Re: [PATCH 3/7] mm/migrate: copy the referenced state via folio_migrate_refs()

[Baoquan He <baoquan.he@linux.dev>]

On 08/18/26 at 01:38pm, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> folio_migrate_flags() copies PG_referenced separately, while
> folio_migrate_refs() copies the rest of the reference state. Make
> folio_migrate_refs() copy the complete state, i.e., PG_referenced plus
> the MGLRU refs counter, in both LRU implementations, and drop the
> open-coded copy so the reference state is transferred in one place.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  include/linux/mm_inline.h | 16 +++++++++++++---
>  mm/migrate.c              |  4 ++--
>  2 files changed, 15 insertions(+), 5 deletions(-)

LGTM,

Reviewed-by: Baoquan He <baoquan.he@linux.dev>

> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 93bf3fa221f8..df62daaa2ee7 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -366,11 +366,19 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>  	return true;
>  }
>  
> +/**
> + * folio_migrate_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_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 */
>  
> @@ -401,6 +409,8 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>  
>  static inline void folio_migrate_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..82307332711f 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -776,8 +776,6 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>  {
>  	int cpupid;
>  
> -	if (folio_test_referenced(folio))
> -		folio_set_referenced(newfolio);
>  	if (folio_test_uptodate(folio))
>  		folio_mark_uptodate(newfolio);
>  	if (folio_test_clear_active(folio)) {
> @@ -807,7 +805,9 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>  	if (folio_test_idle(folio))
>  		folio_set_idle(newfolio);
>  
> +	/* Copy the reference state, including PG_referenced */
>  	folio_migrate_refs(newfolio, folio);
> +
>  	/*
>  	 * Copy NUMA information to the new page, to prevent over-eager
>  	 * future migrations of this same page.
> 
> -- 
> 2.55.0
> 
>