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

[David Hildenbrand]

On 22.08.23 16:39, Jann Horn wrote:
> On Tue, Aug 22, 2023 at 4:51 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > On Mon, 21 Aug 2023, Jann Horn wrote:
> > > On Mon, Aug 21, 2023 at 9:51 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > > > Just for this case, take the pmd_lock() two steps earlier: not because
> > > > it gives any protection against this case itself, but because ptlock
> > > > nests inside it, and it's the dropping of ptlock which let the bug in.
> > > > In other cases, continue to minimize the pmd_lock() hold time.
> > >
> > > Special-casing userfaultfd like this makes me a bit uncomfortable; but
> > > I also can't find anything other than userfaultfd that would insert
> > > pages into regions that are khugepaged-compatible, so I guess this
> > > works?
> >
> > I'm as sure as I can be that it's solely because userfaultfd breaks
> > the usual rules here (and in fairness, IIRC Andrea did ask my permission
> > before making it behave that way on shmem, COWing without a source page).
> >
> > 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'm working on something similar that goes a bit further than just guard 
pages. It also installs PTE markers into page tables, inside existing 
large VMAs.

Initially, I'll only tackle anon VMAs, though.

--
Cheers,

David / dhildenb