linux-kernel - Re: preempt bug in set_pmd

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Message-ID: <20080306125215.GD20173@elte.hu>
Date:	Thu, 6 Mar 2008 13:52:15 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Hugh Dickins <hugh@...itas.com>, "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?

* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> 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.

actually, i think the correct approach is to remove the TLB flushing and 
perhaps to check that the old pte is not present. Do we ever _change_ 
mappings via __set_fixmap()? I think we only ever install them.

but if we ever change them somewhere then the correct approach is to do 
a flush_tlb_all(). It's not just about preemption but about the fact 
that we modified the kernel address space and we must propagate that to 
all CPUs.

the vmalloc() backtrace you sent - how did set_pte_pfn() get into that 
codepath - vmalloc shouldnt be using __set_fixmap().

	Ingo
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