linux-kernel - Re: mmap: is default non-populating behavior stable?

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Message-ID: <20081104162820.644b1487@lxorguk.ukuu.org.uk>
Date:	Tue, 4 Nov 2008 16:28:20 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Friesen <cfriesen@...tel.com>,
	Rik van Riel <riel@...hat.com>,
	"Eugene V. Lyubimkin" <jackyf.devel@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	hugh <hugh@...itas.com>
Subject: Re: mmap: is default non-populating behavior stable?

On Tue, 04 Nov 2008 17:07:00 +0100
Peter Zijlstra <peterz@...radead.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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