linux-kernel - [PATCH] kernel/fork: Increase minimum number of allowed threads

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Message-ID: <20250711230348.213841-1-hauke@hauke-m.de>
Date: Sat, 12 Jul 2025 01:03:48 +0200
From: Hauke Mehrtens <hauke@...ke-m.de>
To: sashal@...nel.org,
	linux-kernel@...r.kernel.org
Cc: frederic@...nel.org,
	david@...hat.com,
	viro@...iv.linux.org.uk,
	paulmck@...nel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Hauke Mehrtens <hauke@...ke-m.de>
Subject: [PATCH] kernel/fork: Increase minimum number of allowed threads

From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

A modern Linux system creates much more than 20 threads at bootup.
When I booted up OpenWrt in qemu the system sometimes failed to boot up
when it wanted to create the 419th thread. The VM had 128MB RAM and the
calculation in set_max_threads() calculated that max_threads should be
set to 419. When the system booted up it tried to notify the user space
about every device it created because CONFIG_UEVENT_HELPER was set and
used. I counted 1299 calles to call_usermodehelper_setup(), all of
them try to create a new thread and call the userspace hotplug script in
it.

This fixes bootup of Linux on systems with low memory.

I saw the problem with qemu 10.0.2 using these commands:
qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic

Cc: stable@...r.kernel.org
Signed-off-by: Hauke Mehrtens <hauke@...ke-m.de>
---
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 7966c9a1c163..388299525f3c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -115,7 +115,7 @@
 /*
  * Minimum number of threads to boot the kernel
  */
-#define MIN_THREADS 20
+#define MIN_THREADS 600

 /*
  * Maximum number of threads
-- 
2.50.1