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

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Message-ID: <424599d3-9bd0-4cd5-9ad7-37cebdc765e5@linux.dev>
Date: Thu, 26 Jun 2025 23:06:11 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: 21cnbao@...il.com, 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 22:39, David Hildenbrand wrote:
[...]
>>>> @@ -1870,12 +1870,13 @@ static inline unsigned int
>>>> folio_unmap_pte_batch(struct folio *folio,
>>>>            return 1;
>>>>        /* ... where we must be able to batch the whole folio. */
>>>
>>> Why is that still required? :)
>>
>> Sorry ... I was still stuck in the "all-or-nothing" mindset ...
>>
>> So, IIUC, you mean we should completely remove the "max_nr < nr_pages"
>> check and just let folio_pte_batch handle whatever partial batch it
>> safely can.
>>
>>>
>>>> -    if (pte_pfn(pte) != folio_pfn(folio) || max_nr !=
>>>> folio_nr_pages(folio))
>>>> +    if (pte_pfn(pte) != folio_pfn(folio) || max_nr < nr_pages)
>>>>            return 1;
>>>> -    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr,
>>>> fpb_flags,
>>>> -                 NULL, NULL, NULL);
>>>> -    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
>>>> +    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, nr_pages,
>>>> +                 fpb_flags, NULL, NULL, NULL);
>>>> +
>>>> +    return (max_nr != nr_pages) ? 1 : max_nr;
>>>
>>> Why is that still required? :)
>>
>> Then simply return the number of PTEs that consecutively map to the
>> large folio. Right?
> 
> Yes. Any part of the large folio. Just return folio_pte_batch() ...
> 

Ah, got it. Thanks!