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

[John Hubbard]

On 1/7/21 2:00 PM, Linus Torvalds wrote:
> 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)

Well, that means that pages that are used for pinned DMA like this, can
not be shared with other processes. Is that an acceptable limitation
for the RDMA users? It seems a bit constraining, at first glance anyway.

> > 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?

Yes, perfectly. I'm going to update Documentation/core-api/pin_user_pages.rst
accordingly, once the dust settles on these discussions.

thanks,
--
John Hubbard
NVIDIA