linux-kernel - Re: [PATCH 1/5] mm: khugepaged: expand the is_refcount

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Message-ID: <c53887da-ebbe-432e-bf81-308085215420@redhat.com>
Date: Mon, 19 Aug 2024 10:36:30 +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 1/5] mm: khugepaged: expand the is_refcount_suitable() to
 support file folios

On 19.08.24 10:14, Baolin Wang wrote:
> Expand the is_refcount_suitable() to support reference checks for file folios,
> as preparation for supporting shmem mTHP collapse.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>   mm/khugepaged.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index cdd1d8655a76..f11b4f172e61 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -549,8 +549,14 @@ static bool is_refcount_suitable(struct folio *folio)
>   	int expected_refcount;
>   
>   	expected_refcount = folio_mapcount(folio);
> -	if (folio_test_swapcache(folio))
> +	if (folio_test_anon(folio)) {
> +		expected_refcount += folio_test_swapcache(folio) ?
> +					folio_nr_pages(folio) : 0;
> +	} else {
>   		expected_refcount += folio_nr_pages(folio);
> +		if (folio_test_private(folio))
> +			expected_refcount++;
> +	}

Alternatively, a bit neater

if (!folio_test_anon(folio) || folio_test_swapcache(folio))
	expected_refcount += folio_nr_pages(folio);
if (folio_test_private(folio))
	expected_refcount++;

The latter check should be fine even for anon folios (although always false)


>   
>   	return folio_ref_count(folio) == expected_refcount;
>   }
> @@ -2285,8 +2291,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>   			break;
>   		}
>   
> -		if (folio_ref_count(folio) !=
> -		    1 + folio_mapcount(folio) + folio_test_private(folio)) {

The "1" is due to the pagecache, right? IIUC, we don't hold a raised 
folio refcount as we do the xas_for_each().

-- 
Cheers,

David / dhildenb