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

[Linus Torvalds]

On Fri, Dec 17, 2021 at 12:36 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> >
> > 5. Take a R/O pin (RDMA, VFIO, ...)
> > -> refcount > 1
> >
> > 6. memset(mem, 0xff, pagesize);
> > -> Write fault -> COW
>
> I do not believe this is actually a bug.
>
> You asked for a R/O pin, and you got one.

If you want a shared pin that actually follows the changes of your
process around, then that is what you should have asked for.

At the time of such a shared pin, you can do what we already do:
re-use the page if it has a refcount of 1. Or do an early COW event
(feel free to avoid the "mark it writable and dirty").

But note: *refcount* of 1. Not "mapcount". Because mapcount would be
broken garbage.

                Linus