linux-kernel - Re: preempt bug in set_pmd

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Message-ID: <Pine.LNX.4.64.0803052027590.32046@blonde.site>
Date:	Wed, 5 Mar 2008 20:40:41 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: preempt bug in set_pmd_pfn?

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
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