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

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Date:   Tue, 2 May 2017 22:19:36 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     mtk.manpages@...il.com, 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 05/02/2017 11:22 AM, Mike Rapoport wrote:
> 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.

Thanks for checking!

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/