Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

[Andrea Arcangeli]

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