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

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Message-ID: <42343413-7089-404e-8ce0-dd0025dca04c@linux.dev>
Date: Thu, 26 Jun 2025 09:17:33 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Barry Song <21cnbao@...il.com>, David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, baolin.wang@...ux.alibaba.com,
 chrisl@...nel.org, kasong@...cent.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, lorenzo.stoakes@...cle.com,
 ryan.roberts@....com, v-songbaohua@...o.com, x86@...nel.org,
 ying.huang@...el.com, zhengtangquan@...o.com,
 Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
 folios during reclamation



On 2025/6/26 05:03, Barry Song wrote:
> On Thu, Jun 26, 2025 at 12:25 AM David Hildenbrand <david@...hat.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!