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From: Jeremy Fitzhardinge <jeremy@goop.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Hugh Dickins <hugh@veritas.com>, "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: Thu, 06 Mar 2008 10:19:27 -0800	[thread overview]
Message-ID: <47D035AF.8080306@goop.org> (raw)
In-Reply-To: <20080306125215.GD20173@elte.hu>

Ingo Molnar wrote:
> 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.
>   

Yes, I think that's the case.  clear_fixmap() exists for clearing out an 
existing mapping, but its only used to clear out the WP test mapping and 
in early_iounmap (if called late).  I couldn't see any instances of 
replacing a mapping.

> 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.
>   

Yes, I was wondering about that.  If __set_fixmap is only used at boot 
time, then a global flush isn't necessary, but if its deemed a 
general-purpose API in a normal running kernel, it needs to deal with 
cross-cpu flushes.

64-bit set_fixmap is __init only, and I'd be OK with that.  The only 
non-__init use in the 32-bit kernel is the compat vdso mapping, and that 
could easily be done by other means (though it would effectively become 
an opencoded set_fixmap, so perhaps that's not a good idea...).

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

No, that's set_pte_at(), which is the real issue in both cases.  
__set_fixmap calls both set_pte_at and flush_tlb_one, which is why it 
gets two backtrackes.

    J

  reply	other threads:[~2008-03-06 18:25 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
2008-03-06 12:52               ` Ingo Molnar
2008-03-06 18:19                 ` Jeremy Fitzhardinge [this message]
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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