kernel/fork.c: avoid division by zero

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 32-bit calculation there is no solution which delivers valid results
for all possible combinations of the parameters.  The code is only called
once.  Hence a 64-bit calculation can be used as solution.

[akpm@linux-foundation.org: use clamp_t(), per Oleg]
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>

1 files changed, 20 insertions, 9 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 01038e6f51a8..c7f2e1a4187a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -88,6 +88,16 @@
 #include <trace/events/task.h>
 
 /*
+ * Minimum number of threads to boot the kernel
+ */
+#define MIN_THREADS 20
+
+/*
+ * Maximum number of threads
+ */
+#define MAX_THREADS FUTEX_TID_MASK
+
+/*
  * Protected counters by write_lock_irq(&tasklist_lock)
  */
 unsigned long total_forks;      /* Handle normal Linux uptimes. */
@@ -258,18 +268,19 @@ void __init __weak arch_task_cache_init(void) { }
  */
 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);
+        u64 threads;
 
         /*
-         * we need to allow at least 20 threads to boot a system
+         * The number of threads shall be limited such that the thread
+         * structures may only consume a small part of the available memory.
          */
-        if (max_threads < 20)
-                max_threads = 20;
+        if (fls64(totalram_pages) + fls64(PAGE_SIZE) > 64)
+                threads = MAX_THREADS;
+        else
+                threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
+                                    (u64) THREAD_SIZE * 8UL);
+
+        max_threads = clamp_t(u64, threads, MIN_THREADS, MAX_THREADS);
 }
 
 void __init fork_init(void)