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

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

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@gmail.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@linux.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?

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/