Re: [PATCH v7 16/16] mm/mglru: Avoid scan anon while swapcache is low for GFP_NOIO allocation

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] mm/mglru: stalled aging in try_to_shrink_lruvec() under
  GFP_NOIO
--

commit c2e07c75b5549aa295c630747e1cecd7625269f0
Author: Bo Zhang <zhangbo56@xiaomi.com>

mm/mglru: Avoid scan anon while swapcache is low for GFP_NOIO allocation

This commit skips anonymous folio scanning in MGLRU when GFP_NOIO is set
and swapcache is low by checking can_reclaim_anon_pages() in
lruvec_evictable_size().

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f36e9a379b62a..4fdbf0c2dcde0 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
[ ... ]
> @@ -4213,6 +4206,8 @@ static void lruvec_evictable_size(struct lruvec *lruvec, struct scan_control *sc
>  	int type;
>  	unsigned long seq;
>  	struct lru_gen_folio *lrugen = &lruvec->lrugen;
> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> +	int nid = lruvec_pgdat(lruvec)->node_id;
>  	DEFINE_MAX_SEQ(lruvec);
>  	DEFINE_MIN_SEQ(lruvec);
>  
> @@ -4220,6 +4215,8 @@ static void lruvec_evictable_size(struct lruvec *lruvec, struct scan_control *sc
>  	nr[LRU_GEN_FILE] = 0;
>  
>  	for_each_evictable_type(type, swappiness) {
> +		if (type == LRU_GEN_ANON && !can_reclaim_anon_pages(memcg, nid, sc))
> +			continue;
>  		for (seq = min_seq[type]; seq <= max_seq; seq++)
>  			nr[type] += lruvec_gen_size(lrugen, type, seq, sc->reclaim_idx + 1);
>  	}

[Severity: Medium]
Can zeroing nr[LRU_GEN_ANON] here while swappiness remains non-zero stall
MGLRU aging during GFP_NOIO reclaim?

When swap space is available and NUMA demotion is not, get_swappiness()
stays in 1..MAX_SWAPPINESS even when can_reclaim_anon_pages() returns
false. Because nr[LRU_GEN_ANON] is 0, evict_folio_lists() only scans file
generations, so min_seq[LRU_GEN_ANON] stays at max_seq - 3 and
try_to_inc_min_seq() clamps min_seq[LRU_GEN_FILE] to max_seq - MIN_NR_GENS:

mm/vmscan.c:try_to_inc_min_seq() {
	...
	if (swappiness && swappiness <= MAX_SWAPPINESS) {
		unsigned long seq = lrugen->max_seq - MIN_NR_GENS;

		if (min_seq[LRU_GEN_ANON] > seq && min_seq[LRU_GEN_FILE] < seq)
			min_seq[LRU_GEN_ANON] = seq;
		else if (min_seq[LRU_GEN_FILE] > seq && min_seq[LRU_GEN_ANON] < seq)
			min_seq[LRU_GEN_FILE] = seq;
	}
}

Once populatable file generations are drained, evict_folio_lists() returns
0 and try_to_shrink_lruvec() breaks out of the loop before setting
no_populatable, while should_run_aging() sees evictable_min_seq() at
min_seq[LRU_GEN_ANON] (max_seq - 3) and lru_gen_imbalanced() still counts
unreclaimable anon pages in old[LRU_GEN_ANON]. As a result, reclaim fails
to age from DEF_PRIORITY down to DEF_PRIORITY / 2 + 1 and then directly
evicts the youngest file generation (max_seq) without page-table aging or
tier protection.

Should get_swappiness() return 0 when !can_reclaim_anon_pages() instead, so
swappiness is 0 consistently across try_to_shrink_lruvec(),
try_to_inc_min_seq(), should_run_aging(), and lru_gen_imbalanced()?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111232603.9934-1-baohua@kernel.org?part=16