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

[Seiji Nishikawa <snishika@redhat.com>]

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.

Signed-off-by: Seiji Nishikawa <snishika@redhat.com>
---
 mm/vmscan.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 76378bc257e3..fb6b4056dcce 100644
--- 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