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

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Message-Id: <20170502094654.GC5910@rapoport-lnx>
Date:   Tue, 2 May 2017 12:46:55 +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 4/5] userfaultfd.2: add note about asynchronios
 events delivery

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.
 
> > +
> >  .\" FIXME elaborate about non-cooperating mode, describe its limitations
> >  .\" for kernels before 4.11, features added in 4.11
> >  .\" and limitations remaining in 4.11
> > 
> 
> Cheers,
> 
> Michael
> 
> 
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/
>