Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending

[Linus Torvalds]

On Thu, Jan 7, 2021 at 1:53 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> >
> > Now, I do agree that from a QoI standpoint, it would be really lovely
> > if we actually enforced it. I'm not entirely sure we can, but maybe it
> > would be reasonable to use that
> >
> >    mm->has_pinned && page_maybe_dma_pinned(page)
> >
> > at least as the beginning of a heuristic.
> >
> > In fact, I do think that "page_maybe_dma_pinned()" could possibly be
> > made stronger than it is. Because at *THAT* point, we might say "we
>
> What exactly did you have in mind, to make it stronger? I think the
> answer is in this email but I don't quite see it yet...

Literally just adding a " && page_mapcount(page) == 1" in there
(probably best done inside page_maybe_dma_pinned() itself)

> Direct IO pins, on the other hand, are more transient. We can probably live
> without tagging Direct IO pages as FOLL_PIN. I think.

Yes. I think direct-IO writes should be able to just do a transient
GUP, and if it causes a COW fault that isn't coherent, that's the
correct semantics, I think (ie the direct-IO will see the original
data, the COW faulter will get it's own private copy to make changes
to).

I think pinning should be primarily limited to things that _require_
coherency (ie you pin because you're going to do some active two-way
communication using that page)

Does that match your thinking?

               Linus