From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1Jys8P-0007dd-Ax
	for qemu-devel@nongnu.org; Wed, 21 May 2008 13:29:53 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1Jys8M-0007as-AW
	for qemu-devel@nongnu.org; Wed, 21 May 2008 13:29:52 -0400
Received: from [199.232.76.173] (port=50850 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1Jys8M-0007ae-2m
	for qemu-devel@nongnu.org; Wed, 21 May 2008 13:29:50 -0400
Received: from host36-195-149-62.serverdedicati.aruba.it
	([62.149.195.36]:52354 helo=mx.cpushare.com)
	by monty-python.gnu.org with esmtps
	(TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60)
	(envelope-from <andrea@qumranet.com>) id 1Jys8L-0000vz-Ua
	for qemu-devel@nongnu.org; Wed, 21 May 2008 13:29:50 -0400
Date: Wed, 21 May 2008 19:01:29 +0200
From: Andrea Arcangeli <andrea@qumranet.com>
Subject: Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with
	cache=off (O_DIRECT)
Message-ID: <20080521170129.GF22488@duo.random>
References: <48341783.3060204@qumranet.com>
	<20080521134154.GA15210@shareable.org>
	<483429EB.7070705@codemonkey.ws> <48342F05.2090603@qumranet.com>
	<48343106.4070801@codemonkey.ws> <48343844.1050107@qumranet.com>
	<20080521153454.GB20527@shareable.org>
	<48344793.2020902@codemonkey.ws>
	<20080521162406.GA21501@shareable.org>
	<48345258.9040004@qumranet.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <48345258.9040004@qumranet.com>
Reply-To: qemu-devel@nongnu.org
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Avi Kivity <avi@qumranet.com>
Cc: Blue Swirl <blauwirbel@gmail.com>, Laurent Vivier <Laurent.Vivier@bull.net>, qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>

On Wed, May 21, 2008 at 07:48:24PM +0300, Avi Kivity wrote:
> Hmm, if remap_file_pages() supports MAP_PRIVATE, that solves all the 
> problems neatly.
>
> Andrea, do you know if that combination works?

No, it only supports MAP_SHARED. Why would anyone want MAP_PRIVATE for
real I/O operations? MAP_PRIVATE is primarily for binaries and things
that are readonly on disk and that we may want to update (like for
dynamic linking w/o hardlinking).

Or is that for -snapshot support, to keep the modifications in
anonymous ram?

I imagine you want remap_file_pages for zerocopy I/O without using
O_DIRECT. If you use O_DIRECT you don't need mmap. However O_DIRECT
work best with kernel asyncio for small seeking blocks so all I/O can
be submitted at the same time. writev also works better than write for
O_DIRECT.