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


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