Re: [PATCH 2/3] mm/mglru: improve scan_folios() exhaustion detection

[Ridong Chen]

On 8/20/2026 12:56 PM, Barry Song (Xiaomi) wrote:
> Commit 16b475d2ac3c ("mm/mglru: avoid reclaim type fall back when
> isolation makes no progress") uses scanned == 0 to determine
> whether scan_folios() has exhausted a reclaim type. However,
> this is not always sufficient. It is possible for scanned > 0,
> while the oldest reclaimable generation is exhausted after the
> first scan_folios() call.
>
> We detect early_stop in scan_folios(). If we stop early for any reason,
> it means the current reclaim type is not exhausted yet. If early_stop is
> never reached, it means we have exhausted the current oldest generation
> without hitting any scanning limit.
>
> Another issue is that if the lruvec has 4 generations, we might have
> exhausted the oldest generation while the second oldest generation is
> still reclaimable. In that case, this type is not exhausted yet.
>
> Add an exhausted output argument to scan_folios() so it can
> explicitly report whether the reclaimable lists for the current
> type have been exhausted.
>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>   mm/vmscan.c | 28 ++++++++++++++++++++++------
>   1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d5cc30b667ad..1f2e574b0061 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4721,7 +4721,8 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
>   
>   static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   		       struct scan_control *sc, int type, int tier,
> -		       struct list_head *list, int *isolatedp)
> +		       struct list_head *list, int *isolatedp,
> +		       bool *exhausted)
>   {
>   	int i;
>   	int gen;
> @@ -4732,12 +4733,15 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   	int skipped = 0;
>   	unsigned long remaining = nr_to_scan;
>   	struct lru_gen_folio *lrugen = &lruvec->lrugen;
> +	bool early_stop = false;
>   
>   	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
>   	VM_WARN_ON_ONCE(!list_empty(list));
>   
> -	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
> +	if (get_nr_gens(lruvec, type) == MIN_NR_GENS) {
> +		*exhausted = true;
>   		return 0;
> +	}
>   
>   	gen = lru_gen_from_seq(lrugen->min_seq[type]);
>   
> @@ -4768,8 +4772,10 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   				skipped_zone += delta;
>   			}
>   
> -			if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH)
> +			if (!--remaining || max(isolated, skipped_zone) >= MIN_LRU_BATCH) {
> +				early_stop = true;
>   				break;
> +			}
>   		}
>   
>   		if (skipped_zone) {
> @@ -4778,8 +4784,10 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   			skipped += skipped_zone;
>   		}
>   
> -		if (!remaining || isolated >= MIN_LRU_BATCH)
> +		if (!remaining || isolated >= MIN_LRU_BATCH) {
> +			early_stop = true;
>   			break;
> +		}
>   	}
>   
>   	item = PGSCAN_KSWAPD + reclaimer_offset(sc);
> @@ -4790,6 +4798,13 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   				scanned, skipped, isolated,
>   				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>   
> +	/*
> +	 * If we didn't stop early, all reclaimable folios in the current
> +	 * generation have been scanned. We are exhausted if this is the last
> +	 * reclaimable generation.
> +	 */
> +	*exhausted = !early_stop &&
> +		     lrugen->min_seq[type] + MIN_NR_GENS == lrugen->max_seq;
>   	*isolatedp = isolated;
>   	return scanned;
>   }
> @@ -4847,11 +4862,12 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   	bool type_fallback_allowed = !is_single_type_reclaim(swappiness);
>   	int type = get_type_to_scan(lruvec, swappiness);
>   	int total_scanned = 0, scanned, tier;
> +	bool exhausted;
>   
>   retry:
>   	tier = get_tier_idx(lruvec, type);
>   	scanned = scan_folios(nr_to_scan, lruvec, sc,
> -			      type, tier, list, isolated);
> +			      type, tier, list, isolated, &exhausted);
>   
>   	total_scanned += scanned;
>   	if (*isolated) {
> @@ -4864,7 +4880,7 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   	 * We are running out of the current reclaim type. Fall back to
>   	 * the other type if allowed.
>   	 */
> -	if (!scanned && type_fallback_allowed) {
> +	if (exhausted && type_fallback_allowed) {
>   		type = !type;
>   		type_fallback_allowed = false;
>   		goto retry;

LGTM.

Reviewed-by: Ridong Chen <ridong.chen@xxxxxxxxx>

--
Best regards
Ridong