linux-kernel - Re: [RFC 06/14] fork: zero vmap stack using clear

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Message-ID: <f5105c5d-59fb-4bca-af70-353e480f2b12@suse.com>
Date: Tue, 12 Mar 2024 09:15:58 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, akpm@...ux-foundation.org, x86@...nel.org, bp@...en8.de,
 brauner@...nel.org, bristot@...hat.com, bsegall@...gle.com,
 dave.hansen@...ux.intel.com, dianders@...omium.org,
 dietmar.eggemann@....com, eric.devolder@...cle.com, hca@...ux.ibm.com,
 hch@...radead.org, hpa@...or.com, jacob.jun.pan@...ux.intel.com,
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 kent.overstreet@...ux.dev, kinseyho@...gle.com,
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Subject: Re: [RFC 06/14] fork: zero vmap stack using clear_page() instead of
 memset()



On 11.03.24 г. 18:46 ч., Pasha Tatashin wrote:
> In preporation for dynamic kernel stacks do not zero the whole span of
> the stack, but instead only the pages that are part of the vm_area.
> 
> This is because with dynamic stacks we might have only partially
> populated stacks.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>   kernel/fork.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 6a2f2c85e09f..41e0baee79d2 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -263,8 +263,8 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm)
>   static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>   {
>   	struct vm_struct *vm_area;
> +	int i, j, nr_pages;
>   	void *stack;
> -	int i;
>   
>   	for (i = 0; i < NR_CACHED_STACKS; i++) {
>   		vm_area = this_cpu_xchg(cached_stacks[i], NULL);
> @@ -282,7 +282,9 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>   		stack = kasan_reset_tag(vm_area->addr);
>   
>   		/* Clear stale pointers from reused stack. */
> -		memset(stack, 0, THREAD_SIZE);
> +		nr_pages = vm_area->nr_pages;
> +		for (j = 0; j < nr_pages; j++)
> +			clear_page(page_address(vm_area->pages[j]));

Can't this be memset(stack, 0, nr_pages*PAGE_SIZE) ?
>   
>   		tsk->stack_vm_area = vm_area;
>   		tsk->stack = stack;