[PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp() versus uffd

[Matthew Wilcox]

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?