Re: [PATCH v7 03/16] mm/mglru: reclaim from the oldest populated gen

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] mm/mglru: uninitialized sc.priority in lru_gen_seq_write() skews
  scan_folios()
--

commit c250a15f7ef946582fda6b278da38054c0ad3a66
Author: Kairui Song <kasong@tencent.com>

mm/mglru: reclaim from the oldest populated gen

This commit updates scan_folios() to scan from the oldest populated
generation when sc->priority is raised from DEF_PRIORITY, passing that
generation down to sort_folio() and folio_inc_gen().

Link: https://lore.kernel.org/linux-mm/20260726122123.7614-4-baohua@kernel.org/ [1]

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f98cd483f014a..20199232dd70c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c

[ ... ]

> @@ -4758,17 +4756,27 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	int scanned = 0;
>  	int isolated = 0;
>  	int skipped = 0;
> +	unsigned long min_seq;
>  	unsigned long remaining = nr_to_scan;
>  	struct lru_gen_folio *lrugen = &lruvec->lrugen;
>  
>  	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
>  	VM_WARN_ON_ONCE(!list_empty(list));
>  
> +	/*
> +	 * Respect the gen hotness distribution at low priority to avoid
> +	 * aging and scan cost, but prefer to satisfy the reclaimers request
> +	 * when under pressure to respect swappiness more and reduce IO cost.
> +	 */
> +	if (sc->priority == DEF_PRIORITY)
> +		min_seq = lrugen->min_seq[type];
> +	else
> +		min_seq = lruvec_populated_min_seq(lruvec, type, lrugen->max_seq - MIN_NR_GENS + 1);

[Severity: Low]
Does debugfs eviction via lru_gen_seq_write() take the raised-priority
branch here because sc.priority is left zero-initialized?

lru_gen_seq_write() allocates struct scan_control sc on the stack without
setting .priority = DEF_PRIORITY, so sc.priority is 0.  When an eviction
command runs while one type's oldest generation is empty and the other
type keeps get_nr_gens() above MIN_NR_GENS, scan_folios() treats the
call as raised priority and advances min_seq up to max_seq - MIN_NR_GENS + 1,
evicting folios from generations younger than the requested min_gen_nr
(including active generations that run_eviction() rejects).

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111232603.9934-1-baohua@kernel.org?part=3