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Date: Sat, 19 Jan 2013 12:24:52 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: rtorrent and recent snapshots - apparent problem with msync()
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On Jan 18 16:24, Christopher Faylor wrote:
> On Fri, Jan 18, 2013 at 04:10:25PM -0500, Christopher Faylor wrote:
> >On Sat, Jan 19, 2013 at 12:18:39AM +0400, Andrey Repin wrote:
> >>Greetings, Corinna Vinschen!
> >>
> >>>> I saw that you made another change to this function.  Is it possible that
> >>>> this might actually fix the "rtorrent problem"?
> >>
> >>> No.  It only adds the MS_SYNC handling.  rtorrent uses MS_ASYNC.
> >>
> >>That made me think... If rtorrent uses MS_ASYNC, shouldn't *rtorrent* be
> >>prepared for consequences? Instead of you trying to satisfy its
> >>expectations?

Me adding MS_SYNC handling has nothing to do with the original problem.
It was just something I realized being missing in our msync
implementation while looking into the rtorrent behaviour.

Actually, what I was really looking into at this time was...

> >It does seem that way.
> 
> Actually, it isn't that clear.  It seems like msync is failing in the
> MS_ASYNC case when it shouldn't be, i.e., rtorrent is within its rights
> to expect the operation to succeed.

...this:  Under Linux, msync might return with errno set to EBUSY if
"MS_INVALIDATE was specified in flags, and a memory  lock exists for the
specified address range."(*)

This sounds a lot like what rtorrent occurs.  So, what I was looking for
is if rtorrent calls msync with the MS_INVALIDATE flag, but it doesn't.

Right now the msync loop does not handle MS_INVALIDATE specificially,
only after the loop is exited is EBUSY returned now.  OTOH, usually the
kernel is not expected to lock a memory page temporarily for its own
dubious purposes *and* let a user mode function call fail due to that.


Corinna

(*) http://www.kernel.org/doc/man-pages/online/pages/man2/msync.2.html

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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