Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[Linus Torvalds]

On Fri, Dec 17, 2021 at 1:47 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> For now I have not heard a compelling argument why the mapcount is
> dubious, I repeat:
>
> * mapcount can only increase due to fork()
> * mapcount can decrease due to unmap / zap
>
> We can protect from the transtition == 1 -> >1 using the mmap_lock.
>
> For COW the mapcount is the only thing that matters *if we take GUP* out
> of the equation. And that's exactly what we

What do you have against just doing what we already do in other parts,
that a/b thing?

Which avoids the whole mmap_sem issue. That was a big issue for the
rdma people, afaik.

                Linus