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

[Lance Yang <lance.yang@linux.dev>]

On 2025/6/26 05:03, Barry Song wrote:
> 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);

Ah, got it. Thanks for pointing that out!