Date: Tue, 4 Nov 2008 16:28:20 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Chris Friesen <cfriesen@nortel.com>, Rik van Riel <riel@redhat.com>,
       "Eugene V. Lyubimkin" <jackyf.devel@gmail.com>,
       linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>,
       hugh <hugh@veritas.com>
Subject: Re: mmap: is default non-populating behavior stable?
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On Tue, 04 Nov 2008 17:07:00 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, 2008-11-04 at 09:56 -0600, Chris Friesen wrote:
> > Rik van Riel wrote:
> > > Peter Zijlstra wrote:
> > 
> > >> The exact interaction of mmap() and truncate() I'm not exactly clear on.
> > > 
> > > Truncate will reduce the size of the mmaps on the file to
> > > match the new file size, so processes accessing beyond the
> > > end of file will get a segmentation fault (SIGSEGV).
> > 
> > I suspect Peter was talking about using truncate() to set the initial 
> > file size, effectively increasing rather than reducing it.
> 
> I was thinking of truncate() on an already mmap()'ed region, either
> increasing or decreasing the size so that part of the mmap becomes
> (in)valid.
> 
> I'm not sure how POSIX speaks of this.
> 
> I think Linux does the expected thing.

I believe our behaviour is correct for mmap/mumap/truncate and it
certainly used to be and was tested.

At the point you do anything involving mremap (which is non posix) our
behaviour becomes rather bizarre.

Alan
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