+ mm-vmscan-account-for-free-pages-to-prevent-infinite-loop-in-throttle_direct_reclaim.patch added to mm-hotfixes-unstable branch

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

The patch titled
     Subject: mm: vmscan: account for free pages to prevent infinite Loop in throttle_direct_reclaim()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-vmscan-account-for-free-pages-to-prevent-infinite-loop-in-throttle_direct_reclaim.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vmscan-account-for-free-pages-to-prevent-infinite-loop-in-throttle_direct_reclaim.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Seiji Nishikawa <snishika@redhat.com>
Subject: mm: vmscan: account for free pages to prevent infinite Loop in throttle_direct_reclaim()
Date: Sun, 1 Dec 2024 01:12:34 +0900

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.

Link: https://lkml.kernel.org/r/20241130161236.433747-2-snishika@redhat.com
Signed-off-by: Seiji Nishikawa <snishika@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmscan.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/mm/vmscan.c~mm-vmscan-account-for-free-pages-to-prevent-infinite-loop-in-throttle_direct_reclaim
+++ a/mm/vmscan.c
@@ -374,7 +374,14 @@ unsigned long zone_reclaimable_pages(str
 	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;
 }
 
_

Patches currently in -mm which might be from snishika@redhat.com are

mm-vmscan-ensure-kswapd-is-woken-up-if-the-wait-queue-is-active.patch
mm-vmscan-account-for-free-pages-to-prevent-infinite-loop-in-throttle_direct_reclaim.patch