On Thu, Jan 07, 2021 at 01:05:19PM -0800, Linus Torvalds wrote:

I think those would very much be worth fixing, so that if
UFFDIO_WRITEPROTECT taking the mmapo_sem for writing causes problems,
we can _fix_ those problems.

But I think it's entirely wrong to treat UFFDIO_WRITEPROTECT as
specially as Andrea seems to want to treat it. Particularly with
absolutely zero use cases to back it up.

Again for the record: there's nothing at all special in
UFFDIO_WRITEPROTECT in this respect.

If you could stop mentioning UFFDIO_WRITEPROTECT and only focus on
softdirty/clear_refs, maybe you wouldn't think my judgment is biased
towards clear_refs/softdirty too.

You can imagine the side effects of page_count doing a COW
erroneously, as corollary of the fact that KSM won't ever allow to
merge two pages if one of them is under GUP pin. Why is that?

Thanks,
Andrea