Re: One (possible) x86 get_user_pages bug

From: Peter Zijlstra
Date: Thu Jan 27 2011 - 11:56:04 EST

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.
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