[PATCH 4/4 v4] kernel/fork.c: memory hotplug updates max_threads

[Heinrich Schuchardt]

Memory may be added or removed while the system is online.

With the patch the value of max_threads is updated accordingly.

The limits of the init process are also updated.
This does not include updating limits of other running processes.

Tested with commands like
  echo 5000 > /proc/sys/kernel/threads-max
  echo 0 > /sys/devices/system/memory/memory7/online
  cat /proc/sys/kernel/threads-max
  echo 1 > /sys/devices/system/memory/memory7/online
  cat /proc/sys/kernel/threads-max

Signed-off-by: Heinrich Schuchardt <xypron.glpk@xxxxxx>
---
 kernel/fork.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 33b084e..7fd7c0a9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -75,6 +75,8 @@
 #include <linux/aio.h>
 #include <linux/compiler.h>
 #include <linux/sysctl.h>
+#include <linux/memory.h>
+#include <linux/notifier.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -295,6 +297,31 @@ static void set_max_threads(unsigned int max_threads_suggested)
 		init_task.signal->rlim[RLIMIT_NPROC];
 }
 
+/*
+ * Callback function called for memory hotplug events.
+ */
+static int memory_hotplug_callback(struct notifier_block *self,
+				   unsigned long action, void *arg)
+{
+	switch (action) {
+	case MEM_ONLINE:
+		/*
+		 * If memory was added, try to maximize the number of allowed
+		 * threads.
+		 */
+		set_max_threads(UINT_MAX);
+		break;
+	case MEM_OFFLINE:
+		/*
+		 * If memory was removed, try to keep current value.
+		 */
+		set_max_threads(max_threads);
+		break;
+	}
+
+	return NOTIFY_OK;
+}
+
 void __init fork_init(void)
 {
 #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
@@ -311,6 +338,8 @@ void __init fork_init(void)
 	arch_task_cache_init();
 
 	set_max_threads(UINT_MAX);
+
+	hotplug_memory_notifier(memory_hotplug_callback, 0);
 }
 
 int __weak arch_dup_task_struct(struct task_struct *dst,
-- 
2.1.4

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