On Mon, May 28, 2012 at 05:20:02PM +0300, Avi Kivity wrote: > The "right thing" we should be doing is running get_page() on every > small page within the frame (we asked for a small page but are > opportunistrically using the pages around it, without a proper ref). > That's a bit slow though, so we cheat. The problem is that we're aligning the pfn. The refcount move just follows the pfn alignment. The spte setting I think need the correct aligned pfn that matches the hugepmd NPT/EPT alignment. So then we're moving the pfn refcount too, otherwise kvm_release_pfn_clean then would run on a different pfn than the one that was returned by gup-fast. If we would drop the refcount before calling __direct_map or use a gup-fast that doesn't even take a pin, we wouldn't need to move the refcount and we could only free the page (without having to do a get_page). > But I guess we can start with your fix. But what about shifting mask by > one bit? Isn't it sufficient? > > - mask = KVM_PAGES_PER_HPAGE(level) - 1; > + mask = KVM_PAGES_PER_HPAGE(level); > + mask *= KVM_HOST_HPAGES_PER_HPAGE; > + mask -= 1; > > This should move the reference to the right place. The pfn passed to mmu_set_spte then would then be aligned at 4M despite the NTP/EPT size is 2M, so I doubt it would be ok. The real thing to check here is that the pfn passed is correct. The refcount just follows. Just doing s/get_page_unless_zero()/get_page()/ should work I think. And good thing there's no chance to get this wrong by testing, it either boots or doesn't boot. -- 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
