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

[Qi Zheng]

Hi Kairui,

On 8/26/26 1:53 AM, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxx>
> ---
>   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);

It seems a bit weird to have this build warning inside this function,
but after I tried using static_assert() and failed, I think I understand
why you put it here.

Besides that, LGTM, so:

Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>

Thanks,
Qi