On Tue, Aug 22, 2023 at 04:39:43PM +0200, Jann Horn wrote: > > Perhaps something else will want that same behaviour in future (it's > > tempting, but difficult to guarantee correctness); for now, it is just > > userfaultfd (but by saying "_armed" rather than "_missing", I'm half- > > expecting uffd to add more such exceptional modes in future). > > Hm, yeah, sounds okay. (I guess we'd also run into this if we ever > wanted to make it possible to reliably install PTE markers with > madvise() or something like that, which might be nice for allowing > userspace to create guard pages without unnecessary extra VMAs...) I don't know what a userspace API for this would look like, but I have a dream of creating guard VMAs which only live in the maple tree and don't require the allocation of a struct VMA. Use some magic reserved pointer value like XA_ZERO_ENTRY to represent them ... seems more robust than putting a PTE marker in the page tables?
