kernel/fork.c: new function for max_threads

PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
THREAD_SIZE.

E.g.  architecture hexagon may have page size 1M and thread size 4096.
This would lead to a division by zero in the calculation of max_threads.

With this patch the buggy code is moved to a separate function
set_max_threads.  The error is not fixed.

After fixing the problem in a separate patch the new function can be
reused to adjust max_threads after adding or removing memory.

Argument mempages of function fork_init() is removed as totalram_pages is
an exported symbol.

The creation of separate patches for refactoring to a new function and for
fixing the logic was suggested by Ingo Molnar.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 23 insertions, 15 deletions

diff --git a/init/main.c b/init/main.c
index a7e969d12f51..2115055faeac 100644
--- a/init/main.c
+++ b/init/main.c
@@ -91,7 +91,7 @@
 static int kernel_init(void *);
 
 extern void init_IRQ(void);
-extern void fork_init(unsigned long);
+extern void fork_init(void);
 extern void radix_tree_init(void);
 #ifndef CONFIG_DEBUG_RODATA
 static inline void mark_rodata_ro(void) { }
@@ -645,7 +645,7 @@ asmlinkage __visible void __init start_kernel(void)
 #endif
         thread_info_cache_init();
         cred_init();
-        fork_init(totalram_pages);
+        fork_init();
         proc_caches_init();
         buffer_init();
         key_init();
diff --git a/kernel/fork.c b/kernel/fork.c
index c507e29bcb01..01038e6f51a8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -253,7 +253,26 @@ EXPORT_SYMBOL_GPL(__put_task_struct);
 
 void __init __weak arch_task_cache_init(void) { }
 
-void __init fork_init(unsigned long mempages)
+/*
+ * set_max_threads
+ */
+static void set_max_threads(void)
+{
+        /*
+         * The default maximum number of threads is set to a safe
+         * value: the thread structures can take up at most one
+         * eighth of the memory.
+         */
+        max_threads = totalram_pages / (8 * THREAD_SIZE / PAGE_SIZE);
+
+        /*
+         * we need to allow at least 20 threads to boot a system
+         */
+        if (max_threads < 20)
+                max_threads = 20;
+}
+
+void __init fork_init(void)
 {
 #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
 #ifndef ARCH_MIN_TASKALIGN
@@ -268,18 +287,7 @@ void __init fork_init(unsigned long mempages)
         /* do the arch specific task caches init */
         arch_task_cache_init();
 
-        /*
-         * The default maximum number of threads is set to a safe
-         * value: the thread structures can take up at most one
-         * eighth of the memory.
-         */
-        max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
-
-        /*
-         * we need to allow at least 20 threads to boot a system
-         */
-        if (max_threads < 20)
-                max_threads = 20;
+        set_max_threads();
 
         init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
         init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;