On Wed, 23 Oct 2019, Yang Shi wrote: > On 10/23/19 12:28 PM, Hugh Dickins wrote: > > > > > + return map_count >= 0 && > > You have added a map_count >= 0 test there. Okay, not wrong, but not > > necessary, and not consistent with what's returned in the PageAnon > > case (if this were called for an unmapped page). > > I was thinking about this too. I'm wondering there might be a case that the > PMD is split and it was the last PMD map, in this case subpage's _mapcount is > also equal to compound_mapcount (both is -1). So, it would return true, then > KVM may setup PMD map in EPT, but it might be PTE mapped later on the host. > But, I'm not quite sure if this is really possible or if this is really a > integrity problem. So, I thought it might be safer to add this check. The mmu_notifier_invalidate_range_start.._end() in __split_huge_pmd(), with KVM's locking and sequence counting, is required to protect against such races. Hugh
