linux-kernel - [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and

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Message-ID: <20251217060505.681923-1-joshua.hahnjy@gmail.com>
Date: Tue, 16 Dec 2025 22:05:03 -0800
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Daniel Palmer <daniel@...f.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Brendan Jackman <jackmanb@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Zi Yan <ziy@...dia.com>,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and_batch

Commit 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
moved the error handling (0-handling) of zone_batchsize from its
callers to inside the function. However, the commit left out the error
handling for the NOMMU case, leading to deadlocks on NOMMU systems.

Since in the NOMMU case the reported-to-user batchsize should still be 0,
we would only like the error handling to exist in the callsites that
set the internal value for the zone (i.e. zone_set_pageset_high_and_batch).

Restore max(1, zone_batchsize(zone)) to the callsite to prevent errors
on NOMMU systems.

Fixes: 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
Reported-by: Daniel Palmer <daniel@...ngy.jp>
Closes: https://lore.kernel.org/linux-mm/CAFr9PX=_HaM3_xPtTiBn5Gw5-0xcRpawpJ02NStfdr0khF2k7g@mail.gmail.com/
Reported-by: Guenter Roeck <linux@...ck-us.net>
Closes: https://lore.kernel.org/all/42143500-c380-41fe-815c-696c17241506@roeck-us.net/
Signed-off-by: Joshua Hahn <joshua.hahnjy@...il.com>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 822e05f1a964..10c1297fd3ea 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6045,7 +6045,7 @@ static void zone_set_pageset_high_and_batch(struct zone *zone, int cpu_online)
 {
 	int new_high_min, new_high_max, new_batch;
 
-	new_batch = zone_batchsize(zone);
+	new_batch = max(1, zone_batchsize(zone));
 	if (percpu_pagelist_high_fraction) {
 		new_high_min = zone_highsize(zone, new_batch, cpu_online,
 					     percpu_pagelist_high_fraction);

base-commit: 40fbbd64bba6c6e7a72885d2f59b6a3be9991eeb
-- 
2.47.3