linux-kernel - [PATCH v2] mm/page_alloc: Report 1 as zone_batchsize for !CONFIG

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Message-ID: <20251218083200.2435789-1-joshua.hahnjy@gmail.com>
Date: Thu, 18 Dec 2025 00:31:59 -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,
	kernel-team@...a.com
Subject: [PATCH v2] mm/page_alloc: Report 1 as zone_batchsize for !CONFIG_MMU

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.

For NOMMU systems, return 1 instead of 0 for zone_batchsize, which restores
the previous deadlock-free behavior.

There is no functional difference expected with this patch before commit
2783088ef24e, other than the pr_debug in zone_pcp_init now printing out
1 instead of 0 for zones in NOMMU systems. Not only is this a pr_debug,
the difference is purely semantic anyways.

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>
---
v1 --> v2:
- Instead of restoring  max(1, zone_batchsize(zone)), just return 1 for NOMMU
  systems since this is simpler and only affects a single pr_debug.
 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..977cbf20777d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5924,7 +5924,7 @@ static int zone_batchsize(struct zone *zone)
 	 * recycled, this leads to the once large chunks of space being
 	 * fragmented and becoming unavailable for high-order allocations.
 	 */
-	return 0;
+	return 1;
 #endif
 }
 

base-commit: 40fbbd64bba6c6e7a72885d2f59b6a3be9991eeb
-- 
2.47.3