[PATCH 2/4 v4] kernel/fork.c: avoid division by zero

Heinrich Schuchardt
2015-02-23T20:16:40Z

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 calculus 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.

The calculation of max_threads is moved to a separate function.
This allows future patches to use the same logic, e.g. when
- max_threads is set by writing to /proc/sys/kernel/threads-max
- when adding or removing memory.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
---
kernel/fork.c | 42 +++++++++++++++++++++++++++++-------------
1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 3a423b2..4d68e1e 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. */
@@ -254,26 +264,32 @@ EXPORT_SYMBOL_GPL(__put_task_struct);
void __init __weak arch_task_cache_init(void) { }

/*
- * set_max_threads
- * The argument is ignored.
+ * set_max_threads tries to set the default limit to the suggested value.
*/
static void set_max_threads(unsigned int max_threads_suggested)
{
- /*
- * The default maximum number of threads is set to a safe
- * value: the thread structures can take up at most half
- * of 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;
+ threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
+ (u64) THREAD_SIZE * 8UL);
+
+ if (threads > max_threads_suggested)
+ threads = max_threads_suggested;
+
+ if (threads > MAX_THREADS)
+ threads = MAX_THREADS;
+
+ if (threads < MIN_THREADS)
+ threads = MIN_THREADS;
+
+ max_threads = (int) threads;

- init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
- init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
+ init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = (int) threads / 2;
+ init_task.signal->rlim[RLIMIT_NPROC].rlim_max = (int) threads / 2;
init_task.signal->rlim[RLIMIT_SIGPENDING] =
init_task.signal->rlim[RLIMIT_NPROC];
}
--
2.1.4