linux-kernel - Re: [PATCH man-pages 1/2] userfaultfd.2: start documenting non-cooperative events

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Message-Id: <20170502092255.GA3022@rapoport-lnx>
Date:   Tue, 2 May 2017 12:22:56 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-man@...r.kernel.org
Subject: Re: [PATCH man-pages 1/2] userfaultfd.2: start documenting
 non-cooperative events

On Mon, May 01, 2017 at 08:34:16PM +0200, Michael Kerrisk (man-pages) wrote:
> 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)" <mtk.manpages@...il.com> 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 <rppt@...ux.vnet.ibm.com>
> >>> ---
> >>>  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?

Yes.
 
> Could you review my changes in commit 522ab2ff6fc9010432a
> to make sure they are okay.

Yes, thats correct and with your updates the text is much clearer. Thanks.
 
> Cheers,
> 
> Michael
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/
>