From owner-freebsd-hackers@FreeBSD.ORG  Sat Apr 12 23:53:07 2008
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id E9B681065678;
	Sat, 12 Apr 2008 23:53:07 +0000 (UTC)
	(envelope-from dillon@apollo.backplane.com)
Received: from apollo.backplane.com (apollo.backplane.com [216.240.41.2])
	by mx1.freebsd.org (Postfix) with ESMTP id AF0688FC13;
	Sat, 12 Apr 2008 23:53:07 +0000 (UTC)
	(envelope-from dillon@apollo.backplane.com)
Received: from apollo.backplane.com (localhost [127.0.0.1])
	by apollo.backplane.com (8.14.1/8.14.1) with ESMTP id m3CNr7aL066380;
	Sat, 12 Apr 2008 16:53:07 -0700 (PDT)
Received: (from dillon@localhost)
	by apollo.backplane.com (8.14.1/8.13.4/Submit) id m3CNr7sR066379;
	Sat, 12 Apr 2008 16:53:07 -0700 (PDT)
Date: Sat, 12 Apr 2008 16:53:07 -0700 (PDT)
From: Matthew Dillon <dillon@apollo.backplane.com>
Message-Id: <200804122353.m3CNr7sR066379@apollo.backplane.com>
To: "Kip Macy" <kip.macy@gmail.com>
References: <20080309212441.GA56523@porthos.spock.org>
	<200804122156.m3CLuot5065753@apollo.backplane.com>
	<b1fa29170804121533q792b6491u106794ce722e78e3@mail.gmail.com>
Cc: hackers@freebsd.org, Jonathan Chen <jon@freebsd.org>
Subject: Re: mlock & COW
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
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: Sat, 12 Apr 2008 23:53:08 -0000


:>     vm_map_lookup() line 3161 and line 3297 (FreeBSD current).  Commenting
:>     those out will allow the COW on the user-wired page:
:>
:>         if ((entry->eflags & MAP_ENTRY_USER_WIRED) &&
:>             (entry->eflags & MAP_ENTRY_COW) &&
:>             (fault_type & VM_PROT_WRITE) &&
:>             (fault_typea & VM_PROT_OVERRIDE_WRITE) == 0) {
:>                 RETURN(KERN_PROTECTION_FAILURE);
:>         }

    Ok, never mind on commenting out that code in vm_map_lookup(),
    it won't work.  In fact, it will crash the machine even faster.

    It's messy both ways, but marginally less messy if you do it in the
    vm_fault() code.

    The problem with fixing it in the fault code is that the vm_fault()
    routine is called by the wiring and unwiring code and also by code which
    undoes failed wirings (where the entry flags do not reflect what the
    caller wants vm_fault() to do), so vm_fault() can't just check the entry
    flags and automatically user-wire.

    To do it without rewriting the whole mess (and rewriting is not a bad
    idea), the platform trap code needs to call vm_fault() with a new
    flag VM_FAULT_AUTOWIRE, then vm_map_lookup() needs to set the contents
    of &fs.wired whether it succeeds or fails, then vm_fault(), upon
    seeing result == KERN_PROTECTION_FAILURE, must check whether 
    VM_FAULT_AUTOWIRE was specified and if so check fs.wired to see if the
    mapping failed due to incompatible protections on a user wired 
    mapping, and THEN it can set VM_FAULT_USER_WIRE in fault_flags and
    continue normally.

    And on top of all of that I'm still not sure whether the originally
    underlying read-only paged that was COW'd will be properly unwired.

    As I said, messy.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>