From owner-freebsd-arch@FreeBSD.ORG  Mon May  4 09:55:03 2015
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: arch@FreeBSD.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id 76F71469;
 Mon,  4 May 2015 09:55:03 +0000 (UTC)
Received: from cell.glebius.int.ru (glebius.int.ru [81.19.69.10])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client CN "cell.glebius.int.ru", Issuer "cell.glebius.int.ru" (not verified))
 by mx1.freebsd.org (Postfix) with ESMTPS id 04870183F;
 Mon,  4 May 2015 09:55:01 +0000 (UTC)
Received: from cell.glebius.int.ru (localhost [127.0.0.1])
 by cell.glebius.int.ru (8.14.9/8.14.9) with ESMTP id t449sxej042498
 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO);
 Mon, 4 May 2015 12:54:59 +0300 (MSK)
 (envelope-from glebius@FreeBSD.org)
Received: (from glebius@localhost)
 by cell.glebius.int.ru (8.14.9/8.14.9/Submit) id t449sxvM042497;
 Mon, 4 May 2015 12:54:59 +0300 (MSK)
 (envelope-from glebius@FreeBSD.org)
X-Authentication-Warning: cell.glebius.int.ru: glebius set sender to
 glebius@FreeBSD.org using -f
Date: Mon, 4 May 2015 12:54:59 +0300
From: Gleb Smirnoff <glebius@FreeBSD.org>
To: Julian Elischer <julian@freebsd.org>
Cc: Konstantin Belousov <kostikbel@gmail.com>, alc@FreeBSD.org,
 arch@FreeBSD.org
Subject: Re: more strict KPI for vm_pager_get_pages()
Message-ID: <20150504095459.GI34544@glebius.int.ru>
References: <20150430142408.GS546@nginx.com>
 <20150504082426.GC2390@kib.kiev.ua> <554740EF.7030808@freebsd.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <554740EF.7030808@freebsd.org>
User-Agent: Mutt/1.5.23 (2014-03-12)
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.20
Precedence: list
List-Id: Discussion related to FreeBSD architecture <freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-arch>,
 <mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch/>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
 <mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 04 May 2015 09:55:03 -0000

On Mon, May 04, 2015 at 05:50:39PM +0800, Julian Elischer wrote:
J> > IMO, the response from vm_pager_haspages() is only advisory, since
J> > filesystem might not control the external entities which are the source
J> > of the required data.
J> 
J> Also since the backing object is not locked, a truncate() may be performed
J> between the operations making the prior return information invalid.
J> Certainly in remote filesystems, possibly on local ones too.

Of course, the object shouldn't be unlocked between vm_pager_haspage()
and vm_pager_get_pages(). All current consumers do not unlock.

-- 
Totus tuus, Glebius.