Re: [PATCH v6 09/15] mm/mglru: Avoid placing rejected folios below populated gen

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found no new issues
in this patch, but noticed 1 related pre-existing issue it does not
introduce:
- [Medium] mm: uncleared LRU_REFS_FLAGS in lru_gen_add_folio() [1]
--

commit 8ef17c06cc7c92ed5b07f7cf40e28066a3ac2fb0
Author: Barry Song (Xiaomi) <baohua@kernel.org>

mm/mglru: Avoid placing rejected folios below populated gen

This commit passes the scanned generation sequence number from
scan_folios() to move_folios_to_lru() so rejected folios are placed above
the populated generation or in the youngest generation, and removes the
previous folio_set_active() call from evict_folios().

> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index ab69b9930893f..f6f4bc9a704fe 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -332,7 +332,8 @@ static inline unsigned long lru_gen_folio_seq(const struct lruvec *lruvec,
>  	return max(READ_ONCE(lrugen->max_seq) - gen + 1, READ_ONCE(lrugen->min_seq[type]));
>  }
>  
> -static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
> +static inline bool __lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio,
> +		unsigned long scanned_seq, bool reclaiming)
>  {
>  	unsigned long seq;
>  	unsigned long flags;
> @@ -347,6 +348,8 @@ static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio,
>  		return false;
>  
>  	seq = lru_gen_folio_seq(lruvec, folio, reclaiming);
> +	if (scanned_seq != -1UL)
> +		seq = max(seq, min(scanned_seq + 1, lrugen->max_seq));
>  	gen = lru_gen_from_seq(seq);
>  	flags = (gen + 1UL) << LRU_GEN_PGOFF;
>  	/* see the comment on MIN_NR_GENS about PG_active */

[Severity: Medium]
This problem wasn't introduced by this patch, but should set_mask_bits()
in __lru_gen_add_folio() also clear LRU_REFS_FLAGS [1]?

include/linux/mm_inline.h:__lru_gen_add_folio() {
	...
	set_mask_bits(folio_flags(folio, 0), LRU_GEN_MASK | BIT(PG_active), flags);
	...
}

Omitting LRU_REFS_FLAGS from the mask leaves stale PG_referenced and
LRU_REFS_MASK bits set when a folio is placed into a new generation,
inflating its tier access count and causing cold folios to be protected
from reclaim at the expense of hotter ones.

[1] https://sashiko.dev/#/bug/linux-d2209941-6149-4b37-a97a-08c70d875d2c

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261110202025.25736-1-baohua@kernel.org?part=9