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From: Hugh Dickins <hugh@veritas.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <ak@suse.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: preempt bug in set_pmd_pfn?
Date: Wed, 5 Mar 2008 20:40:41 +0000 (GMT)	[thread overview]
Message-ID: <Pine.LNX.4.64.0803052027590.32046@blonde.site> (raw)
In-Reply-To: <47CEF20D.5070608@goop.org>

On Wed, 5 Mar 2008, Jeremy Fitzhardinge wrote:
> 
> Fixmap slots are global, not percpu; you may be thinking of kmap_atomic, which
> reserves percpu fixmap slots for its use.

Ah yes, of course you're right, I was misremembering: thanks.

> Most uses of set_fixmap are early
> in boot, where preemption (or other CPUs) isn't a factor.  The exception is
> mapping the compat vdso global mapping.   However, that is special-cased
> anyway, since the set_fixmap is followed by an explicit all-cpu tlb flush.

Mmm, yes, I hadn't noticed that one: rather a weird case.

(If root had a mind to do so, she could be flipping compat_vdso
back and forth on all cpus concurrently; but I don't see an actual
problem of getting a worse outcome than she deserves for doing so -
the pages involved never get freed.)

> It seems to me that the correct fix is to just make __set_fixmap disable
> preemption for its duration; it probably doesn't make much difference for the
> native case, and it makes Xen happy.

I'd have thought it better done just in that one peculiar case,
with a comment that it's to keep Xen happy; but I'd better back
out now and leave it to you and Ingo.

Hugh

  reply	other threads:[~2008-03-05 20:41 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-04 21:13 Jeremy Fitzhardinge
2008-03-04 21:28 ` Ingo Molnar
2008-03-04 21:27   ` Jeremy Fitzhardinge
2008-03-05  6:48     ` Ingo Molnar
2008-03-05 14:29       ` Hugh Dickins
2008-03-05 16:48         ` Jeremy Fitzhardinge
2008-03-05 17:38           ` Hugh Dickins
2008-03-05 19:18             ` Jeremy Fitzhardinge
2008-03-05 20:40               ` Hugh Dickins [this message]
2008-03-06 12:52               ` Ingo Molnar
2008-03-06 18:19                 ` Jeremy Fitzhardinge
2008-03-05 16:45       ` Jeremy Fitzhardinge
2008-03-05  0:06 ` Andi Kleen
2008-03-05  0:07   ` Jeremy Fitzhardinge
2008-03-05  0:16     ` Andi Kleen
2008-03-05  0:19       ` Jeremy Fitzhardinge
2008-03-05  1:28         ` Andi Kleen

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