Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

[Barry Song <21cnbao@gmail.com>]

On Thu, Jun 26, 2025 at 12:25 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.06.25 14:20, Lance Yang wrote:
[...]
> > Hmm... I have a question about the reference counting here ...
> >
> >               if (vma->vm_flags & VM_LOCKED)
> >                       mlock_drain_local();
> >               folio_put(folio);
> >               /* We have already batched the entire folio */
> >
> > Does anyone else still hold a reference to this folio after folio_put()?
>
> The caller of the unmap operation should better hold a reference :)
>
> Also, I am not sure why we don't perform a
>
> folio_put_refs(folio, nr_pages);

Because we've already called folio_ref_sub(folio, nr_pages - 1);
Looking back, it’s kind of ugly, huh.

discard:
                if (unlikely(folio_test_hugetlb(folio))) {
                        hugetlb_remove_rmap(folio);
                } else {
                        folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
                        folio_ref_sub(folio, nr_pages - 1);
                }

I assume Lance will send a patch? If so, remember to remove this
when switching to folio_put_refs(folio, nr_pages);

Thanks
Barry