Re: [PATCH v3 3/6] mm/migrate: copy all referenced state via folio_migrate_lru_refs

[Ridong Chen <ridong.chen@linux.dev>]

On 8/26/2026 1:53 AM, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> folio_migrate_flags() copies PG_referenced separately from the MGLRU
> refs counter, which folio_migrate_refs() transfers. Yet under MGLRU,
> PG_referenced and the refs counter bits together describe the
> referenced status of a folio.
> 
> Consolidate the two: rename folio_migrate_refs() to
> folio_migrate_lru_refs() and let it copy the complete referenced
> status, i.e., the MGLRU refs count including PG_referenced, or just
> PG_referenced for the active/inactive LRU. Drop the open-coded
> PG_referenced copy so the referenced status is transferred in one
> place. No behavior change is intended: under the active/inactive LRU
> the extra bits are unused, so operating on them is a noop.
> 
> Transfer the reference state first, before the destination folio is
> marked uptodate, so a concurrent lockless reader cannot have its
> reference update overwritten by the copy.
> 
> Reviewed-by: Baoquan He <baoquan.he@linux.dev>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>   include/linux/mm_inline.h | 20 +++++++++++++++-----
>   mm/migrate.c              |  6 +++---
>   2 files changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index edfaf2661812..7f91a89b5ba3 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -365,11 +365,19 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>   	return true;
>   }
>   
> -static inline void folio_migrate_refs(struct folio *new, const struct folio *old)
> +/**
> + * folio_migrate_lru_refs - copy the reference state to a new folio
> + * @new: the destination folio
> + * @old: the source folio
> + *
> + * Transfer the reference state to @new during migration: the MGLRU
> + * refs count, including PG_referenced, or just PG_referenced for the
> + * active/inactive LRU.
> + */
> +static inline void folio_migrate_lru_refs(struct folio *new, const struct folio *old)
>   {
> -	unsigned long refs = READ_ONCE(old->flags.f) & LRU_REFS_MASK;
> -
> -	set_mask_bits(&new->flags.f, LRU_REFS_MASK, refs);
> +	BUILD_BUG_ON(LRU_REFS_MASK & BIT(PG_referenced));
> +	folio_set_lru_refs(new, folio_lru_refs(old));
>   }
>   #else /* !CONFIG_LRU_GEN */
>   
> @@ -398,8 +406,10 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>   	return false;
>   }
>   
> -static inline void folio_migrate_refs(struct folio *new, const struct folio *old)
> +static inline void folio_migrate_lru_refs(struct folio *new, const struct folio *old)
>   {
> +	if (folio_test_referenced(old))
> +		folio_set_referenced(new);
>   }
>   #endif /* CONFIG_LRU_GEN */
>   
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 15b45832bcfa..a369d0c95c38 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -776,8 +776,9 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>   {
>   	int cpupid;
>   
> -	if (folio_test_referenced(folio))
> -		folio_set_referenced(newfolio);
> +	/* Copy the reference state, including PG_referenced */
> +	folio_migrate_lru_refs(newfolio, folio);
> +
>   	if (folio_test_uptodate(folio))
>   		folio_mark_uptodate(newfolio);
>   	if (folio_test_clear_active(folio)) {
> @@ -807,7 +808,6 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>   	if (folio_test_idle(folio))
>   		folio_set_idle(newfolio);
>   
> -	folio_migrate_refs(newfolio, folio);
>   	/*
>   	 * Copy NUMA information to the new page, to prevent over-eager
>   	 * future migrations of this same page.
> 

Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Thanks.

-- 
Best regards
Ridong