linux-kernel - Re: [PATCH 2/5] mm: khugepaged: use the number of pages in the folio to check the reference count

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Message-ID: <3802c50b-325e-491f-9f8d-8ab309121935@linux.alibaba.com>
Date: Mon, 19 Aug 2024 18:05:01 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: hughd@...gle.com, willy@...radead.org, 21cnbao@...il.com,
 ryan.roberts@....com, shy828301@...il.com, ziy@...dia.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] mm: khugepaged: use the number of pages in the folio
 to check the reference count



On 2024/8/19 17:40, David Hildenbrand wrote:
> On 19.08.24 10:14, Baolin Wang wrote:
>> Use the number of pages in the folio to check the reference count as
>> preparation for supporting shmem mTHP collapse.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>>   mm/khugepaged.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index f11b4f172e61..60d95f08610c 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1994,7 +1994,7 @@ static int collapse_file(struct mm_struct *mm, 
>> unsigned long addr,
>>           /*
>>            * We control three references to the folio:
> 
> ^ "three" is wrong now.

Ah, good catch. Will change to '2 + nr_pages'.

> 
>>            *  - we hold a pin on it;
>> -         *  - one reference from page cache;
>> +         *  - nr_pages reference from page cache;
>>            *  - one from lru_isolate_folio;
>>            * If those are the only references, then any new usage
>>            * of the folio will have to fetch it from the page
>> @@ -2002,7 +2002,7 @@ static int collapse_file(struct mm_struct *mm, 
>> unsigned long addr,
>>            * truncate, so any new usage will be blocked until we
>>            * unlock folio after collapse/during rollback.
>>            */
>> -        if (folio_ref_count(folio) != 3) {
>> +        if (folio_ref_count(folio) != 2 + folio_nr_pages(folio)) {
>>               result = SCAN_PAGE_COUNT;
>>               xas_unlock_irq(&xas);
>>               folio_putback_lru(folio);
>> @@ -2185,7 +2185,7 @@ static int collapse_file(struct mm_struct *mm, 
>> unsigned long addr,
>>           folio_clear_active(folio);
>>           folio_clear_unevictable(folio);
>>           folio_unlock(folio);
>> -        folio_put_refs(folio, 3);
>> +        folio_put_refs(folio, 2 + folio_nr_pages(folio));
>>       }
>>       goto out;
> 
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks for reviewing.