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Subject: Re: One (possible) x86 get_user_pages bug
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Jan Beulich <JBeulich@novell.com>
Cc: Xiaowei Yang <xiaowei.yang@huawei.com>, Nick Piggin <npiggin@kernel.dk>,
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Date: Thu, 27 Jan 2011 17:56:43 +0100
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On Thu, 2011-01-27 at 16:07 +0000, Jan Beulich wrote:
> 
> Nick, based on your doing of the initial implementation, would
> you be able to estimate whether disabling get_user_pages_fast()
> altogether for Xen would be performing measurably worse than
> adding the locks (but continuing to avoid acquiring mm->mmap_sem)
> as suggested by Xiaowei? That's of course only if the latter is correct
> at all, of which I haven't fully convinced myself yet. 

Also, I don't think taking the pte_lock is sufficient in your case..
since x86 mmu_gather frees the page tables themselves too, your
dereference of the pgd/pud/pmd will be subject to the same deref after
free problems.