Re: [PATCH resend 2/2] mm: vmscan: stop scanning ineligible folios after max_nr_skipped

[Kunwu Chan <kunwu.chan@gmail.com>]

On Tue,  1 Sep 2026 16:47:06 +0800 john <love_goo@163.com> wrote:

> From: Wupeng Ma <mawupeng1@huawei.com>
> 
> When reclaiming for a lower zone, isolate_lru_folios() accounts folios
> from higher zones as skipped and, once max_nr_skipped hits
> SWAP_CLUSTER_MAX_SKIPPED, force-isolates the remaining ineligible folios
> to keep the loop from spinning on the skipped ones.  Those folios are
> reclaimed even though they can never satisfy the current allocation, so
> nr_reclaimed is inflated into a false progress that keeps resetting
> no_progress_loops in should_reclaim_retry() and delays the OOM.
> 
> Stop scanning once max_nr_skipped is reached instead of force-isolating
> the ineligible folios.  The skipped folios are already accounted in
> total_scan, so shrink_lruvec() can charge the overshoot against its scan
> budget (see the previous commit) and will not rescan them.
> 
> Fixes: 1c7b17cf0594 ("mm/vmscan: fix hard LOCKUP in function isolate_lru_folios")
> Signed-off-by: Wupeng Ma <mawupeng1@huawei.com>
> ---
>  mm/vmscan.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 823af9e86efd3..375f7cd5aa441 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1701,12 +1701,20 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
>  		nr_pages = folio_nr_pages(folio);
>  		total_scan += nr_pages;
>  
> -		/* Using max_nr_skipped to prevent hard LOCKUP*/
> -		if (max_nr_skipped < SWAP_CLUSTER_MAX_SKIPPED &&
> -		    (folio_zonenum(folio) > sc->reclaim_idx)) {
> +		/*
> +		 * Using max_nr_skipped to prevent hard LOCKUP.
> +		 * Once the cap is hit, stop rather than force-isolating:
> +		 * reclaiming ineligible folios only inflates nr_reclaimed
> +		 * into a false progress.
> +		 */
> +		if (folio_zonenum(folio) > sc->reclaim_idx) {
>  			nr_skipped[folio_zonenum(folio)] += nr_pages;
> -			move_to = &folios_skipped;
>  			max_nr_skipped++;
> +			if (max_nr_skipped >= SWAP_CLUSTER_MAX_SKIPPED) {
> +				list_move(&folio->lru, &folios_skipped);
> +				break;

One question about stopping the scan once max_nr_skipped reaches 
SWAP_CLUSTER_MAX_SKIPPED.

Previously, reaching this limit caused the remaining ineligible folios 
to be force-isolated so that the scanner would not keep looping on skipped 
folios. With this change, we break out of isolate_lru_folios() instead.

Could this cause us to stop before reaching eligible folios later in the LRU? 
Is SWAP_CLUSTER_MAX_SKIPPED intended to be sufficient to determine that 
continuing the scan cannot make useful reclaim progress?

Thanks,
KunWu

> +			}
> +			move_to = &folios_skipped;
>  			goto move;
>  		}
>  
> -- 
> 2.53.0
> 
> 

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