> If you unmap something (resulting in pte_none()) where previously > something used to be mapped in a page table, you might suddenly inform > the user space fault handler about a page fault that it doesn't expect, > because it previously placed a page and did not zap that page itself > (MADV_DONTNEED). > > So at least with userfaultfd I think we have to be careful. Not sure if > there are other corner cases (again, KSM behavior is interesting) > > -- > Thanks, > > David / dhildenb We can implement it such that if userfaultfd is enabled on a VMA then instead of unmapping the zero page, we will map to a read only zero page. The original patch from Yu Zhao frees zero pages only on reclaim, I am not sure it needs to be this restricted though. In use cases where immediately freeing zero pages does not work we can dedupe similar to how KSM does it.
