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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Jan 7, 2021 at 12:59 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> The problem is it's not even possible to detect reliably if there's
> really a long term GUP pin because of speculative pagecache lookups.

So none of the normal code _needs_ that any more these days, which is
what I think is so nice. Any pinning will do the COW, and then we have
the logic to make sure it stays writable, and that keeps everything
nicely coherent and is all fairly simple.

And yes, it does mean that if somebody then explicitly write-protects
a page, it may end up being COW'ed after all, but if you first pinned
it, and then started playing with the protections of that page, why
should you be surprised?

So to me, this sounds like a "don't do that then" situation.

Anybody who does page pinning and wants coherency should NOT TOUCH THE
MAPPING IT PINNED.

(And if you do touch it, it's your own fault, and you get to keep both
of the broken pieces)

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
know a pinned page always must have a page_mapcount() of 1" - since as
part of pinning it and doing the GUP_PIN, we forced the COW, and then
subsequent fork() operations enforce it too.

So I do think that it might be possible to make that clear_refs code
notice "this page is pinned, I can't mark it WP without the pinning
coherency breaking".

It might not even be hard. But admittedly I'm somewhat handwaving
here, and I might not have thought of some situation.

              Linus