From owner-freebsd-fs Sun Sep 13 01:33:36 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id BAA02937 for freebsd-fs-outgoing; Sun, 13 Sep 1998 01:33:36 -0700 (PDT) (envelope-from owner-freebsd-fs@FreeBSD.ORG) Received: from implode.root.com (implode.root.com [198.145.90.17]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id BAA02932 for ; Sun, 13 Sep 1998 01:33:32 -0700 (PDT) (envelope-from root@implode.root.com) Received: from implode.root.com (localhost [127.0.0.1]) by implode.root.com (8.8.5/8.8.5) with ESMTP id BAA19667; Sun, 13 Sep 1998 01:29:06 -0700 (PDT) Message-Id: <199809130829.BAA19667@implode.root.com> To: Don Lewis cc: freebsd-fs@FreeBSD.ORG Subject: Re: vm system interaction with nullfs In-reply-to: Your message of "Sun, 13 Sep 1998 00:01:42 PDT." <199809130701.AAA22846@salsa.gv.tsc.tdk.com> From: David Greenman Reply-To: dg@root.com Date: Sun, 13 Sep 1998 01:29:06 -0700 Sender: owner-freebsd-fs@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org > >Since the vm system keeps track of what it has in memory by (vnode, offset), >how is this supposed to work when stackable filesystems are in use which >create multiple vnodes for a single filesytem object, or is this broken? >Unless this works right, it looks like you'll end up with multiple copies >of the same disk blocks in memory and in memory copies may all be different. > >It would seem that in the case of nullfs and similar transparent filesytems, >the vm system should always use the lowest vnode, but this doesn't seem to >be implemented (though I could just be getting lost in the maze of twisty >little passages). It's even messier if the layer isn't transparent, >like an encryption layer. Yes, that's the fundamental reason why nullfs is broken. It's been known for years that the solution is to always reference the same underlying VM object, but noone has yet gotten around to implementing that correctly. -DG David Greenman Co-founder/Principal Architect, The FreeBSD Project To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message