From owner-freebsd-hackers@FreeBSD.ORG  Wed Mar 30 02:18:40 2005
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id C05B816A4CE; Wed, 30 Mar 2005 02:18:40 +0000 (GMT)
Received: from VARK.MIT.EDU (VARK.MIT.EDU [18.95.3.179])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 5712843D4C; Wed, 30 Mar 2005 02:18:40 +0000 (GMT)
	(envelope-from das@FreeBSD.ORG)
Received: from VARK.MIT.EDU (localhost [127.0.0.1])
	by VARK.MIT.EDU (8.13.3/8.13.1) with ESMTP id j2U2IWjc032980;
	Tue, 29 Mar 2005 21:18:32 -0500 (EST)
	(envelope-from das@FreeBSD.ORG)
Received: (from das@localhost)
	by VARK.MIT.EDU (8.13.3/8.13.1/Submit) id j2U2IWr9032979;
	Tue, 29 Mar 2005 21:18:32 -0500 (EST)
	(envelope-from das@FreeBSD.ORG)
Date: Tue, 29 Mar 2005 21:18:32 -0500
From: David Schultz <das@FreeBSD.ORG>
To: Richard Sharpe <rsharpe@richardsharpe.com>
Message-ID: <20050330021831.GA26006@VARK.MIT.EDU>
Mail-Followup-To: Richard Sharpe <rsharpe@richardsharpe.com>,
	freebsd-hackers@FreeBSD.ORG, alc@FreeBSD.ORG
References: <Pine.LNX.4.58.0503291533320.2471@durable>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Pine.LNX.4.58.0503291533320.2471@durable>
cc: alc@FreeBSD.ORG
cc: freebsd-hackers@FreeBSD.ORG
Subject: Re: Possible problems with mmap/munmap on FreeBSD ...
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 30 Mar 2005 02:18:41 -0000

On Tue, Mar 29, 2005, Richard Sharpe wrote:
> Hi,
> 
> I am having some problems with the tdb package on FreeBSD 4.6.2 and 4.10.
> 
> One of the things the above package does is:
> 
>    mmap the tdb file to a region of memory
>    store stuff in the region (memmov etc).
>    when it needs to extend the size of the region {
>      munmap the region
>      write data at the end of the file
>      mmap the region again with a larger size
>    }
> 
> What I am seeing is that after the munmap the data written to the region
> is gone.
> 
> However, if I insert an msync before the munmap, everything is nicely
> coherent. This seems odd (in the sense that it works without the msync
> under Linux).
> 
> The region is mmapped with:
> 
>    mmap(NULL, tdb->map_size,
>         PROT_READ|(tdb->read_only? 0:PROT_WRITE),
>         MAP_SHARED|MAP_FILE, tdb->fd, 0);
> 
> What I notice is that all the calls to mmap return the same address.
> 
> A careful reading of the man pages for mmap and munmap does not suggest
> that I am doing anything wrong.
> 
> Is it possible that FreeBSD is deferring flushing the dirty data, and then
> forgets to do it when the same starting address is used etc?

It looks like all of the underlying pages are getting invalidated
in vm_object_page_remove().  This is clearly the right thing to do
for private mappings, but it seems wrong for shared mappings.
Perhaps Alan has some insight.