From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751744AbdEASea (ORCPT ); Mon, 1 May 2017 14:34:30 -0400 Received: from mail-wr0-f194.google.com ([209.85.128.194]:36035 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751717AbdEASeY (ORCPT ); Mon, 1 May 2017 14:34:24 -0400 Subject: Re: [PATCH man-pages 1/2] userfaultfd.2: start documenting non-cooperative events To: Mike Rapoprt References: <1493302474-4701-1-git-send-email-rppt@linux.vnet.ibm.com> <1493302474-4701-2-git-send-email-rppt@linux.vnet.ibm.com> <190E3CFC-492F-4672-9385-9C3D8F57F26C@linux.vnet.ibm.com> Cc: mtk.manpages@gmail.com, Andrea Arcangeli , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-man@vger.kernel.org From: "Michael Kerrisk (man-pages)" Message-ID: <3cff5638-cb15-50e6-f5a4-d9a0fce643c5@gmail.com> Date: Mon, 1 May 2017 20:34:16 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <190E3CFC-492F-4672-9385-9C3D8F57F26C@linux.vnet.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Mike, On 04/28/2017 11:45 AM, Mike Rapoprt wrote: > > > On April 27, 2017 8:26:16 PM GMT+03:00, "Michael Kerrisk (man-pages)" wrote: >> Hi Mike, >> >> I've applied this, but have some questions/points I think >> further clarification. >> >> On 04/27/2017 04:14 PM, Mike Rapoport wrote: >>> Signed-off-by: Mike Rapoport >>> --- >>> man2/userfaultfd.2 | 135 >> ++++++++++++++++++++++++++++++++++++++++++++++++++--- >>> 1 file changed, 128 insertions(+), 7 deletions(-) >>> >>> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2 >>> index cfea5cb..44af3e4 100644 >>> --- a/man2/userfaultfd.2 >>> +++ b/man2/userfaultfd.2 >>> @@ -75,7 +75,7 @@ flag in >>> .PP >>> When the last file descriptor referring to a userfaultfd object is >> closed, >>> all memory ranges that were registered with the object are >> unregistered >>> -and unread page-fault events are flushed. >>> +and unread events are flushed. >>> .\" >>> .SS Usage >>> The userfaultfd mechanism is designed to allow a thread in a >> multithreaded >>> @@ -99,6 +99,20 @@ In such non-cooperative mode, >>> the process that monitors userfaultfd and handles page faults >>> needs to be aware of the changes in the virtual memory layout >>> of the faulting process to avoid memory corruption. >>> + >>> +Starting from Linux 4.11, >>> +userfaultfd may notify the fault-handling threads about changes >>> +in the virtual memory layout of the faulting process. >>> +In addition, if the faulting process invokes >>> +.BR fork (2) >>> +system call, >>> +the userfaultfd objects associated with the parent may be duplicated >>> +into the child process and the userfaultfd monitor will be notified >>> +about the file descriptor associated with the userfault objects >> >> What does "notified about the file descriptor" mean? > > Well, seems that I've made this one really awkward :) > When the monitored process forks, all the userfault objects > associated​ with it are duplicated into the child process. For each > duplicated object, userfault generates event of type UFFD_EVENT_FORK > and the uffdio_msg for this event contains the file descriptor that > should be used to manipulate the duplicated userfault object. > Hope this clarifies. Yes, it's clearer now. Mostly what was needed here was a forward reference that mentions UFFD_EVENT_FORK explicitly. I added that, and also enhanced the text on UFFD_EVENT_FORK a little. Also, it's not just fork(2) for which UFFD_EVENT_FORK is generated, right? It can also be a clone(2) cal that does not specify CLONE_VM, right? Could you review my changes in commit 522ab2ff6fc9010432a to make sure they are okay. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/