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: <966bfe10-6123-48db-95b7-ade2f794700d@redhat.com>
Date: Mon, 19 Aug 2024 11:40:41 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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 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.

>   		 *  - 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>

-- 
Cheers,

David / dhildenb