linux-kernel - Re: [PATCH man-pages 4/5] userfaultfd.2: add note about asynchronios events delivery

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Message-ID: <a8acfb52-4e86-b05f-3915-17d2417f81bb@gmail.com>
Date:   Tue, 2 May 2017 22:27:53 +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 4/5] userfaultfd.2: add note about asynchronios
 events delivery

On 05/02/2017 11:46 AM, Mike Rapoport wrote:
> On Mon, May 01, 2017 at 08:33:45PM +0200, Michael Kerrisk (man-pages) wrote:
>> Hi Mike,
>>
>> On 05/01/2017 07:43 AM, Mike Rapoport wrote:
>>> Signed-off-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>
>>
>> Thanks. Applied. One question below.
>>
>>> ---
>>>  man2/userfaultfd.2 | 12 ++++++++++++
>>>  1 file changed, 12 insertions(+)
>>>
>>> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
>>> index 8b89162..f177bba 100644
>>> --- a/man2/userfaultfd.2
>>> +++ b/man2/userfaultfd.2
>>> @@ -112,6 +112,18 @@ created for the child process,
>>>  which allows userfaultfd monitor to perform user-space paging
>>>  for the child process.
>>>  
>>> +Unlike page faults which have to be synchronous and require
>>> +explicit or implicit wakeup,
>>> +all other events are delivered asynchronously and
>>> +the non-cooperative process resumes execution as
>>> +soon as manager executes
>>> +.BR read(2).
>>> +The userfaultfd manager should carefully synchronize calls
>>> +to UFFDIO_COPY with the events processing.
>>> +
>>> +The current asynchronous model of the event delivery is optimal for
>>> +single threaded non-cooperative userfaultfd manager implementations.
>>
>> The preceding paragraph feels incomplete. It seems like you want to make
>> a point with that last sentence, but the point is not explicit. What's
>> missing?
> 
> I've copied both from Documentation/vm/userfaulftfd.txt, and there we also
> talk about possibility of addition of synchronous events delivery and
> that makes the paragraph above to seem crippled :)
> The major point here is that current events delivery model could be
> problematic for multi-threaded monitor. I even suspect that it would be
> impossible to ensure synchronization between page faults and non-page
> fault events in multi-threaded monitor.

Okay -- thanks for the info. I've noted it, but won't make changes 
any changes to the page for now.

Cheers,

Michael



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