Re: [PATCH] mm: migrate LRU_REFS_MASK bits in folio_migrate_flags

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 25 Sep 2024 11:02:25 +0800 "zhaoyang.huang" <zhaoyang.huang@unisoc.com> wrote:

> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> Bits of LRU_REFS_MASK are not inherited during migration which lead to
> new_folio start from tier0. Fix this by migrate the bits domain.

I'm having trouble understanding this, sorry.  Please more fully
describe the runtime effects of this flaw.

> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -291,6 +291,12 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>  	return true;
>  }
>  
> +static inline void folio_migrate_refs(struct folio *new_folio, struct folio *folio)
> +{
> +	unsigned long refs = READ_ONCE(folio->flags) & LRU_REFS_MASK;
> +
> +	set_mask_bits(&new_folio->flags, LRU_REFS_MASK, refs);
> +}
>  #else /* !CONFIG_LRU_GEN */
>  
>  static inline bool lru_gen_enabled(void)
> @@ -313,6 +319,8 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>  	return false;
>  }
>  
> +static inline void folio_migrate_refs(struct folio *new_folio, struct folio *folio)
> +{}
>  #endif /* CONFIG_LRU_GEN */
>  
>  static __always_inline
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 923ea80ba744..60c97e235ae7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -618,6 +618,7 @@ 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.
> -- 
> 2.25.1