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

[Linus Torvalds]

On Tue, Dec 21, 2021 at 10:51 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> For that purpose the pincount would already kind-off work. Not precise,
> but at least something ("this page cannot possibly have been pinned").

That part actually exists already, ie put_page_refs() has this:

  #ifdef CONFIG_DEBUG_VM
        if (VM_WARN_ON_ONCE_PAGE(page_ref_count(page) < refs, page))
                return;
  #endif

And yeah, it shouldn't have that '#ifdef CONFIG_DEBUG_VM' there, but I
think it's because the non-CONFIG_DEBUG_VM #define for
VM_WARN_ON_ONCE_PAGE() is broken, and doesn't return 0.

                   Linus