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

On Fri, Dec 17, 2021 at 3:34 AM David Hildenbrand david@redhat.com wrote:

• • If the child takes a read-only pin on such a page (i.e., FOLL_WRITE is not
• • set) and then unmaps the target page, we have:
• • • page has mapcount == 1 and refcount > 1

All these games with mapcount makes me think this is still broken.

mapcount has been a horribly broken thing in the past, and I'm not convinced it's not a broken thing now.

•   vmf->page = vm_normal_page(vmf->vma, vmf->address, vmf->orig_pte);
  

•   if (vmf->page && PageAnon(vmf->page) && !PageKsm(vmf->page) &&
  

•       page_mapcount(vmf->page) > 1) {
  

What keeps the mapcount stable in here?

And I still believe that the whole notion that "COW should use mapcount" is pure and utter garbage.

If we are doing a COW, we need an *exclusive* access to the page. That is not mapcount, that is the page ref.

mapcount is insane, and I think this is making this worse again.

Linus