Re: [PATCH v3 2/6] mm/mglru: introduce helpers for manipulating gen and refs flags

[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>
> 
> Instead of doing bit ops on folio->flags.f, introduce helpers for
> adjusting a folio's refs and generation info, making the code easier
> to debug and understand.
> 
> No functional change is intended: some combined atomic operations are
> split into two, which only creates harmless transient states. There is
> no measurable performance impact, and some paths even look slightly
> better in the generated assembly.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>   include/linux/mm_inline.h | 76 ++++++++++++++++++++++++++++++++++++++++++-----
>   include/linux/mmzone.h    |  1 +
>   mm/folio.c                | 19 +++++++-----
>   mm/vmscan.c               | 61 ++++++++++++++++++++-----------------
>   4 files changed, 114 insertions(+), 43 deletions(-)
> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 621c8653d8f7..edfaf2661812 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -142,10 +142,42 @@ static inline int lru_tier_from_refs(int refs, bool workingset)
>   	return workingset ? MAX_NR_TIERS - 1 : order_base_2(refs);
>   }
>   
> -static inline int folio_lru_refs(const struct folio *folio)
> +/**
> + * lru_gen_from_flags - Return the LRU generation number from folio flags.
> + * @flags: folio flags
> + *
> + * Returns: A number between 0 and (MAX_NR_GENS - 1), inclusive. Returns
> + * -1 if the flags indicate the folio is off the list (e.g., isolated).
> + */
> +static inline int lru_gen_from_flags(unsigned long flags)
> +{
> +	int gen = ((flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF);
> +
> +	BUILD_BUG_ON(LRU_GEN_MASK & LRU_REFS_MASK);
> +	gen -= 1;
> +	VM_WARN_ON_ONCE(gen != -1 && gen >= MAX_NR_GENS);
> +	return gen;
> +}
> +
> +/**
> + * lru_gen_set_flags - Set the LRU generation number to specified folio flags.
> + * @flags: pointer to the folio flags
> + * @gen: generation number, between 0 and (MAX_NR_GENS - 1), inclusive.
> + */
> +static inline void lru_gen_set_flags(unsigned long *flags, int gen)
>   {
> -	unsigned long flags = READ_ONCE(folio->flags.f);
> +	VM_WARN_ON_ONCE(gen >= MAX_NR_GENS || gen < 0);
> +
> +	*flags &= ~LRU_GEN_MASK;
> +	*flags |= (gen + 1UL) << LRU_GEN_PGOFF;
> +}
>   
> +/**
> + * lru_refs_from_flags - Return LRU referenced / access count from folio flags.
> + * @flags: folio flags
> + */
> +static inline int lru_refs_from_flags(unsigned long flags)
> +{
>   	if (!(flags & BIT(PG_referenced)))
>   		return 0;
>   	/*
> @@ -155,11 +187,40 @@ static inline int folio_lru_refs(const struct folio *folio)
>   	return ((flags & LRU_REFS_MASK) >> LRU_REFS_PGOFF) + 1;
>   }
>   
> -static inline int folio_lru_gen(const struct folio *folio)
> +/**
> + * lru_refs_set_flags - Set the LRU referenced / access count to specified folio flags.
> + * @flags: pointer to the folio flags
> + * @refs: referenced / access count number, between 0 and LRU_REFS_MAX, inclusive.
> + */
> +static inline void lru_refs_set_flags(unsigned long *flags, unsigned int refs)
> +{
> +	VM_WARN_ON_ONCE(refs > LRU_REFS_MAX);
> +	BUILD_BUG_ON(LRU_REFS_MAX != (LRU_REFS_MASK >> LRU_REFS_PGOFF) + 1);
> +
> +	*flags &= ~LRU_REFS_FLAGS;
> +	if (!refs)
> +		return;
> +	*flags |= (BIT(PG_referenced) | ((refs - 1UL) << LRU_REFS_PGOFF));
> +}
> +
> +static inline int folio_lru_refs(const struct folio *folio)
>   {
> -	unsigned long flags = READ_ONCE(folio->flags.f);
> +	return lru_refs_from_flags(READ_ONCE(*const_folio_flags(folio, 0)));
> +}
> +
> +static inline void folio_set_lru_refs(struct folio *folio, unsigned int refs)
> +{
> +	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
> +
> +	do {
> +		new_flags = old_flags;
> +		lru_refs_set_flags(&new_flags, refs);
> +	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
> +}
>   
> -	return ((flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> +static inline int folio_lru_gen(const struct folio *folio)
> +{
> +	return lru_gen_from_flags(READ_ONCE(*const_folio_flags(folio, 0)));
>   }
>   
>   static inline bool lru_gen_is_active(const struct lruvec *lruvec, int gen)
> @@ -270,7 +331,7 @@ static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio,
>   	gen = lru_gen_from_seq(seq);
>   	flags = (gen + 1UL) << LRU_GEN_PGOFF;
>   	/* see the comment on MIN_NR_GENS about PG_active */
> -	set_mask_bits(&folio->flags.f, LRU_GEN_MASK | BIT(PG_active), flags);
> +	set_mask_bits(folio_flags(folio, 0), LRU_GEN_MASK | BIT(PG_active), flags);
>   
>   	lru_gen_update_size(lruvec, folio, -1, gen);
>   	/* for folio_rotate_reclaimable() */
> @@ -295,7 +356,7 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>   
>   	/* for folio_migrate_flags() */
>   	flags = !reclaiming && lru_gen_is_active(lruvec, gen) ? BIT(PG_active) : 0;
> -	flags = set_mask_bits(&folio->flags.f, LRU_GEN_MASK, flags);
> +	flags = set_mask_bits(folio_flags(folio, 0), LRU_GEN_MASK, flags);
>   	gen = ((flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
>   
>   	lru_gen_update_size(lruvec, folio, gen, -1);
> @@ -339,7 +400,6 @@ 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)
>   {
> -
>   }
>   #endif /* CONFIG_LRU_GEN */
>   
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 94f9c3ff5416..c9ecf370cd9f 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -497,6 +497,7 @@ enum lruvec_flags {
>   
>   #define LRU_GEN_MASK		((BIT(LRU_GEN_WIDTH) - 1) << LRU_GEN_PGOFF)
>   #define LRU_REFS_MASK		((BIT(LRU_REFS_WIDTH) - 1) << LRU_REFS_PGOFF)
> +#define LRU_REFS_MAX		BIT(LRU_REFS_WIDTH)
>   
>   /*
>    * For folios accessed multiple times through file descriptors,
> diff --git a/mm/folio.c b/mm/folio.c
> index c02dcea9c03c..a932059057ac 100644
> --- a/mm/folio.c
> +++ b/mm/folio.c
> @@ -353,26 +353,28 @@ static void __lru_cache_activate_folio(struct folio *folio)
>   
>   static void lru_gen_inc_refs(struct folio *folio)
>   {
> -	unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
> +	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
> +	int refs;
>   
>   	if (folio_test_unevictable(folio))
>   		return;
>   
>   	/* see the comment on LRU_REFS_FLAGS */
> -	if (!folio_test_referenced(folio)) {
> -		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
> +	if (!folio_lru_refs(folio)) {
> +		folio_set_lru_refs(folio, 1);
>   		return;
>   	}
>   

Do we still need this branch now?

If folio_lru_refs(folio) == 0, that means refs = lru_refs_from_flags(old_flags) 
== 0, correct?

If so, the do-while loop below already handles this case, making this early 
return redundant.

Or am I missing something here?

>   	do {
> -		if ((old_flags & LRU_REFS_MASK) == LRU_REFS_MASK) {
> +		new_flags = old_flags;
> +		refs = lru_refs_from_flags(old_flags);
> +		if (refs == LRU_REFS_MAX) {
>   			if (!folio_test_workingset(folio))
>   				folio_set_workingset(folio);
>   			return;
>   		}
> -
> -		new_flags = old_flags + BIT(LRU_REFS_PGOFF);
> -	} while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
> +		lru_refs_set_flags(&new_flags, refs + 1);
> +	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
>   }
>   
>   static bool lru_gen_clear_refs(struct folio *folio)
> @@ -384,7 +386,8 @@ static bool lru_gen_clear_refs(struct folio *folio)
>   	if (gen < 0)
>   		return true;
>   
> -	set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS | BIT(PG_workingset), 0);
> +	folio_set_lru_refs(folio, 0);
> +	folio_clear_workingset(folio);
>   
>   	rcu_read_lock();
>   	seq = READ_ONCE(folio_lruvec(folio)->lrugen.min_seq[type]);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 73a81b4a3e16..9ee9f8dc6805 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -843,19 +843,22 @@ static bool lru_gen_set_refs(struct folio *folio, const vma_flags_t *vma_flags)
>   	if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
>   		/* Activate file-backed executable folios after first usage. */
>   		if (is_exec_file_folio(folio, vma_flags)) {
> -			set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_workingset));
> +			folio_set_workingset(folio);
> +			folio_set_lru_refs(folio, 0);
>   			return true;
>   		}
>   
> -		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
> +		folio_set_lru_refs(folio, 1);
>   		return false;
>   	}
>   
>   	/* Promote on second access */
> -	if (folio_lru_refs(folio) > 1)
> -		set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_workingset));
> -	else
> +	if (folio_lru_refs(folio) > 1) {
> +		folio_set_workingset(folio);
> +		folio_set_lru_refs(folio, 0);
> +	} else {
>   		folio_mark_accessed(folio);
> +	}
>   	return true;
>   }
>   #else
> @@ -3266,11 +3269,10 @@ static bool positive_ctrl_err(struct ctrl_pos *sp, struct ctrl_pos *pv)
>    ******************************************************************************/
>   
>   /* promote pages accessed through page tables */
> -static int folio_update_gen(struct folio *folio, int gen, const vma_flags_t *vma_flags)
> +static int folio_update_gen(struct folio *folio, int new_gen, const vma_flags_t *vma_flags)
>   {
> -	unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
> -
> -	VM_WARN_ON_ONCE(gen >= MAX_NR_GENS);
> +	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
> +	int old_gen;
>   
>   	/*
>   	 * See the comment on LRU_REFS_FLAGS, and activate file-backed
> @@ -3279,20 +3281,24 @@ static int folio_update_gen(struct folio *folio, int gen, const vma_flags_t *vma
>   	 */
>   	if (!folio_test_referenced(folio) && !folio_test_workingset(folio) &&
>   	    !is_exec_file_folio(folio, vma_flags)) {
> -		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
> +		folio_set_lru_refs(folio, 1);
>   		return -1;
>   	}
>   
>   	do {
> +		old_gen = lru_gen_from_flags(old_flags);
> +		new_flags = old_flags;
> +
>   		/* lru_gen_del_folio() has isolated this page? */
> -		if (!(old_flags & LRU_GEN_MASK))
> -			return -1;
> +		if (old_gen < 0)
> +			break;
>   
> -		new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_FLAGS);
> -		new_flags |= ((gen + 1UL) << LRU_GEN_PGOFF) | BIT(PG_workingset);
> -	} while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
> +		lru_gen_set_flags(&new_flags, new_gen);
> +		lru_refs_set_flags(&new_flags, 0);
> +		new_flags |= BIT(PG_workingset);
> +	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
>   
> -	return ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> +	return old_gen;
>   }
>   
>   /* protect pages accessed multiple times through file descriptors */
> @@ -3301,21 +3307,20 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
>   	int type = folio_is_file_lru(folio);
>   	struct lru_gen_folio *lrugen = &lruvec->lrugen;
>   	int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
> -	unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
> -
> -	VM_WARN_ON_ONCE_FOLIO(!(old_flags & LRU_GEN_MASK), folio);
> +	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
>   
>   	do {
> -		new_gen = ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> +		new_gen = lru_gen_from_flags(old_flags);
> +
>   		/* folio_update_gen() has promoted this page? */
>   		if (new_gen >= 0 && new_gen != old_gen)
>   			return new_gen;
>   
> +		new_flags = old_flags;
>   		new_gen = (old_gen + 1) % MAX_NR_GENS;
> -
> -		new_flags = old_flags & ~(LRU_GEN_MASK | LRU_REFS_FLAGS);
> -		new_flags |= (new_gen + 1UL) << LRU_GEN_PGOFF;
> -	} while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
> +		lru_gen_set_flags(&new_flags, new_gen);
> +		lru_refs_set_flags(&new_flags, 0);
> +	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
>   
>   	lru_gen_update_size(lruvec, folio, old_gen, new_gen);
>   
> @@ -4716,7 +4721,7 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
>   
>   	/* see the comment on LRU_REFS_FLAGS */
>   	if (!folio_test_referenced(folio))
> -		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, 0);
> +		folio_set_lru_refs(folio, 0);
>   
>   	success = lru_gen_del_folio(lruvec, folio, true);
>   	VM_WARN_ON_ONCE_FOLIO(!success, folio);
> @@ -4932,8 +4937,10 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   		}
>   
>   		/* don't add rejected folios to the oldest generation */
> -		if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
> -			set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS, BIT(PG_active));
> +		if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type]) {
> +			folio_set_lru_refs(folio, 0);
> +			folio_set_active(folio);
> +		}
>   	}
>   
>   	move_folios_to_lru(&list);
> 

-- 
Best regards
Ridong