Re: [PATCH] mm: re-enable kswapd when memory pressure subsides or demotion is toggled

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

On Mon, 8 Sep 2025 19:04:10 +0900 Chanwon Park <flyinrm@gmail.com> wrote:

> If kswapd fails to reclaim pages from a node MAX_RECLAIM_RETRIES in a
> row, kswapd on that node gets disabled. That is, the system won't wakeup
> kswapd for that node until page reclamation is observed at least once.
> That reclamation is mostly done by direct reclaim, which in turn enables
> kswapd back.
> 
> However, on systems with CXL memory nodes, workloads with high anon page
> usage can disable kswapd indefinitely, without triggering direct
> reclaim. This can be reproduced with following steps:
> 
>    numa node 0   (32GB memory, 48 CPUs)
>    numa node 2~5 (512GB CXL memory, 128GB each)
>    (numa node 1 is disabled)
>    swap space 8GB
> 
>    1) Set /sys/kernel/mm/demotion_enabled to 0.
>    2) Set /proc/sys/kernel/numa_balancing to 0.
>    3) Run a process that allocates and random accesses 500GB of anon
>       pages.
>    4) Let the process exit normally.

hm, OK, I guess this is longstanding misbehavior?

> 
> Since kswapd_failures resets may be missed by ++ operation, it is
> changed from int to atomic_t.

Possibly this should have been a separate (earlier) patch.  But I
assume the need for this conversion was inroduced by this patch, so
it's debatable.

> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1411,7 +1411,7 @@ typedef struct pglist_data {
>  	int kswapd_order;
>  	enum zone_type kswapd_highest_zoneidx;
>  
> -	int kswapd_failures;		/* Number of 'reclaimed == 0' runs */
> +	atomic_t kswapd_failures;	/* Number of 'reclaimed == 0' runs */

This caused a number of 80-column horrors!  I had a fiddle, what do you
think?

--- a/mm/page_alloc.c~mm-re-enable-kswapd-when-memory-pressure-subsides-or-demotion-is-toggled-fix
+++ a/mm/page_alloc.c
@@ -2860,29 +2860,29 @@ static void free_frozen_page_commit(stru
 		 */
 		return;
 	}
+
 	high = nr_pcp_high(pcp, zone, batch, free_high);
-	if (pcp->count >= high) {
-		free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
-				   pcp, pindex);
-		if (test_bit(ZONE_BELOW_HIGH, &zone->flags) &&
-		    zone_watermark_ok(zone, 0, high_wmark_pages(zone),
-				      ZONE_MOVABLE, 0)) {
-			struct pglist_data *pgdat = zone->zone_pgdat;
-			clear_bit(ZONE_BELOW_HIGH, &zone->flags);
+	if (pcp->count < high)
+		return;
 
-			/*
-			 * Assume that memory pressure on this node is gone
-			 * and may be in a reclaimable state. If a memory
-			 * fallback node exists, direct reclaim may not have
-			 * been triggered, leaving 'hopeless node' stay in
-			 * that state for a while. Let kswapd work again by
-			 * resetting kswapd_failures.
-			 */
-			if (atomic_read(&pgdat->kswapd_failures)
-			    >= MAX_RECLAIM_RETRIES &&
-			    next_memory_node(pgdat->node_id) < MAX_NUMNODES)
-				atomic_set(&pgdat->kswapd_failures, 0);
-		}
+	free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
+			   pcp, pindex);
+	if (test_bit(ZONE_BELOW_HIGH, &zone->flags) &&
+	    zone_watermark_ok(zone, 0, high_wmark_pages(zone),
+			      ZONE_MOVABLE, 0)) {
+		struct pglist_data *pgdat = zone->zone_pgdat;
+		clear_bit(ZONE_BELOW_HIGH, &zone->flags);
+
+		/*
+		 * Assume that memory pressure on this node is gone and may be
+		 * in a reclaimable state. If a memory fallback node exists,
+		 * direct reclaim may not have been triggered, causing a
+		 * 'hopeless node' to stay in that state for a while.  Let
+		 * kswapd work again by resetting kswapd_failures.
+		 */
+		if (atomic_read(&pgdat->kswapd_failures) >= MAX_RECLAIM_RETRIES &&
+		    next_memory_node(pgdat->node_id) < MAX_NUMNODES)
+			atomic_set(&pgdat->kswapd_failures, 0);
 	}
 }
 
--- a/mm/show_mem.c~mm-re-enable-kswapd-when-memory-pressure-subsides-or-demotion-is-toggled-fix
+++ a/mm/show_mem.c
@@ -278,8 +278,8 @@ static void show_free_areas(unsigned int
 #endif
 			K(node_page_state(pgdat, NR_PAGETABLE)),
 			K(node_page_state(pgdat, NR_SECONDARY_PAGETABLE)),
-			str_yes_no(atomic_read(&pgdat->kswapd_failures)
-				   >= MAX_RECLAIM_RETRIES),
+			str_yes_no(atomic_read(&pgdat->kswapd_failures) >=
+				   MAX_RECLAIM_RETRIES),
 			K(node_page_state(pgdat, NR_BALLOON_PAGES)));
 	}
 
_