661a8a..c30176e67900 100644 --- a/Documentation/admin-guide/mm/userfaultfd.rst +++ b/Documentation/admin-guide/mm/userfaultfd.rst @@ -108,6 +108,57 @@ UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an half copied page since it'll keep userfaulting until the copy has finished. +Notes: + +- If you requested UFFDIO_REGISTER_MODE_MISSING when registering then + you must provide some kind of page in your thread after reading from + the uffd. You must provide either UFFDIO_COPY or UFFDIO_ZEROPAGE. + The normal behavior of the OS automatically providing a zero page on + an annonymous mmaping is not in place. + +- None of the page-delivering ioctls default to the range that you + registered with. You must fill in all fields for the appropriate + ioctl struct including the range. + +- You get the address of the access that triggered the missing page + event out of a struct uffd_msg that you read in the thread from the + uffd. You can supply as many pages as you want with UFFDIO_COPY or + UFFDIO_ZEROPAGE. Keep in mind that unless you used DONTWAKE then + the first of any of those IOCTLs wakes up the faulting thread. + +- Be sure to test for all errors including (pollfd[0].revents & + POLLERR). This can happen, e.g. when ranges supplied were + incorrect. + +Write Protect Notifications +--------------------------- + +This is equivalent to (but faster than) using mprotect and a SIGSEGV +signal handler. + +Firstly you need to register a range with UFFDIO_REGISTER_MODE_WP. +Instead of using mprotect(2) you use ioctl(uffd, UFFDIO_WRITEPROTECT, +struct *uffdio_writeprotect) while mode = UFFDIO_WRITEPROTECT_MODE_WP +in the struct passed in. The range does not default to and does not +have to be identical to the range you registered with. You can write +protect as many ranges as you like (inside the registered range). +Then, in the thread reading from uffd the struct will have +msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP set. Now you send +ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect) again +while pagefault.mode does not have UFFDIO_WRITEPROTECT_MODE_WP set. +This wakes up the thread which will continue to run with writes. This +allows you to do the bookkeeping about the write in the uffd reading +thread before the ioctl. + +If you registered with both UFFDIO_REGISTER_MODE_MISSING and +UFFDIO_REGISTER_MODE_WP then you need to think about the sequence in +which you supply a page and undo write protect. Note that there is a +difference between writes into a WP area and into a !WP area. The +former will have UFFD_PAGEFAULT_FLAG_WP set, the latter +UFFD_PAGEFAULT_FLAG_WRITE. The latter did not fail on protection but +you still need to supply a page when UFFDIO_REGISTER_MODE_MISSING was +used. + QEMU/KVM ======== -- 2.21.0