Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support

[Andy Lutomirski]

On Wed, Aug 24, 2016 at 9:51 AM, Josh Cartwright <joshc@xxxxxx> wrote:
> Hey Andy-
>
> Small non-critical/potential future optimization comment below:
>
> On Thu, Aug 11, 2016 at 02:35:21AM -0700, Andy Lutomirski wrote:
>> If CONFIG_VMAP_STACK is selected, kernel stacks are allocated with
>> vmalloc_node.
>>
>> grsecurity has had a similar feature (called
>> GRKERNSEC_KSTACKOVERFLOW) for a long time.
>>
>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> ---
> [..]
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 52e725d4a866..05f7ef796fb4 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -158,19 +158,39 @@ void __weak arch_release_thread_stack(unsigned long *stack)
>>   * Allocate pages if THREAD_SIZE is >= PAGE_SIZE, otherwise use a
>>   * kmemcache based allocator.
>>   */
>> -# if THREAD_SIZE >= PAGE_SIZE
>> -static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
>> -                                               int node)
>> +# if THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK)
>> +static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>>  {
>> +#ifdef CONFIG_VMAP_STACK
>> +     void *stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
>> +                                        VMALLOC_START, VMALLOC_END,
>> +                                        THREADINFO_GFP | __GFP_HIGHMEM,
>> +                                        PAGE_KERNEL,
>> +                                        0, node,
>> +                                        __builtin_return_address(0));
>> +
>> +     /*
>> +      * We can't call find_vm_area() in interrupt context, and
>> +      * free_thread_stack can be called in interrupt context, so cache
>> +      * the vm_struct.
>> +      */
>> +     if (stack)
>> +             tsk->stack_vm_area = find_vm_area(stack);
>
> This is annoying, we end up having to walk the vm_area tree twice (once
> for the allocation, then here to get a handle on area).
>
> Perhaps it's time the vmalloc code learned an allocation API that
> returned the vm_area handle as well.
>

Agreed.  I may do this once everything else lands.