Re: [PATCH] KVM: MMU: fix huge page adapted on non-PAE host

[Avi Kivity <avi@redhat.com>]

On 05/28/2012 05:32 PM, Andrea Arcangeli wrote:
> Hi,
> 
> On Mon, May 28, 2012 at 04:53:38PM +0300, Avi Kivity wrote:
>> As far as I can tell __get_user_pages_fast() will take the reference
>> count in the page head in the first place.
> 
>     mask = KVM_PAGES_PER_HPAGE(level) - 1;
> 
> The BUG would trigger if the above KVM mask is 2M (that is the NPT/EPT
> pmd size), but the hugepage size in the host is 4M (noPAE 32bit).
> 
> The refcount is taken only in the head page for heads, and in both for
> tails.
> 
> Because we've mmu notifier, we never keep the pages mapped by sptes
> refcounted, we drop them all. So all we need to do is just to move the
> refcount on the same exact pfn that is then freed by mmu_set_spte
> (kvm_release_pfn_clean at the end).
> 
> The adjustement is not done for the refcounting, the issue here is, we
> want to adjust the "pfn" passed to mmu_set_spte, and in turn we've to
> move the refcounting too, because the kvm_release_pfn_clean will run
> on that "pfn" (not on the pfn returned by gup-fast anymore).
> 
> So it looks fine to just do get_page and the patch looks correct (not
> sure if the mmio the mmio check is needed or if we can just do
> get_page) as long as the "pfn" that is returned through &pfn parameter
> and then passssed to mmu_set_sptes is the same one were we do get_page.
> 
> The reason it was a get_page_unless_zero() is that it wanted to check
> that there was no THP split and the head page was still there. Problem
> is that with a 4M host page size and 2M NTP/EPT pmd size, we need to
> get_page a tail page half of the time, and get_page_unless_zero()
> won't be a correct refcount for tail pages, not equivalent to a full
> get_page.
> 
> Overall the most important thing is that the pfn returned is the
> correct one that matches the alignment of the NPT/EPT hugepmd size,
> the refcounting just closely follows that aligned "pfn".

Yes, I see it now.  Adjusting mask is incorrect since we won't have the
same adjustment on release.  I'll apply the patch for 3.5.

-- 
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