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

[Josh Cartwright]

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.

  Josh

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