On Tue, 1 Dec 2020, Andrea Arcangeli wrote: > > Any suggestions on how to have the per-vaddr per-mm _PAGE_UFFD_WP bit > survive the pte invalidates in a way that remains associated to a > certain vaddr in a single mm (so it can shoot itself in the foot if it > wants, but it can't interfere with all other mm sharing the shmem > file) would be welcome... I think it has to be a new variety of swap-like non_swap_entry() pte, see include/linux/swapops.h. Anything else would be more troublesome. Search for non_swap_entry and for migration_entry, to find places that might need to learn about this new variety. IIUC you only need a single value, no need to carve out another whole swp_type: could probably be swp_offset 0 of any swp_type other than 0. Note that fork's copy_page_range() does not "copy ptes where a page fault will fill them correctly", so would in effect put a pte_none into the child where the parent has this uffd_wp entry. I don't know anything about uffd versus fork, whether that would pose a problem. Hugh