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

From: Seiji Nishikawa
Date: Sat Nov 30 2024 - 11:13:06 EST


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@xxxxxxxxxx>
---
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