Re: [PATCH] mm: vmscan: account for free pages to prevent infinite Loop in throttle_direct_reclaim()

[Andrew Morton <akpm@linux-foundation.org>]

On Sun,  1 Dec 2024 01:12:34 +0900 Seiji Nishikawa <snishika@redhat.com> wrote:

> The kernel hangs due to a task stuck in throttle_direct_reclaim(),
> caused by a node being incorrectly deemed balanced despite pressure in
> certain zones, such as ZONE_NORMAL. This issue arises from
> zone_reclaimable_pages() returning 0 for zones without reclaimable file-
> backed or anonymous pages, causing zones like ZONE_DMA32 with sufficient
> free pages to be skipped.
> 
> The lack of swap or reclaimable pages results in ZONE_DMA32 being
> ignored during reclaim, masking pressure in other zones. Consequently,
> pgdat->kswapd_failures remains 0 in balance_pgdat(), preventing fallback
> mechanisms in allow_direct_reclaim() from being triggered, leading to an
> infinite loop in throttle_direct_reclaim().
> 
> This patch modifies zone_reclaimable_pages() to account for free pages
> (NR_FREE_PAGES) when no other reclaimable pages exist. This ensures
> zones with sufficient free pages are not skipped, enabling proper
> balancing and reclaim behavior.

We'll want to backport a fix for this into -stable kernels.  For that
it's best to be able to identify a suitable Fixes: target, to tell
others whether their kernel needs the fix.  Are you able to help
identify that commit?

Thanks.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -374,7 +374,14 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>  	if (can_reclaim_anon_pages(NULL, zone_to_nid(zone), NULL))
>  		nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
>  			zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);
> -
> +	/*
> +	 * If there are no reclaimable file-backed or anonymous pages, 
> +	 * ensure zones with sufficient free pages are not skipped. 
> +	 * This prevents zones like DMA32 from being ignored in reclaim 
> +	 * scenarios where they can still help alleviate memory pressure.
> +	 */
> +	if (nr == 0)
> +	    nr = zone_page_state_snapshot(zone, NR_FREE_PAGES);
>  	return nr;
>  }
>  
> -- 
> 2.47.0