linux-kernel - Re: [PATCH] mm: khugepaged: fix the incorrect statistics when collapsing large file folios

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Message-ID: <74467d4c-3ffc-44c0-8e9c-4d631ef9162e@redhat.com>
Date: Mon, 14 Oct 2024 13:32:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: 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] mm: khugepaged: fix the incorrect statistics when
 collapsing large file folios

On 14.10.24 12:24, Baolin Wang wrote:
> Khugepaged already supports collapsing file large folios (including shmem mTHP)
> by commit 7de856ffd007 ("mm: khugepaged: support shmem mTHP collapse"), and the
> control parameters in khugepaged: 'khugepaged_max_ptes_swap' and 'khugepaged_max_ptes_none',
> still compare based on PTE granularity to determine whether a file collapse is
> needed. However, the statistics for 'present' and 'swap' in hpage_collapse_scan_file()
> do not take into account the large folios, which may lead to incorrect judgments
> regarding the khugepaged_max_ptes_swap/none parameters, resulting in unnecessary
> file collapses.
> 
> To fix this issue, take into account the large folios' statistics for 'present'
> and 'swap' variables in the hpage_collapse_scan_file().
> 
> Fixes: 7de856ffd007 ("mm: khugepaged: support shmem mTHP collapse")
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>   mm/khugepaged.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index ba28ba09fe89..6f8d46d107b4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2267,7 +2267,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>   			continue;
>   
>   		if (xa_is_value(folio)) {
> -			++swap;
> +			swap += 1 << xas_get_order(&xas);
>   			if (cc->is_khugepaged &&
>   			    swap > khugepaged_max_ptes_swap) {
>   				result = SCAN_EXCEED_SWAP_PTE;
> @@ -2314,7 +2314,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>   		 * is just too costly...
>   		 */
>   
> -		present++;
> +		present += folio_nr_pages(folio);
>   
>   		if (need_resched()) {
>   			xas_pause(&xas);

Looks correct to me:

Acked-by: David Hildenbrand <david@...hat.com>


-- 
Cheers,

David / dhildenb