Re: [PATCH 3/5] fork: Remove assumption that vm_area->nr_pages equals to THREAD_SIZE

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 06 May 2025 09:57:55 +0200 Linus Walleij <linus.walleij@linaro.org> wrote:

> From: Pasha Tatashin <pasha.tatashin@soleen.com>
> 
> In many places number of pages in the stack is detremined via
> (THREAD_SIZE / PAGE_SIZE). There is also a BUG_ON() that ensures that
> (THREAD_SIZE / PAGE_SIZE) is indeed equal to vm_area->nr_pages.
> 
> Consistently use vm_area->nr_pages to determine the actual number
> of pages allocated in the stack instead, so it is clear what is
> going on here.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Link: https://lore.kernel.org/20240311164638.2015063-5-pasha.tatashin@soleen.com
> [Rebased, also skipped intermediary helper variable nr_pages]

I like

[linus.walleij@linaro.org: rebased, also skipped intermediary helper variable nr_pages]

> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -254,9 +254,7 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm_area)
>  	int ret;
>  	int nr_charged = 0;
>  
> -	BUG_ON(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
> -
> -	for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
> +	for (i = 0; i < vm_area->nr_pages; i++) {

Well, constants are fast and small.  I assume this will add a few
instructions and a bit of runtime overhead?

>  		ret = memcg_kmem_charge_page(vm_area->pages[i], GFP_KERNEL, 0);
>  		if (ret)
>  			goto err;
> @@ -518,7 +516,7 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
>  		struct vm_struct *vm_area = task_stack_vm_area(tsk);
>  		int i;
>  
> -		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
> +		for (i = 0; i < vm_area->nr_pages; i++)
>  			mod_lruvec_page_state(vm_area->pages[i], NR_KERNEL_STACK_KB,
>  					      account * (PAGE_SIZE / 1024));
>  	} else {