+ mm-page_alloc-report-1-as-zone_batchsize-for-config_mmu.patch added to mm-hotfixes-unstable branch

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

The patch titled
     Subject: mm/page_alloc: report 1 as zone_batchsize for !CONFIG_MMU
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-page_alloc-report-1-as-zone_batchsize-for-config_mmu.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-report-1-as-zone_batchsize-for-config_mmu.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: Joshua Hahn <joshua.hahnjy@gmail.com>
Subject: mm/page_alloc: report 1 as zone_batchsize for !CONFIG_MMU
Date: Thu, 18 Dec 2025 00:31:59 -0800

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.

Link: https://lkml.kernel.org/r/20251218083200.2435789-1-joshua.hahnjy@gmail.com
Fixes: 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Reported-by: Daniel Palmer <daniel@thingy.jp>
Closes: https://lore.kernel.org/linux-mm/CAFr9PX=_HaM3_xPtTiBn5Gw5-0xcRpawpJ02NStfdr0khF2k7g@mail.gmail.com/
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/all/42143500-c380-41fe-815c-696c17241506@roeck-us.net/
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Guenetr Roeck <linux@roeck-us.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/page_alloc.c~mm-page_alloc-report-1-as-zone_batchsize-for-config_mmu
+++ a/mm/page_alloc.c
@@ -5924,7 +5924,7 @@ static int zone_batchsize(struct zone *z
 	 * recycled, this leads to the once large chunks of space being
 	 * fragmented and becoming unavailable for high-order allocations.
 	 */
-	return 0;
+	return 1;
 #endif
 }
 
_

Patches currently in -mm which might be from joshua.hahnjy@gmail.com are

mm-page_alloc-report-1-as-zone_batchsize-for-config_mmu.patch