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. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
