From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
xen-devel <xen-devel@lists.xensource.com>,
Thomas Gleixner <tglx@linutronix.de>,
Hugh Dickins <hugh@veritas.com>, Zachary Amsden <zach@vmware.com>,
kvm-devel <kvm-devel@lists.sourceforge.net>,
Virtualization Mailing List <virtualization@lists.osdl.org>,
Rusty Russell <rusty@rustcorp.com.au>,
Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 0 of 4] mm+paravirt+xen: add pte read-modify-write abstraction
Date: Sat, 24 May 2008 10:25:43 -0700 (PDT) [thread overview]
Message-ID: <alpine.LFD.1.10.0805241022210.3081@woody.linux-foundation.org> (raw)
In-Reply-To: <48372C2F.1090704@goop.org>
On Fri, 23 May 2008, Jeremy Fitzhardinge wrote:
>
> Well, you could use it to update the page and protection bits while preserving
> the AD bits, but I don't think that's very useful.
It's not that it's not "very useful" - it's that it would be TOTALLY
WRONG.
If you change the page and the AD bits can change randomly while you do
so, that means that the AD bits are now _undefined_. Which of the two
pages did it happen to? The old one? The new one? Nobody can know. So
you'd effectively have dirty bits that could be associated with the wrong
physical page, which means that some page may be dirty, but the kernel
would have it marked clean.
That would be beyond bad. That would be a *serious* VM bug, with very
subtle data corruption issues.
So no, you cannot, and absolutely must not use this to update the actual
page mapping. Ever. It is only valid for the protection bits.
Linus
next prev parent reply other threads:[~2008-05-24 17:27 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-05-23 14:20 Jeremy Fitzhardinge
2008-05-23 14:20 ` [PATCH 1 of 4] mm: add a pte_rmw transaction abstraction Jeremy Fitzhardinge
2008-05-23 14:20 ` [PATCH 2 of 4] paravirt: add hooks for pte_rmw_start/commit Jeremy Fitzhardinge
2008-05-23 14:20 ` [PATCH 3 of 4] xen: implement pte_rmw_start/commit Jeremy Fitzhardinge
2008-05-23 14:20 ` [PATCH 4 of 4] xen: add mechanism to extend existing multicalls Jeremy Fitzhardinge
2008-05-23 18:27 ` [PATCH 0 of 4] mm+paravirt+xen: add pte read-modify-write abstraction Zachary Amsden
2008-05-23 20:32 ` Jeremy Fitzhardinge
2008-05-23 23:25 ` Zachary Amsden
2008-05-31 0:13 ` Jeremy Fitzhardinge
2008-06-02 20:09 ` Zachary Amsden
2008-05-23 18:57 ` Linus Torvalds
2008-05-23 20:42 ` Jeremy Fitzhardinge
2008-05-24 17:25 ` Linus Torvalds [this message]
2008-05-24 20:44 ` Jeremy Fitzhardinge
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