Re: [PATCH v4 07/17] mm/mglru, gup: mark folios referenced via a fast helper

[Barry Song]

On Fri, Oct 16, 2026 at 2:15 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> Introduce folio_inc_lru_refs_fast(): raise refs from 0 to
> LRU_REFS_REFERENCED via cmpxchg, or just set PG_referenced for the
> classical LRU as before. No counter update, promotion, and guarteens
> no other complex operations in following commits.
>
> Convert the gup fast-path sites. No behavior change for classical LRU.
>
> Tested-by: zhaozhengzhuo <zhaozhengzhuo@xxxxxxxxxxxxx>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  include/linux/mm_inline.h | 25 +++++++++++++++++++++++++
>  mm/gup.c                  |  6 +++---
>  2 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 65415eeac802..effa13c63632 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -454,6 +454,31 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>  }
>  #endif /* CONFIG_LRU_GEN */
>
> +/**
> + * folio_inc_lru_refs_fast - Bump folio refs count without promotion.
> + * @folio: the folio
> + *
> + * Raise refs from 0 to LRU_REFS_REFERENCED and leave hotter folios
> + * untouched. For the classical LRU, a plain PG_referenced set.
> + */
> +static __always_inline void folio_inc_lru_refs_fast(struct folio *folio)
> +{
> +       unsigned long new_flags, old_flags;
> +
> +       if (!lru_gen_enabled()) {
> +               folio_set_referenced(folio);
> +               return;
> +       }
> +
> +       old_flags = READ_ONCE(*folio_flags(folio, 0));
> +       do {
> +               if (lru_get_refs_flags(old_flags))
> +                       break;
> +               new_flags = old_flags;
> +               lru_set_refs_flags(&new_flags, LRU_REFS_REFERENCED);
> +       } while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
> +}

What is the actual purpose of this patch? Is it to skip
cmpxchg() when the folio already has non-zero refs for
better performance?

Thanks,
Barry